PHOTOGRAPHY

in this article photography is treated under four different heads. the first is introductory and general covering, among other topics, amateur, professional and aerial photography, radiography and amateur cinematog- raphy. the second deals with photographic manufacture; the third, with colour photography; and the fourth is an account of the general principles of photography in the light of recent knowledge. i. introductory and general photography is now applied to very many fields. it 1s em- ployed in astronomy—in which photographic has almost super- seded visual observation—microscopy, spectroscopy and many other fields of science, for a discussion of which reference must be made to the corresponding articles. amateur photography.—the use of photography for making personal records has extended very greatly since portable cameras and sensitive materials were introduced, and the tendency is continually towards a simplification of the work to be done by the user and a diminution in the amount of expert knowledge required to get good results. this has resulted in the establish- ment of commercial firms to develop and print photographs made by the amateur, so that the great majority of photographs are now taken by those whose knowledge of the subject is limited to that necessary for making the exposures. the cameras used are chiefly of the folding type using a cartridge of film and often of very small size. the film can be loaded into the camera without the use of a dark room and removed after exposure for development. after an exposure has been made, an inscription may be written with a stylus on the red paper protecting the aueographic film. the 125 pressure of the stylus removes the dark-coloured wax from the “carbon ” paper. an opening in the back of the camera where the writing is done is then exposed to light, which penetrates the translucent red paper and prints the inscription on the film. for development, the films are hung by clips in deep tanks and are then printed by artificial light upon gaslight or developing- out papers which are made in difierent degrees of contrast to suit the negatives obtained. as a general rule the photographic dealers will deliver prints from an exposed roll of film within two days. enlargements are made upon bromide paper, fre- quently. by means of enlarging cameras in which the focus is automatically maintained correct while the scale of magnification is varied (see fig. 1 on plate). in addition to roll films, films in packs are largely used, each separate unit having a tab of paper attached to it by which it can be withdrawn after exposure. a small number of cameras still use plates or flat cut out films in suitable holders. almost all the cameras used in amateur photography are de- signed primarily for use in the hand and are only occasionally used upon tripods. in addition to the small portable cameras, many amateur photographers use reflex cameras and folding cameras fitted with focal plane shutters. a number of makers have introduced lenses working at very large apertures f/2 and f/2-5 (see fig. 4 on plate) and special cameras fitted with these lenses have been used for photography in ordinary rooms and under poor light conditions (see optical grass). such equip- ment is very valuable under extreme conditions, but has the limitation that large aperture lenses have little depth of focus and consequently must be focused very exactly if good results are to be obtained. professional photography.—this may be divided into the two great fields of portraiture and commercial photography. por- trait photographers are tending more and more to work by artificial light, thus rendering themselves independent of the variations of daylight. large units employing mercury vapour lamps, arc lamps and nitrogen tungsten incandescent lamps are now available. the materials used by the portrait photographer are plates or flat films. there is a growing tendency to make negatives not larger than 5 in. x 7 in. and to make the prints by projection. asa printing material, the gelatino-chloride printing- out paper has been supplanted almost entirely by developing-out papers. in england, bromide paper is very generally used, while in the united states a slower chloride paper is employed pro- fessionally, some grades of this paper having a long scale suitable for printing from the high quality, professional negatives. portrait prints are no longer intended for insertion in albums and are supplied generally in card folders. lightings are of the most varied type, and of recent years many unusual lightings have been introduced as a result of the influence of the motion picture upon the work of the professional photographer. commercial photography is now very wide in its field, a large number of advertisements being illustrated photographically and photographs often taking the place of samples in the hands of salesmen. commercial photographs are printed usually upon glossy papers and are mounted upon linen in such a way that they can be handled and filed conveniently. they are not in- frequently tinted by hand, a practice which has almost gone out of use jor other fields of photography. aerial photegraphy.—photography from the air is utilized for military purposes as well as for surveying. its use during the world war was very extensive, thousands of negatives being taken every day by the armies. the first aeroplane cameras were modi- fications of standard cameras with usually a focal plane shutter, and the material was generally plates carried in holders and later in magazines adapted for quick changing. hand-operated cameras using magazines were later developed and were used largely in 1916 and 1919. later, semi-automatic plate cameras were designed in which the observer released the shutter while the mechanism operated the changing of the plates. in the last years of the war these cameras were made of very considerable size and weight, sometimes as many as 50 plates 18 cm. x 24 cm. in size being carried on a camera at one time. towards the end 126 of the war film cameras were developed carrying rolls of film giving roo exposures 18 x 24 centimetres. the great advantage of such cameras is, of course, the light weight of the material used. in 1918 entirely automatic film cameras were made, the changing of the film and the exposure being carried out by means of a motor, either electrical or driven by the wind (sce fig. 3 on plate). in 1926 the automatic film cameras were used almost exclusively in the united states while plate cameras—either automatic or hand-operated—were still employed by the european air services. i'rom the photographic point of view the most important phenomenon encountered when working from a great height, as in an aeroplane, is the scattering of the light by the atmosphere, an effect which is generally known as fuze. since this scattering is greater for shorter wave-lengths of light, it can be eliminated to a considerable extent by the removal of the blue light and by the use of only longer wave-lengths. in order to accomplish this, the materials used are generally sensitive to red light, and filters can be used with satisfactory results as to exposure; under such conditions great penetration through hazy atmosphere is possi- ble. excellent photographs have been taken from the greatest height which an aeroplane can attain. while acrial photography is an immense aid to the military both for the detection of enemy operations and also for the preparation of maps, its application to precision surveying is subject to limitations. it is not possible to note with suflicient accuracy the height and angle of the aerial camera for a map to be made without correction for errors, and consequently aerial surveying requires the provision of bases of known position on the ground which can be included and used to scale and correct the photograph. this limits its use in surveying (q.v.) since such measured bases are not always available. nevertheless there are many purposes for which aerial surveying is very well adapted. a fire survey of a city can be made with suflicient accuracy by aerial photography at less than one-tenth of the cost of a corre- sponding ground survey. this makes it possible to repeat the fire surveys of rapidly growing cities at much more frequent intervals than could be done otherwise. surveys of lakes and forests can be carried out by the acroplane rapidly and with sufficient accuracy for many purposes. (sce archaeology: -li7r survey; surveying.) radiography (see radiotherapy; rontgen rays).—in this branch of photography very special conditions of work are in- volved. from its introducticn in 1896, when x-rays were dis- covered by rentgen, the field of radiography—almost entirely in connection with its medical application—has grown until at the present time approximatcly as many negatives are taken by means of x-rays as are made in portrait studies. the earlier radiographs were made on plates of the same type as those used for portraiture; improvements were made by the introduction of large quantities of silver salt into the emulsion, while later attempts were made to load the emulsion by the addition of salts of other metals in such a way that the absorption of the x-rays would be increased and high sensitiveness obtained. there is reason to believe that these attempts were based on an incorrect understanding of the laws of the absorption of the x-rays. a considerable advance in the technique was made when double-coated film was introduced, a special x-ray emul- sion being coated on both sides of the film, so that the image was formed half on the front and half on the back. in 1926 it was customary to use such films in combination with one or more, generally two, so-called intensifying screens. these intensifying screens consist of a layer containing calcium tungstate coated on a suitable support. the calcium tungstate fluoresces under the influence of the x-rays, transforming a portion of the x-rays into light which is active photographically. the general method of working, therefore, was in 1926 to expose the film in a cassette; that is, a holder in which the film is pressed into contact with an intensifying screen on each sicle. one of the great difficulties in obtaining radiographs of good quality, especially when photographing through portions of the body of considerable thickness, is the presence of scattered radiation or secondary x-rays produced by the scattering of the photography primary x-rays in the tissues. measurements have shown that the scattered radiation generally accounts for as much as three- fourths of the entire density of a radiograph taken through the body, and since this scattered radiation does not contribute to the formation of the image, it naturally produces a great lowering of contrast and general loss of detail and quality. the only method found to eliminate this is the use of what is known as the potter bucky diaphragm in which a number of strips of lead are adjusted so that they point toward the source of the x-rays. in order to prevent their forming shadows, the diaphragm is moved during exposure, and under these conditions the strips of lead cut out much of the scattered radiation, which is not proceeding from the source of x-rays but is scattered in all directions, and thus enable the image to be formed to a much greater extent by the direct x-rays coming from the focal point (see fig. 2 on plate). for dental work the film is supplied in special packages to be held inside the mouth, usually with a backing of lead foil to stop secondary radiation and to enable the package to be moulded to the shape of the mouth. dental radiography has been extending very rapidly, and in 1926 radiographs of the tecth were among the most valuable guides to the operations of the dental surgeon. amateur cinematogra phy.—cameras have been placed on the market by means of which amateurs can take motion pictures without the use of the elaborate and expensive apparatus em- ployed by the professional cinematographer. some of these cameras, while of small size, use motion-picture film of standard width (35 mm.) and differ from professional apparatus only in the short length of the film which they take and in their low cost. cheaper forms of projectors taking standard film are also sup- plied for the use of amateurs, but the cost of the films and their development and printing have prevented any rapid extension of the use of this apparatus among the general public. cameras have been introduced taking sub-standard film particularly intended for use in amateur cinematography. ‘two sizes have been introduced: in trance, a film 9} mm. wide has been intro- duced by the pathe co.: 30 metres are supplied in magazines for use in a small camera (see fig. 5 on plate), and the film after ex- posure is developed by the user or by photographic dealers by means of a reversal process which transforms the original nega- tive into a positive which is then ready for use in a projector. the eastman kodak co. in 1923 introduced a film 16 mm. wide to make a picture approximately one-sixth of the area of standard pictures, and they and other makers have placed on the market cameras and projectors adapted to take this small size of film (see fig. 6 on plate). the film is packed with paper lcaders at- tached to the end in a special form of light-tight packing so that it can be placed in the camera without the use of a dark room, and after exposure the processing is done by means of a special reversal process which transforms the negative into a positive ready for projection and which it is claimed enables errors in exposure to be corrected toa certain extent during the processing, a sufficient degree of latitude in exposure thus being retained. duplicates can be made from the original positives, standard size film can be reduced to the 16 mm. size and the 16 mm. film can be enlarged to give pictures of standard size, although in doing this a certain amount of graininess in the resulting print is inevitable. the reversal process gives to the original prints very great freedom from grain, so that when projected they are entirely satisfactory upon the screen. the cameras supplied for this branch of photography are usually portable and are motor-driven so that once threaded, they require no attention until the film is finished. for all these amateur instruments copies of theatrical films from various libraries are available for use in the home, and it is probable that such small and portable projectors may also find an applica- tion in the school and lecture-hall. il. manufacture of photographic materials the manufacture of photographic sensitive materials is 4 very specialised industry. approximately 40,000 people are employed in the whole photographic industry throughout the world, about photography plate i. 3 °f: ms op 88 #2 + 70 fic. 1. autofocus enlarger; the size of the enlargement is varied by raising or lowering the lens, the focus being held automatically at the correct point. the negative is placed in the holder between the bellows and the lamphouse above it. fic. 2. radiographs made with (left) and without (right) the bucky diaphragm. fic. 3. automatic aerial film camera with zeiss tessar lens. fic. 4. large aperture camera, taking pictures 2} in. by 3} in. on plates through 10 cm. focusing lens, working at f:2:0. fic. 5. pathe amateur motion picture camera, pictures 9 mm. by 73 mm. fic. 6. pictures 10 mm. by 73 mm, on 16 mm. film. (fig. 3. courtesy of fairchild aerial camera corp., n.y.) photography 20,000 being engaged in the manufacture of materials sensitive to light, with which this section deals, and the remainder in the | manufacture of cameras and in the wholesale distribution of the products to the retailer. with the introduction of the gelatine dry plate, the preparation of materials by photographers for themselves diminished very rapidly, and the manufacture of ready-prepared material took its place. the first dry plates appear to have been placed upon the english market in 1877. in 1880 the manufacture of gelatine dry plates was general in england and was commencing in belgium, germany, france and the united states. the prototype of the modern photographic film was made by coating upon paper emulsion of the kind used for making dry plates, the paper negative being printed, generally after prepara- tion with oil or wax to make it more translucent. in 1885 paper in a roll intended for making negatives was introduced together with a roll holder fitting the back of the existing cameras, the paper being coated on a special coating machine from which was derived the present film and paper coating machines. later, paper was coated with soluble gelatine so that the gelatine emul- sion could be stripped from the paper base, and finally in 1889 emulsion coated upon a flexible film support made of nitro- cellulose was placed on the market. this film was first made by spreading upon a glass table a solution of nitrocellulose which when dried was coated with a substratum to make the emulsion adhere and then coated with gelatine emulsion, the whole film being stripped from the table and cut up for use. in 1891 a method of packing the film to make it daylight loading was devised, this being done by winding it inside a pro- tective sheet of black paper of such a length that the camera could be loaded without exposing the sensitive film to light, and in 1903 the film used in cartridges was coated on the back with gelatine so as to counteract the tendency to curl produced by the coating of emulsion on one side. in 1885 the coating of bromide emulsion paper by a continu- ous coating-machine was commenced, and about 1890 gelatine- chloride printing-out paper was introduced by abney. in this paper the sensitive salts were a mixture of silver chloride and silver citrate containing an excess of free silver nitrate and also free citric acid. a similar paper prepared with collodion as the vehicle instead of gelatine was known in the united states as aristotype paper and in europe as cellodio-chloride printing-out paper. with these printing-out papers a visible image was ob- tained by printing under a negative in bright light for a con- siderable time, and this image was then toned by the deposition of gold before fixation. in 1897 chloride emulsion papers were introduced; they were made without an excess of silver nitrate and therefore were suitable for development in the same way as bromide paper but differed from bromide paper in that the salts produced by the reaction of the silver nitrate with the soluble halide were not removed by washing. these papers are about 200 times slower than bromide paper and are known in the united states as developing-out paper and in england as gaslight papers. until 1900 the photographic industry was carried on in small factories under the direct personal control of the owner. at the present time, however, photographic manufacture 1s organised chiefly in modern factories manufacturing on a large scale, and using specially designed machinery at every step of the process. in the largest of these factories, the output of motion-picture film alone exceeds 150,000 m. in a year. nearly 5,000,000 ib. of cot- tonare used each year for the manufacture of film, and over three tons of pure silver bullion are used each week. the total power required exceeds 20,000 h.p., and the consumption of coal is over 500 tons a day. the process of manufacture may be divided into three distinct sections: 1. the preparation of the base; 2. the preparation of the emulsion; 3. the coating of the emulsion on the base and the cutting and packing of the material so prepared. preparation of the base.—photographic glass is the specially selected, best quality of thin sheet glass made in a factory which prepares sheet glass for all purposes. even under the best con- 127 ditions only about a quarter of the sheets made are fit for photo- graphic use, the remainder being used for glazing. photographic glass is made very largely in belgium in the factories near charleroi, and in england at st. helens. a considerable amount is also made in the united states. the glass is supplied cut to sizes which are multiples of those used by photographers. the glass arrives packed in straw in boxes holding usually about two gross sheets, and after unpacking and preliminary inspection, is cleaned by an automatic machine into which it is fed. the glass plate is received between rollers and passes through a strong solution of soda between revolving brushes which remove the dirt from the surface, and at the end of the machine is coated with a substratum, usually a weak solution of gelatine containing chrome alum, by means of which the emulsion is made to adhere to the glass. the glass is then dried in a heated oven, examined for defects, and if passed is packed in a tray and transferred to the coating-room. ; in the manufacture of film base the first step is the preparation of cotton linters for nitrating, the treatment consisting of cleans- ing the linters very thoroughly and drying them. they are then nitrated in centrifugal machines and when they have absorbed the necessary amount of nitrogen—12-13 %—the resulting nitro- cotton is soused in water and the washing commenced. this washing is a long process, and is continued until the material is entirely free from absorbed acid, after which it is dehydrated, the water being displaced by the alcohol. the dehydrated nitro- cotton is then dissolved in tumbling barrels or mixers in suit- able solvents, those commonly employed being acetone and methyl alcohol, and at the same time the so-called softeners, such as camphor, are added, these resulting in a flexible film. the viscous nitro-cotton solution, which appears rather like honey, is known in the united states as dope. this is now spread out into a thin film by coating it on a travelling surface. in the united states the coating is generally done onto large whecls about four feet across and about 20 ft. in diameter which rotate slowly about their axes so that by the time one rotation is completed the sol- vents have been evaporated sufficiently to set the film, which can be stripped off the wheel, and dried further by passing through drums. the operation is thus continuous, the film base, as it is called, being prepared in rolls of about 2,c00 feet. in europe it is customary to coat a film about 24 in. wide upon a metallic belt from which the film can be stripped after one rotation in the same way as from adrum. the surfaces of these belts and drums must necessarily be prepared very carefully, the wheels being usually made with a highly polished nickel surface, while the makers of the belt machines recommend that they be coated with gelatine. non-inflammable filn.—since cellulose nitrate is highly in- flammable, supplying itself the greater part of the oxygen which it requires for combustion, there has been much work done on the preparation of film which is not inflammable. the most promising substance for this purpose is cellulose acetate prepared by treating hydrated cellulose with acetic acid and acetic an- hydride in the presence of a suitable catalyst. acetate film is slow burning, burning with about as much difficulty as thick paper, and should it catch fire it is very easy to extinguish. it has the disadvantage that on keeping it tends to dry out more rapidly than nitrate film and thus becomes brittle and while it is very suitable for use where proper precautions against fire cannot be taken, as where films are used in homes or schools, it has not the wearing qualities necessary for use in the motion-picture theatres, where the high speed of projection, the great heat of the projection lamps and the continued operation is a great strain upon the film. during or after manufacture the film base is coated with a substratum to make the emulsion adhere and is then ready for coating. the thickness of the film base depends upon the use to which it is to be put. base for use in the roll film cartridge is 3+ thousandths of an inch thick; motion-picture film, 53 thousandths; and portrait and x-ray film used flat, about 74 thousandths. paper base-—vhotographic paper base is generally made by mills which specialise in its manufacture, the chief factories for 128 this purpose being in belgium, france and germany; but in the united states the bulk of the photographic paper base is made in the same factory where the rest of the production of photo- graphic paper is carried on. the paper is prepared very largely from rag stock, although a small amount of wood pulp is some- times added. after inspection the raw rag stock is washed and digested and is then treated in the beater until its preparation is such that it will make a homogeneous, strong sheet. the making is done on fourdrinier machines, and the sheet is given a tub sizing in addition to the sizing given in the beater. great care is necessary to exclude traces of impurities and especially of metals such as copper and iron from the paper stock. the paper is usually given a coating of baryvta before it re- ceives the photographic emulsion. in this process a suspension of baryta (barium sulphate) in gelatine is coated over the surface of the paper, several coatings being sometimes applied, and not infrequently the paper is calendered after baryta coating in order to get a perfectly smooth glossy surface. baryta coated paper is transferred to the emulsion coating-room in rolls of about 2,000 feet. emulstons—emulsions are of two general types: washed and unwashed. emulsions to be coated on glass or film are always washed. in the first case the precipitation of the silver salts is efiected by adding a solution (about 10% in strength) of silver nitrate to a small quantity of gelatine dissolved in a solution of the halide salts which, in the case of negative emulsions, are usually potassium bromide containing a small proportion of potassium iodide. when precipitation is complete, the emulsion is digested in order to produce an increase of sensitiveness, and the bulk of the gelatine is added so that when cooled it will set toa jelly. instead of increasing the sensitiveness of the emulsion by holding it at a high temperature for some time, it may be digested at a lower temperature in the presence of ammonia, and not infrequently the silver nitrate is precipitated with ammonta and then re-dissolved in excess before the precipitation of the halide is commenced. the set emulsion is cut up into small portions and is washed in running water in order to remove any excess of halide salts and also the soluble nitrate produced in the reaction. after washing it is given a final digestion and is ready for coating upon the base. the emulsions used for printing papers are of three general types: bromide papers, which are made with a washed emulsion essentially identical with that used for coating upon glass and film for use in making lantern slides and positive motion pictures; unwashed chloride emulsions in which the emulsion is coated on the paper in the form in which it is mixed without any removal of the soluble salts by washing; and emulsions also made with- out washing but in which the silver salts are a mixture of chlor- ide and citrate, and in which there is free silver nitrate present, these papers darkening visibly when exposed to light and thus giving a printed-out image which is toned and fixed. coating and packing —dry plates are coated by passing them on a travelling belt under some device through which the emulsion is allowed to spread out over the surface of the glass. this may take the form, for instance, of a trough with a slit in the bottom or of a stepped weir down which the emulsion flows, reaching the plate by means of a flap which rests lightly on the surface of the plate. the coated glass travels forward on to a belt or rollers which are kept wet with ice-cold water, and there the emulsion sets. at the other end of the machine the plates are taken off, placed on the racks, and dried slowly and evenly. they are then cut to the requisite sizes by means of machines using diamonds and are packed in boxes of one dozen for supply to the market. paper is coated by passing from a stock roll around the coating roll which is immersed in about half its depth in a trough of emulsion, the paper as it travels upward carrying the emulsion with it until it passes over a large roll supplied with cold water, which chills the emulsion so that it sets, the setting being com- pleted by passage through a refrigerated box. the coated paper then passes forward to a drying-room in which it is hung up in large loops varying in length from five to 20 ft., the loops travelling down the room continuously by means of a chain and the paper photography drying as it travels, so that at the other end the coated and dried paper can be reeled up into a roll. this roll of paper is then cut into sheets, and the sheets stacked on the top of each other are cut by means of guillotines into sheets of the size required for the market, each sheet being inspected for defects before packing. film is coated in almost exactly the same way as paper. film for roll cartridges is coated with gelatine on the back to prevent its curling and is slit to the requisite width and then cut into the lengths required for the cartridges. the film is then spooled on to a core having metal ends so as to protect the edges of the film from light, the film being wound in with black paper or with red paper having an insert of “‘ carbon” paper similar to that used for making duplicates on the typewriter, this type of film being known as autographic. film packs.—film is also supplied in film packs, in which the film is cut into sheets of the required size, and to each sheet is attached a tab, the tabs being numbered consecutively. a set of tabs is then assembled into a pack so that as each tab is pulled, the film is removed from the front of the pack, leaving the next film in position for exposure, the tabs being torn off and thrown away. radiographic film —film of greater thickness, coated with gelatine on the back is used in the flat form in holders similar to those used for plates and thus is cut and packed exactly as plates. for radiography, film is coated on both sides with emulsion, since the x-ray can penetrate the emulsion with very little loss of density. for dental radiography small films are supplied in special packets ready for use without any further protection from the light. motion-picture film.—motion-picture film is slit from the original roll to the exact width required, 35 mm. in the case of standard film, and before packing is perforated on the edges in accordance with the standard gauge used for the cameras and projection machines. motion-picture film comes on to the market in two general varieties: negative film used in the cameras and positive film on which the pictures are printed and from which they are projected. negative film is supplicd in ordinary and superspeed varieties and also made colour sensitive under the name of “ panchromatic ” film. positive film is supplied either on nitrate base, that used most generally, or on acetate safety base, which is slow-burning. for amateur motion-picture photography the film is supplied in the united states and england, 16 mm. in width made on slow-burning acetate base only, and is supplicd in lengths of 100 ft. packed with paper leaclers at the ends and on a special protective spool so that the cameras can be loaded in daylight. this film is made to be reversed directly to a positive so that the same film as is used in the camera is employed for projection. (g, ea.) lil. colour photography the methods which are used at the present time for reproduc- ing objects in colours by photography are dependent upon the suggestion made by clerk maxwell. according to young’s theory any colour may be considered to be due to the stimulation of the three primary sensations of red, green and violet. in order to reproduce any colour, therefore, we may analyse the different proportions of these three primary colours in it and then syn- thesise the colour by superimposing the three primaries upon a scrcen in projection. to illustrate this, maxwell took three photographs of a coloured ribbon, one through a red solution, one through a green solution, and a third through a blue solution. irom these three negatives, three transparencies were made which were projected cach by means of the coloured light by which it was taken, maxwell adding a fourth picture taken and projected by yellow light. in 1869 ducos du hauron published a small book on colour photography in which he laid down the principles of three-colour photography, on which all later work has been based; and at approximately the same time charles cros published an article in which he had independently come to practically the same re- sults. du hauron’s book is astonishingly complete and contains a very clear account of the two fundamental processes of colour plate it. photography ‘sdurpying aq uapply ajaqe[dui09 st asanod sz ‘91n3d1d ay} ul ‘yznoyie ‘premyou suni puke suldeaq almpeoig yoiym woiy bore peodijs |jeuls e ‘usai5) buljmog sun1 (punoisai0j yal) ysieg a19}eg jo jguio. puey-yysu1 joddn oy} wo °s}991}3s pvoig pu [[ea\ jo jouiod dy} uo (ysiyy “3} 6£s) jsnil, sjoyueg jy} jo jam0} oyi[-prwiesad ay} si suipjing ajqeyinby ay3 jo yuo’ uy *(yysryy 3} z19) zuipjing ja8urs oy} syunouins 31 aq uappry a]j1ed pue surpying 1933] ay} jo 49] 9y} 03 suiseadde 10m0} vy “yoo]q a11qua 9uo saidndz0 puke ysiyy *3j sgp st ‘suipying ajqe} -inbqy 9u} ‘31 jo juoij ut 91n}oniys sie] oy], “p[jom dy} 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jo sjudwaied~ap snoliea oy jo siazzenbpeay ay} a1v yoiym punoze pure ul ‘jjeyaryay st ‘arenbs quaweryseg wosj surpesy 3jj0] 243 ug ‘peyovodun sem ssunseyy usseay ojoymm ‘][ph joisuiutisaa\ jo ,,snjny jo [je] ,, 24} si jamo] yo[d oy} yyauoq jsoui[yy *j0y}o dy} je jamo] viiozoia 94} pul pua duo je udg sig suiutejuod jomot yio[d 94} yim juswietlivg jo sosnoh{ 94} si jai oy} sulovy pue abpiiq ay} jo 24811 dy} 0} ssurpying jodnoi3 ayy, “sowey yy ay} sassos0 ajay yoy sprig joysuiwi3s9,\\ 0 <0udy} pue oenbs jusutetivy out suni j99i}s piiojiia “poling o1e pvap snolsjsnij! jay pue paumoid oie ssuly s,pulr]sugq j194 ], *siamo} u193isam a]gou om $71 aq pastusode1 9q ud yoiya aaqqy joqsuiwysoa\ sed suruuns ‘399138 c140jdia si viigwigy ul jo 91}u9d dy} jo moia [llj9e s14} ut aiefysnos0yy} ure ay] linawnuaaod hsiling ahl jo sualuvnogvah ahl :‘waalsninlsam (‘pit ‘supifoda y ) photography photography including their application by different methods. these processes are known as the “ additive ” (that employed by maxwell) and the “ subtractive ” processes. the practical development of colour photography was, how- ever, delayed for many years by the difficulty that the photo- graphic materials available were sensitive only to the blue and violet of the spectrum; it was with the greatest difficulty that materials sensitive to the visible spectrum could be obtained and with such materials very long exposures were required. the introduction of colour sensitising by vogel in 1873 first made colour photography possible, and the introduction of the isocyanine and carbocyanine sensitisers at the beginning of the 2oth century made it practical. f. e. ives was the first to develop maxwell’s process in practical photography. he took photo- graphs through three dyed gelatine filters upon materials sensi- tised with the best dyes available at that time and projected the transparencies by means of triple lanterns devised for the purpose, these synthetic colour pictures obtained by the additive process giving extremely good colour reproduction upon the screen. he also devised the apparatus known by him as the kromskop, in which the three pictures could be viewed super- imposed upon each other and illuminated by light of suitable hue, so that a combined additive image produced by the three colours could be obtained without the use of a projection lantern. principle of the subtractive process—the “ subtractive ” process, which was employed by ives, depends upon the produc- tion of colours by subtractive absorption instead of by the addi- tion of the colours to each other in projection. suppose that over a sheet of yellow gelatine, which will absorb the blue light, is put a sheet of magenta gelatine, which will absorb the green light; then, since the yellow absorbs the blue light, and the magenta the green light, only the red light is transmitted; a red image can thus be obtained either by projecting it through a red filter or by putting a magenta image on the top of a yellow one. in the same way a green image can be obtained by putting a blue-green one on the top of a yellow one, when the yellow will cut the blue out of the blue-green and leave only green. in working the subtractive process, the three negatives are printed in coloured dyes, the picture taken through the red filter being printed on gelatine dyed blue-green, the one taken through the green filter on gelatine dyed magenta, and the one taken through the blue filter on gelatine that was dyed yellow. now, if the three are cemented together in register, the resulting trans- parent colour picture will reproduce the colours of the original subject and will be a transparency which can be viewed in the hand, or examined in front of an artificial light, or projected in a lantern. screen plate process.—in his book, ducos du hauron suggested one modification of the additive process which has since become of practical importance and is known as the screen plate process. he suggested that the surface of the glass plate or film might be covered with tiny filters—red, green and blue, the sensitive emulsion being coated on top of these and photographed through the filters. this was carried out in practice by joly, who made plates by ruling alternate lines of red, green and blue-violet by means of a ruling-machine, and later by lumitre, who made the autochrome plates by covering the glass with starch grains dyed in the three primary colours. in addition to the processes of ruled lines worked out by joly and by macdonough in chicago, and the lumiere process, a process invented by christensen dependent upon the use of dye particles of dammar emulsified in a suitable liquid has been exploited commercially in germany, and plates have been made in england by printing the lines in bichromated colloids. when such colloids as gelatine, fish-glue, etc., are treated with a solution of bichromate, dried and exposed to light, they become insoluble in water. if, then, a glass plate be coated with a solu- tion of bichromated fish-glue, and the coated plate be exposed to light under a black line screen which allows only one-third to be exposed at a time, two-thirds being protected by the black lines, and then washed in water, one-third of the entire surface will be‘covered by lines of fish-glue, which can be dyed. after the 129 dye has been mordanted to prevent it running, the plate can be recoated with glue, registered in position on the printing screen, and printed so as to get a second glue line which can be dyed in the second colour, provided that the first line has been protected from the action of the dye by coating it with a varnish or other- wise. the third line can be printed by exposing the plate through the back, when the first two lines will act as a screen for the third, and in this way we can build up a three-colour line screen. in germany, krayn used thin sheets of coloured celluloid of the three primary colours and piled them on top of one another to get a pile of alternate layers of red, green and blue. by cutting sections down through this pile, sheets of celluloid made up of strips of each of these red, green and blue layers were obtained. screens have also been made by mechanical printing or a com- bination of this with dyeing, and a number of other methods are possible and have been proposed for making the screens. all the various processes of colour photography which have been described are applicable to motion-picture work, though, owing to the long exposures required and the loss of light by absorption, no practical process utilising the principle of the screen plate for cinematography has been developed. the motion- picture process, however, like other processes of colour photog- raphy, divided clearly into additive and subtractive processes, and, as in the case of the photography of stationary objects, the additive processes developed first. additive methods.—xn the case of motion pictures, there are two methods of additive synthesis. either one may project on the screen the three pictures simultaneously, as is done in the triple projection lantern, or the colour units may be projected successively, relying upon persistence of vision for the blending of colours. kinemacolour process —the oldest practical cinematograph colour process is a two-colour process utilising persistence of vision for the addition of the pictures. by means of a rotating disc of colour filters placed in front of the camera, pictures are taken on panchromatic film alternately through a red and green filter, the pictures being taken at twice the normal speed, so that for each complete picture, two negatives are made, one through each filter. the positives from these negatives are projected through a machine, similarly equipped with a rotating shutter, which is made to operate synchronously with the picture, so that the pictures taken through the red filter are again pro- jected through a red filter, and the green pictures similarly through a green filter. the succession of red and green pictures upon the screen produces complete synthesis by persistence of vision and gives the effect of a two-colour additive picture. this process is known as the kinemacolor process and enjoyed a considerable success. the gaumont process —the most complete and satisfactory process of colour cinematography is undoubtedly the simultane- ous three-colour additive process worked out in its complete form by gaumont. three lenses are used in the camera, and three pictures are taken through the primary colour filters upon the same strip of film. the positives are projected in a machine fitted with a corresponding pull down and gate, and with three condensers and three sets of objectives fitted with special registering devices. the results given by this process are admirable. the subiractive process —an advantage of the subtractive colour process is that the film so produced can be shown in any ordinary machine, and this advantage is even greater in the case of motion pictures than in the case of ordinary lantern slides. the obvious method of making subtractive motion pictures would be to make three negatives and to superpose the negatives upon each other in printing, recoating the film cach time and using a dyed bichromatic process or a process in which the silver image is made to mordant the dye and so is transformed into a colour image. the recoating and re-registering, however, present very great difficulties in practice and would make the process very costly. the practical subtractive processes of colour cinematography use only two colours: these two colours may be either in the one emulsion layer, or in two emulsion layers on 130 opposite sides of double-coated film, a method which appears at the present time to offer considerable advantages in the direction of simplicity. it is clear that since there are two sides to a film, it is possible to coat an emulsion on each side of the film and to print the red image on one side and the green on the other. this process has been worked out to a practical end in several ways, some of the colour images being produced by the mordanting of a dve upon an image obtained by the conversion of the silver into some suitable substance or by the imbibition of a dye into the gelatine or by toning by chemical means to coloured images. _ prisma process.—of the two-colour subtractive processes, the prizma process, depending on the use of mordanted images, was successful for a number of years. this is a typical two-colour process, the red and green negatives being printed on opposite sides of double-coated film and the coloured images produced bv mordanting dyes on to images obtained by replacement of the original silver image by materials capable of mordanting the dve. technicolor process ——in the technicolor process, the images on opposite sides of the film are gelatine relicfs prepared bv printing through the back of thin films and then cementing the two films back to back, developing, bleaching and washing awav the gelatine which during bleaching has been left soft as a result of the absence of an image. the gelatine relief so obtained is coloured with suitable dyes. kodachrome process —in the kodachrome process positives are printed from the original negatives and these are printed on opposite sides of double-coated film to produce negative images. these silver images are removed by a solution which renders the gelatine impermeable where the silver grains were acted on by the solution. after fixing, washing and drying, the film is dyed on both sides in the appropriate colours, the dyes penetrat- ing the permeable gelatine only where no negative image was present and thus producing a positive image consisting of pure dye. iv. general principles modern photographic emulsions consist of a precipitate in gelatine of flat crystals of silver bromide, generally hexagonal or triangular in shape. these crystals have a cubic lattice, and the addition of iodide to the emulsion produces crystals in which the lodine replaces bromine directly in the lattice, and slightly in- creases the lattice spacing. the distribution of different sizes of crystals depends upon the method by which the emulsion is produced, and the properties of the emulsion depend upon that distribution (see fig. 1). the silver halide grains act as units both toward light and toward the action of the developer, so | go b =halide area in (§2x lo“4 per sq.cm. a= frequency * |0~4 per sq.cm. 0-1 0°5 oo lo 0:5 kx 2,000 approx. 17,351 grains photography that each individual grain acted upon by lhght becomes de- velopable when a sufficient amount of action has taken place, and it will be completely reduced by the developer; but the exposure of one grain will not affect any other grain unless the chemical action of development can spread by contact from one to the other {see fig. 2). if we suppose that the same amount of photo-chemical product (latent image) makes either a large or a small grain developable, then, other things being equal, the larger grain contributes a larger amount of silver to the image and thus gives more density for the same amount of light action than a smaller grain. for this reason alone, emulsions containing large grains would be more sensitive than those containing small grains, but experiment has shown that this is not the only factor and that larger grains require less light action to become developable than small grains. further it has been shown that the seat of this sensitiveness is concentrated in specks in the grains, and the sensitiveness of the grain depends upon the presence and the number of these sensi- tive specks. being different in nature from the silver bromide itself, it was suspected that the specks were derived from the gelatine, since some gelatines give emulsions which are much more sensitive than others. a material was extracted from gelatine which was identified by dr. s. e sheppard with allyl mustard oil, which in ammoniacal solution becomes allyl thiocarbamide. allyl thiocar- bamide reacts with silver bromide, and the compound of the two in alkaline solution produces silver sulphide, so that the sensitis- ing specks may be ascribed to the presence of minute traces of silver sulphide on the surface of the silver bromide grain. the ripening process characteristic of the preparation of gelatine bromide emulsions may be ascribed partly to the development of grains of larger sizes from those of small dimensions and partly to the production of the sensitising specks of silver sulphide on the surface of the grain, the exact action of light upon the grains is not well understood, though much research work is being done on the subject. for a discussion of the various theories which have been proposed, the literature must be consulted. the photographic properties of an emulsion are expressed by its characteristic curve, the present form of which was originally suggested by hurter and driffield. in this curve, which is shown in fig. 3, the density is plotted against the logarithm of the ex- posure. the density is the logarithm of the reciprocal of the transparency (d=log 1/t) and has been shown experimentally to be proportional to the mass of the silver per unit area. the amount of silver which will produce a density of unity, corre- sponding to a transmission of 1/roth of the incident light, is measured — = a — ee ey, — oe om o — ee ee eee 50 55 60 25 $area ine mean class size diamin | fic. 1.—distribution of sizes of grains. photomicrograph of emulsion showing grains of silver bromide, developed and undeveloped, and curve plotted from measurement of such grains. photography, colour ; y' es 1. ro - 4, + - three examples of direct colour photography. the top left-hand view is from a snapshot taken at 6.0, a. m. (photographs by h. a. key ) photography termed the ‘‘ photometric constant.” for ordinary photographic emulsions about 10 milligrammes of silver will produce a density of unity on one square decimetre of surface. the curve obtained by plotting the density against the logarithm of the exposure shows three fairly well-defined regions: the initial part, convex to the log e axis, termed the region of wuder-exposure; the middle part, which approximates to a straight line, termed the region the unde- fic. 2.—undeveloped and developed silver grains. veloped silver bromide crystals are transformed into amorphous black grains of metallic silver. of correct exposure; and the upper part, concave to the exposure axis, termed the region of over-exposure. if we prolong the straight line portion to cut the exposure axis at a point j (this was termed by hurter and drifheld [fig. 3], the iserita of the matertal) and the reciprocal of the inertia is taken as a measure of the sensitiveness of the material, the slope of the straight line portion, that is, the tangent of the angle @, is termed the development factor, known in photography as ¥. the value of y increases with time of development, the in- crease tending to a limit termed yoo (gamma infinity), which measures the extreme contrast of which a plate is capable. we shall return to the significance of these quantities for photo- 131 graphic printing and reproduction later; meanwhile, we may note that the straight-line portion of the characteristic curve can be represented by the equation, d=y (log 1 £ —log 191) where f is the exposure (intensity of light x time) to light, ¥ is the development factor, z.c., tan @ in fig. 3, and 7 is a constant, | oft 1-5 oo 1-0 log exposure fic. 3.—iiurter and driffield curve. the density of the negative plotted against the logarithm of the exposure to show the char- acteristics of the emulsion. —s ies) the exposure at which the straight line cuts the axis. although this equation can be regarded as only an approximation, it is a very useful one, furnishing a reference standard for the phenome- na of both development anc exposure. it will be seen that as long as this equation holds for different degrees of development d=ky, £ being constant, that is, any function representing change of density with degree of development will be true also for y. this is expressed in hurter and driffield’s law of con- stant density ratios. the normal manner in which the density and gamma change with time of development, if this law holds, is represented in fig, 4, t=0 tg ts : t. d do 4 ts oa ° es —— | | tan.ley 8] 6 | 9 12 15 18 214 24 27 30 log ioe fic. 4.—change of density and gammia with time of development. as the time of development is increased, density and gamma increase. the characteristic curves intersect at a point (log1ji) on the logio/#-axis. the value of y increases with time of develop- ment, but less and less as development proceeds, and ultimately reaches a limit which is characteristic of the plate but also depends to some extent on the developer. if we now develop (for different times) plates which have been given a series of exposures increas- ing in geometric proportion, measure the densities and plot the 132 resulting curves and then plot y (or density d) as a function of time, we get a curve of the type shown in fig. 5. it will be seen that y (and density) increase rapidly at first, then more slowly, finally reaching a limit. making certain simplifving assumptions, the mathematical function representing this curve can be obtained. let us suppose, 2:0 pate of development 0 20 40 time in mins. curve calc. fic. 5.—yt curve. growth of y with increasing time of development. 60 80 100 120 in agreement with the approach of the curve to a limit, that d*, the limiting density, corresponds to the total developable silver bromide for any fixed exposure. the density undeveloped at any time ? will be dtm d, where d is the density developed at time ¢. supposing other factors—such as the concentration of the developer in the film, its rate of diffusion, temperature, etc.— 'd constant, we can then put the rate of development, ore, dd —=k (d>d this equation gives on integration pro- portional to the undeveloped density 1.e., where & is a constant. ie de k= 0b. p= the constant & is termed the velocity-constant of development. when bromide is present in a developer, the straight line portions of the characteristic curve when produced do not meet me log a (i—ekt);y=y* (i—e**) pye 1g | au os a ig 6 | > logiok fic. 6.—bromide depression curve. bromide in the developer causes a depression of the convergence point. upon the exposure axis but at a point somewhat below it, the effect of bromide being equivalent to a constant depression of density in the straight line portion of the curve (see fig. 6.). this depression of the intersection point shown as } can be used for evaluating the action of bromide, and since an inverse functional relation exists between the reduction potential of the developer photography and the bromide concentration which will produce a given de- pression of the intersection point, a scale can be obtained ex- pressing the relative energies of developers. the following table from a. nietz gives what are probably the most reliable data. relative energy ferrous oxalate ; : ; : ; , 03 p-phenylene diamine, hydrochloride (no alkali) 0-3 p-phenylene diamine hydrochloride (alkalt) ; . of hydroquinone . ; ; (standard) 1-0 p-pheny! glycine 1-6 hydroxylamine ; 2-0 toluhydroquinone . : 2:2 p-aminophenol (hydrochloride) 6:0 chlorhydroquinone ‘ : 7-0 dimethyl p-aminophenol (sulphate) 10:0 monomethyl ; ; 20-0 diamidophenol 30°40 since the inertia is expressed in exposure units (m.c.s.) its numerical value depends upon the light source employed in the measurement of the characteristic curve. hurter and driffield used originally the standard candle and then a pentane lamp; at the present time electric-light sources and acetylene burners are employed. the international congress of photography in 1925 recommended as the standard light source for photographic work the adoption of one visual candle-power of light of colour | sean ee lo 12 64 16 18 20 22 24 2628 30 32 6 8 racteristic curves of papers with increasing times of increasing time of development produces regression of inertia without affecting y except during induction period. fic. 7.—che development. temperature 2,360° absolute. many workers prefer to use a light which has been modified by passage through a filter which is reducing its quality approximately to that of sunhght, corre- sponding to a colour temperature of approximately 5,000° absolute. the so-called “ii & d” speeds of photographic materials thus depend entirely upon the light source and other conditions used in testing, and the speeds given by one maker are not comparable on casual inspection with those from another source. the photographic characteristics of printing papers can be measured by a procedure similar to that used for negative materials, but in this case it is not the mass of reduced silver which is of chief importance but its relative retlecting power. the retlection density is thus the logarithm of the reciprocal of the reflecting power (id =logi9 1/r), the reflecting power being measured with illumination at 45° to the plane of the paper. the characteristic curve obtained with different times of develop- ment is shown in fig. 7, and it will be seen that the effect of increase in time of development is to produce a regression of the inertia without change in y except in the very earliest stages, where the curves are usually of a distorted form. the following constants may be derived:— constant symbol significance (a) maximum (reflection dmax highest attainable density. density) . : : : (b) contrast element (re- dd differential. flection density) dlog.l (c) gamma element {re- yr value of (b) for straight- flection density) line portion, or at inflex- 10n-point. photography significance range of intensittes repro- constant symbol (d) latitude element (re- l flection density) duced on straight-line portion. (e) rendering power (re- flection density) . ; r 1ol s range of intensitics repro- ducible by density differ- ences. the exposure in m.c.s. necessary to print through a negative density of 2-0. (f) total scale (reflection 5 density) . ; , ; (g) standard exposure (re- es flection density) ; from a study of the characteristic curves of the materials used in making the negative and of the material used for printing the negative, the reproduction of the tones of the original subject in the photograph may be traced. by ‘ tone” is understood the variation from light to shade in the subject or the gradation of luminosity apart from colour. this problem is the central one of photographic theory and is fundamental to practically every application. physical tone may be taken as equivalent to light intensity or photometric brightness, whether this be intrinsic or reflected. physiologically, it is apparent brightness that matters, and this can conveniently be treated as physical brightness modified and interpreted by the human eye. the physical range of tones in a subject may be determined photometrically. in ordinary landscapes a range up to 1 to 10 is low; 1 to 30 or 1 to 4o is a normal contrast; 1 to 60 and upwards, high; 1 to 250, extreme. the limits of correct rendering are fixed by the straight-line portions of the characteristic curves. for methods of computing the tone reproduction given in any photographic operation, the papers by f. f. renwick and l. a. jones must be consulted. the resolving power of photographic materials may be defined as the distance between two closely adjacent images which can just be distinguished. it is limited primarily by what is termed the irradiation or spreading of the image due to diffusion of the light in the film. this spreading of the image has been investi- gated by astronomers and used as a basis for a method of stellar photometry. | in addition to the irradiation factor, the resolving power de- pends upon the development factor attainable in the material. the sharpness of an image, in fact, is theoretically equal to the development factor divided by the turbidity factor. both of these factors are functions of the wave-length of the light used in exposure, so that the resolving power is naturally dependent upon the colour of the light. in addition to these two factors, a psychological factor enters into resolution dependent upon the graininess of the image. photographic images on devel- opment show with lower power magnification, as used in projec- tion and enlarging, a granulation which is objectionable in such operations. l. a. jones and n. deisch have shown that it may be measured and numerically ex- pressed on the assumption that g, the graininess, is directly proportional to the distance at which it becomes just visually imperceptible, this being com- pared with the distance at which a structure of known period, e.g, a fine cross-line screen, just disappears. suppose a screen of : 2,000 lines to the inch to be equivalent in this way to a given granularity of the developed image, the latter may be said to have a graininess of 2,000. fic. 8.—fan test object, used for measuring the resolv- ing power of photographic materials. 133 the practical resolving power of photographic materials can be measured by photographing the reduced image of a fan-shaped black and white grating (see fig. 8) and measuring the extent to which the lines are resolved. colour sensitiveness—silver bromo-iodide materials: are sensitive to the ultra-violet, violet and blue of the spectrum, the limit of sensitivity in the green being at about 550 yu. recent in- vestigations in which the sensitivity has been plotted against wave-length for equal amounts of energy show as far as measure- ments have yet been made that the sensitivity rises toa maximum at about 480 yu. in the case of materials of high sensitiveness and is constant throughout the violet and ultra-violet. when the spectrum below 300 is photographed, the absorption by the gelatine becomes important, and below 200 ordinary photo- graphic materials are practically useless. schumann suggested the use of plates made without any gelatine, silver bromide being precipitated and allowed to settle at the bottom of a deep dish on to glass plates. a method which has been used recently for the photography of the short wave-length ultra-violet is a fluorescent coating of mineral oil which transforms the short ordinary panchromatic fic. 9.—spectra of ordinary, ortho and panchromatic materials, taken in a diffraction grating spectrograph in front of the slit of which is placed a black grass wedge ultra-violet waves into blue light and these produce an effect upon the plate. the sensitising effect of dyes discovered by vogel has been developed very greatly. erythrosine, that is the tetra-iodo derivative of fluorescein, was found by eder to be the most suitable dye for sensitising in the green and yellow and has been adopted by photographic manufacturers throughout the world, the materials so sensitised being known as orthochromatic materials. in fig. 9 are shown the spectra of an ordinary material, an orthochromatic one made by the use of erythrosine, and a panchromatic one made by the use of the dyes introduced by the hoechst dye works soon after 1900. a new period in the development of the sensitivity of photo- graphic materials arose with the discovery of the sensitising power of ethyl red by miethe and traube in 1902, ethyl red being a dye produced by spalteholtz in 1883 by the combination of quinaldine ethiodide with quinoline ethiodide in the presence of alkali. e. koenig at the hoechst dye works introduced several dyes analogous to ethyl red which are known as the isocyanines; and in 1906 homolka produced another class of dyes now known as the carbocyanines by effecting the condensa- tion in the presence of formaldehyde, the first of these dyes to be introduced being pinacyanol. by the use of pinacyanol, generally in combination with some of the isocyanine dyes, materials are rendered panchromatic; that is, sensitive to the whole visible spectrum up to and including the red. soon after 134 the introduction of pinacyanol, the hoechst dye works intro- duced dicyanine, which sensitises far out into the infra-red but is difficult to use. it has, however, been employed with success for the photography of spectra up to xgoo. in 191g adams and haller discovered a new dye, kryptocyanine, with its maximum sensitiveness at 4760, and this dye makes it possible to take photographs by infra-red light with a very short exposure; it is employed even for motion-picture photography. a new dye discovered by h. t. clarke in 1925 makes it possible to photo- graph the spectrum without difficulty up to a goo, the maximum sensitiveness of this dye being at a820. (cob ke m2) photosynthesis (see 21.748)—the term photosynthesis is very commonly applied to the process by which green plants build up organic food material under the action of light. it is not altogether a satisfactory term, for as the literal meaning is “synthesis under the action of light ” it would include any chemical synthetic process of this type. also it is not at all certain that the actual systhetic process is due to the action of light. the term carbon-assimilation is also employed though it carries with it the suggestion that carbon itself rather than car- bon dioxide is assimilated. the expression ‘ photosynthetic assimilation of carbon clioxide ” is probably the most accurate description. during recent years there has been a very large output of work on this branch of plant physiology and the field is a very wide one. the subject has been admirably reviewed by stiles. | pigments.—the process of photosynthesis in plants is associ- ated with certain pigments which absorb the incident light; the energy so obtained 1s employed in the building up of complex organic substances from carbon dioxide and water, oxygen being at the sume time evolved. it should be realised that the whole life of the globe is dependent on the photosynthetic power of green plants; they alone are able to manufacture their own food material. non-green plants and animals have not this power, but are entirely dependent for their food on the green plant. in the case of the higher plants, the pigments of the green leaf, as we know mainly from the work of willstatter and his collaborators,? are four in number — chlorophyll a, csslinosnamg, a blue- black crystalline substance, greenish blue in solution; chlore- phyll b, csshmognam[g, a green-black substance, green in solution; carotin, cass, an orange-red substance found also in carrots, and xauthophyll, cahlseo2, a yellow substance. the two first are the green pigments and are often termed “ chloro- phyll,” the other two are the yellow pigments. the amounts of these do not vary very greatly in different leaves. the average amounts are given below: percent of fresh percent of dry weight weight 0:2 0-075 o-o17 0-033 chiorophyll @ chlorophyll 8 carotin : xanthophyll in green algae the same four pigments are present; in the brown and red algae and blue-green algae chlorophyll is present in association with other pigments which give these plants their peculiar colour. the assimilating pigments are not dissolved generally in the cell but are associated with denser portions of the protoplasm of definite form, known as plastids; it is apparenily in these plastida that the special physical and chemical processes characteristic of photosynthesis occur. it 1s generally accepted that the two green pigments are the most important in the proc- ess of photosynthesis. one of the functions of the green pig- ments is clearly that of absorbing the necessary energy for the decomposition of carbon dioxide, and their solutions show very characteristic absorption bands particularlv.in the red-orange and the violet end of the spectrum. whether in addition the chlorophyll reacts chemically with the carbon dioxice is still in doubt. willstatter and stoll *? hold that the carbon dioxide combines with the chlorophyll! to form a definite compound on which light acts; it is thus converted into a substance of the nature of a peroxide which can be acted upon by an enzyme. photosynthesis warburg ‘ holds that by the action of light upon the chloro- phyll there is formed oxygen and a “ photochemical primary product,” which then reacts with a derivative of carbon dioxide; this derivative, the ‘‘ acceptor ” is reduced, and the chlorophyll is reformed. products of photosynthesis—sachs (1862) was the first to relate the carbohydrates appearing in the leaf to the process of assimilation; he was responsible for the dictum that starch was the “ first visible product of assimilation.” this was based on the fact that mn the case of many plants it appeared in the plastids of the leaf in the light and disappeared again in the dark. jt is now generally agreed that carbohydrates of some kind are the main products of photosynthesis though many monocotyledons form little or no starch, but much sugar in the form of glucose, fructose and cane sugar. which carbohydrate is first formed is a matter still in dispute. in green leaves exposed to light cane sugar accumulates markedly, and this has led to the view that it is the first-formed sugar. a much more plausible hypothesis would scem to be that hexose sugars (glucose and fructose) are first produced, and that when these reach a certain concentration cane sugar is formed from them. this view is supported by the work of weevers.5 the starch arises in all probability from glucose; it would seem likely that there is a critical concentration of sugar in the chloroplast above which starch is formed from the sugar, and below which the starch is converted back into sugar. this is confirmed by the fact that by floating them on sugars of high concentration such leaves as are normally starch-free will produce that substance in their chloroplasts. formaldehyde hypothesis ——baeyer in 1870 put forward the suggestion that in the process of photosynthesis formaldehyde (cho) is produced with the help of chlorophyll under the action of light, and that the formaldehyde then polymerises to form sugar. since that time the view that formaldehvde is an inter- mediate product in photosynthesis has been very commonly held. although formaldehyde can be produced from carbon dioxide and water under certain conditions and hexose sugars can arise from it, yet there is no clear evidence of the presence of formaldehyde in the green leaf. the formaldehyde theory, though attractive in its simplicity, cannot be said to rest on any very secure foundation. effect of various conditions on the rate of photosynthesis.— the rate of this process ts affected by the concentration of carbon dioxide available, by the amount of water, by the intensity of the light and by its wave length, by the chlorophyll content, by the supply of oxygen and of mineral salts and also by many other internal conditions, or factors as they are'called. it used to be supposed that all these factors acted independently, but it was shown by the work of f. f. blackman § that the factors are closely inter-related. he put forward the “‘ theory of limiting factors,” that in any set of conditions the rate of the process was determined by the factor present in the lowest intensity. the view that under any set of conditions it was only by the increase of the intensity of one factor that the rate of process could be increased requires some modification in the light of recent work; it is clear; however, that if any factor a is present at very low intensity, it will prevent an increase of another factor b having the marked effect upon the rate of the process which it weuld have produced if the factor a were acting at a higher intensity. bearing this relationship of the factors in mind, we find that with increasing concentration of carbon dioxide the rate of assimilation goes up proportionally, if the concentration is not too high and light and temperature are sufficient. similarly with increasing light-intensity the rate of assimilation goes up proportionally if the temperature and concentration of carbon dioxide are sufficient. with higher light intensities there 1s probably a falling off in the rate, as with higher concentrations of carbon dioxide. with a moderate temperature, if other con- ditions are favourable, the rate of assimilation obeys the van't hoff rule, approximately doubling for cach rise of temperature of 10° c. at higher temperatures somewhere in the neigh- bourhood of 25° c., the injurious effect of high temperature ap- pears and the rate begins to fall off. the process of assimilation phototielegraphy is little sensitive to the concentration of oxygen, reduction to 1-rtocth of the norma] amount having no effect but complete absence of oxygen acts injuriously. a number of inferzal factors must markedly affect the rate of photosynthesis, but the only one that has been closcly investi- gated (by willstatter and stoll) is that of the chlorophyll- content. it was found that the photosynthetic rate per unit of chlorophyll (the “assimilation number ”’) varied markedly in different plants and even in the same plant. leaves of yellow varietics were found to show very high rates in proportion to the amount of chlorophyll present, though the rate in these leaves is less influenced by temperature than is that of fully green leaves. clearly there is some internal factor (or factors) other than chlorophyll content which is controlling the rate. this may be termed the “ protoplasmic factor.” willstatter and stoll conclude that the factor is an enzyme. a reduction in the water centent of the leaf has also been shown to be associated with a certain amount of reduction in the rate of photosynthesis. effect of quality of light on photosyuthesis —the question of the effectiveness in photosynthesis of light of different wave lengths has been hotly debated by physiologists for many years. the question is a womplicated one since in white light there are great differences in the energy-value of the rays of diffcrent wave lengths, and the different rays are absorbed to a very different degree by the chlorophyll and the colourless parts of the leaf. in the case of the leaf there is little doubt that with equal intensity of incident light the red rays are more effective than the blue. the work of warburg and negelcin quoted below indicates also that for equal quantities of light energy absorbed by the chloroplast, the red rays are considerably more effective than the blue. efficiency of the photosynthetic process —\t is known that the green leaf is comparatively inefficient as a producer of assimila- tion material. brown and escombe* attempted to determine the efficiency of the green jeaf by comparing the energy of the light absorbed with the absorption of carbon dioxide, the assump- tion being made that glucose was produced in the leaf from the carbon dioxide. ‘their results are only very approximate but they indicate that the efficiency is only about 1%, increasing however with low intensilies of light to about 4%. a large proportion of the energy absorbed is used in transpiration, and with the higher intensities of light the concentration of carbon dioxide is too low to allow of the light being fully effective. ‘the most accurate experiments on the efficiency of the photosyn- thetic process are those of warburg and negelein® with the unicellular alga, chlorella. with such a plant there is no question of transpiration, and the experimental arrangements were such that practically the whole of the light received was absorbed by the chloroplasts of the cells; in the case of the leaf a considerable amount of the light passes through and a certain amount is reflected. warburg and negelein show that with decrease in the intensity of the light the efficiency goes up, 7.e., a larger proportion of the light absorbed is made use of in photosynthesis, the rate of the latter process being measured by the production of oxygen. in their earlier experiments they obtained an efficiency of 71% when using yellow and yellow-red rays; the methods employed for calculating this were, however, not altogether satisfactory. in their later work they obtained results of 59° for red light, and 34°% for blue light. whether these percentages are the maxima that can be obtained is not yet certain; internal factors may have been present which reduced the rate. the results do show, however, that under these special conditions (carbon dioxide 4%, temperature 1o° c. and a very low intensity of red light) the photosynthetic process in the green plant may exhibit a higher level of efficiency than that of any photochemical process studied in the laboratory. the apparent inefficiency of the green leaf is no doubt largely due to the fact that much of the energy of the light is used in transpiration and also to the fact that there is a large amount of absorption by structures in the leaf other than the chloroplasts. 135 bipiiograrny.—( (i) w. stiles, photasynthesis—the assimilation of carbon by green plants (1925). (2) r. willstatter and a. stoh, uintersuchungen tuber chlorapivll (1913). (3) r. willstdtter and a. stoll, untersuchungen tiber die assimilation der kahlensdure (1918). (4) o. warburg, biochemrsche zeitschrift, pd. 100 (1919), bd. 103 (1920). (5) t. weevers, kon, akad. welexsch. amsterdant proc., vol. 27 (1924). (6) f. f. blackman, annals of botany, vol. 19 (1905). (7) h. t. brown and f. escombe, proc. rey. sec., vol. b. 76 (1905). (8) o. warburg and fe. negelein, zertschr. fiir physikalische cheutie, bd. 102 (1922), and bd. 106 (1923). (v. hb.) phototelegrapery.—the problem of transmitiing dia- grams, pictures and particularly photographs, has engaged the attention of communication engineers since the earliest days of the telegraphic art. it has, however, only been within the last few vears that practice] success has been achieved, leading to the inauguration of regular commercial picture transmission services on established communication systems. elements of the problem.—the problem of transmitting pic- tures to a distance may be considered as divided into five steps. the first step is to analyse the picture into its elements. or to scan it. the second step is to provide means for translating the variations of light and shade of the picture into transmittable signals—for instance, electrical impulses or variations of current strength. the third step is to provide a transmitting system such as wires for carrying electrical current, capable of faith- fully conveying the signals to a distant point. the fourth step consists in retranslating the transmitted signals into various values of light and shade. the fifth step is provision for sore means of synchronisation, that is, of assuring that the recom- position of the transmitted signals places the received picture clements in their proper relative positions. lnafysing er scanning means.—any picture may be resolved into small clements, each of uniform tone valuc. the problem of analysing the picture for transmission purposcs consists in running over or scanning each of the minute clements of the picture in some regular manner. several different devices have been suggested and used for this purpose. prominent among these are cylinders on which the picture, usually in transparent form. as for instance on a celluloid filra. is wrapped. the analys- ing point, such as a stylus or a small spot of light, is located at a fixed point in relation to the apparatus and the cylinder is given a screw motion about its axis whereby every point of the picture is tn turn passed over the analysing point. other de- vices used, where the picture signal is initiated by the action of light, employ mirrors and prisms. a frequently suggested device consists of two mirrors arranged to oscillate about axes at right angles to each other—one mirror scanning the picture ina right and left direction, and the other in an up and down direction. tor the same purpose, prisms of variable angle have been used, whereby the spot of light or an image of the picture is oscillated in two directions. translation of picture tone values into electrical current variations. —the most primitive method of going over fram the light and shade of a picture to variations of electric current can- sists in the division of the picture into small elements, and the estimation or measurement of the average tone value of each such clement. this estimate or measurement may then be trans- mitted as a number or symbol in a code, for subsequent inter- pretation at the receiving end. a later development was to pro- duce the translation by means of a photographic relief, such for instance as a bichromated gelatine photograph, in which the diferent lights and shades of the pictures are represented by different elevations. another development of the same idea is embodied in the production of a line or dot picture of the “ half tone” type in which the various shades are reproduced as lines of variable width suitable for the production of electrical con- tacts of varying duration. the more modern forms of picture transmission apparatus employ photo-sensitive devices, such as light sensitive resistances (e¢.g., selenium q.z.) or photoclectric cells. these devices vary the strength of an electric current in more or less approximation to the strength of the light. se- lenium celis and similar devices which figured prominently in earlier suggestions of this sort are unsatisfactory because their response to light variations is non-proportional and is subject 136 to considerable lag. the alkali metal photoelectric cell is the most satisfactory form of light-sensitive device as its response -is proportional to the intensity of the light and is instantancous. the currents furnished are, however, quite small, and the practi- cal application of photoelectric cells was delayed until the devel- opment of the vacuum tube amplificr, by means of which the photoelectric cell currents may be amplified to a value suitable for use on commercial transmission lines. the electrical signals derived from any of the methods men- tioned for translating light and shade into electrical current varia- tions may be applicd directly to a transmission line or may be used to make an intermediate record, such as a perforated tape of the kind used in telegraphy. the use of an intermediate record of this sort possesses advantages where the character of the transmission line is such that the order of a succession of signals may be differentiated while their intensity is difficult to retain. transmission.—in the present state of the communication art, it is essential that the picture signals should be of such intensity and quality that they may be transmitted over existing communi- cation systems. such systems may be submarine telegraph cable, wire telephone or tclegraph, or radio telephone or telegraph. the greatest success has thus far been achieved on wire lines because of their relative freedom from static, variations of transmission level, and interferencein general. given a picture- transmitting apparatus which provides signals of a sort appro- priate for existing transmission lines, the transmission of pictures becomes a problem differing little from the transmission of tele- graph or telephone currents, requiring, however, a high degree of constancy and freedom from interference, because of the per- manent injury done to a picture by any disturbances, however brief, occurring during its transmission (see telephony). translation of electric signals info @ picture-—the signals received over the transmission line may be retranslated into values of light and shade by any one of several methods. the various tones may be reproduced by variable sized structures, as in the half tone process, by the use of a pen under varying pressure, or by the use of dots of various sizes produced by a printing device, such as a typewriter controlled by the incoming signals. the picture tones may also be reproduced by fixed-sized areas of different density, as by a controlled air brush, or by chemi- cal action induced by the current, or by varied exposure of a photographic sensitive surface. variation of the exposure of a photographic sensitive surface may be accomplished by passing the variable incoming picture current through a light-producing device, such as an incandescent filament or a vacuum tube lamp, or by varying an opening through which light is incident on the film. devices of the latter type are called “ light valves.’ they consist in gencral of strings or ribbons positioned in front of small apertures and of such small mass as to vibrate with satisfactory amplitude when actuated by the picture signals, __ synchronisation.—the accurate synchronisation of the send- ing and receiving ends may be accomplished by some form of clocks at the two ends, kept at identical rates by the occasional transmission of synchronising signals. synchronisation may also be accomplished by sending signals during the transmission of the pictures. these may either be sent at those times when the scanning device temporarily passes off the edge of the picture, bringing the apparatus at the two ends into step, or they may be sent continuously during the process of transmission. in the latter case the synchronising signals must be transmitted over a channel separate from the picture channel. practical processes —picture transmission over communica- tion lines of short length has been accomplished during the past 25 or 30 years by various experimenters, among whom may be noted korn, amstutz, thorne baker and belin. recent systems, which have been worked out for use on modern communication channels, include the ‘‘ telcpix,” the bartlane, and the system of the american telephone and telegraph co., the latter now in commercial operation between some of the principal cities of the united states. in this system the picture to be transmitted is put in the form of a positive photographic print on celluloid phototelegraphy film. this is placed on a transparent cylinder which is advanced and rotated by a screw motion. a minute spot of light is fo- cused on the surface of the film and passes through it on to an starting mechanism oseitlatar fic. 1.—schematic diagram of sending end apparatus for trans- mission of pictures by the american telephone and telegraph com- pany system. the picture to be sent is in the form of a celluloid film transparency, wrapped around a transparent cylinder. a beam of light passes through the film and falls on a photoelectric cell. the electric current thus produced is amplified by vacuum tube ampli- fiers, and is used to modulate a voice frequency alternating current, produced by a vacuum tube oscillator. the entire picture 1s scanned by advancing and rotating the transparent cylinder by a screw motion, the motive power being furnished by an impulse motor con- trolled by a tuning fork. the fork also acts as a modulator for low frequency alternating current to be sent to the receiving end to con- trol the speed of the receiving cylinder. both picture and speed control signals go on the same pair of wires. alkali metal photoelectric cell. the current from the photo- electric cell is amplified and then used to “ modulate ” or vary the strength of a “ carrier” alternating current of voice fre- quency suitable for efficient transmission over telephone lines (sec fig. 1). at the re- ceiving end, the incoming current operates a hght valve (see fig. 2) the light from which falls on a_ sensitive photographic film moved in synchronism with the film at the sending end. the trans- mission of a 5-in. by 7-in. picture built up of adjacent fic. 2.—detailsofa “‘light valve,” used in apparatus for receiving elec- strips 1/100 in. in width is completed in about seven minutes. synchronism of the sending and receiving ends is secured by transmitting low frequency impulses from a tuning fork to impulse motors at the two ends. the synchronising pulses are transmitted on a carrier fre- quency lower than that used for the picture, and the two different carrier currents (of approximately 1,300 and 400 cycles per second frequency), which go over a single pair of wires, are separated from trically transmitted pictures. 7 is a fine ribbon of electrically con- ducting material, through which the incoming current flows. theribbon stands in a magnetic field produced by an electro-magnet, one of whose pole pieces isshown at p. the pole pieces are pierced by a narrow hole, through which passes a beam of light whose size is limited by he adjustable jaws 7. when current flows through 7, the ribbon is de- tlected to one side by an amount proportional to the current, thus permitting the beam of light to fall on a photographic sensitive film, which is being movedin exact syn- chronism with the picture at the sending end, each other at the recciving end by electrical filters (see fig. 3). the system is adapted for the transmission of all kinds of pictures and diagrams. news photo- graphs form a considerable part of the traffic, but fnger prints, mechanical drawings, advertising material, autograph signatures, etc., are also transmitted. the system just described is capable of use not only on wire circuits, but on wireless circuits. these latter are, however, in- herently less stable and are used only where wire channels are not available. other systems using radio for picture transmis- photo-telegraphy fic. 1.—sending apparatus. (radio corp. of america.) a special constant speed motor (a) actuates the gearing (b) which drives the glass cylinder (c) back and forth in front of the photoelectric cell box (d). magnets (e) advance the camera box (d) down the length of the glass cylinder to cover the complete picture rotating on the cylinder line upon line. fic. 2. electrically transmitted finger print. (american telephone and telegraph company system.) fic. 3. electrically transmitted autograph material, japanese-american treaty of 1853. (am. t. and t. co. sys.) fic. 4. receiving apparatus. (am. t. and t. co. sys.) (m) synchronous driving motor, driven by tuning fork, in box, f, controlled by plate low frequency carrier current over telephone line. (c) cylinder on which sensi- tive film is wrapped. (v) light valve, actuated by picture signals transmitted over telephone line on voice frequency carrier, and controlling the amount of light falling on film from light source s. fic. 5. inauguration of president coolidge, march 4 1925. picture as trans- mitted over am. t. and t. co. lines from washington to new york, chicago and san francisco. fic. 6. enlarged portion of electrically transmitted picture, as built up by strips of varying density. (am. t. and t. co. sys.) fic. 7. enlarged portion of electrically transmitted picture, as built up by lines of varying width, suitable for direct use in typographic printing. (am. t. and t. co. sys.) phthisis—physical chemistry sion have been developed by c. f. jenkins, belin and the radio corp. of america. the system employed by the radio corp. has been worked out with particular view to minimising static and other disturbances peculiar to radio. the tone values of the picture are determined by the number of pulses per unit picture channel filter i starting mechanism lerminal al amplifier controf channel| _| filter amplifie rectifier fic. 3.—schematic diagram of receiving end apparatus for trans- mission of pictures by the american telephone and telegraph company system. the incoming picture and speed control signals, each on its own “carrier” frequency, are separated by electrical filters. the speed control signals drive an impulse motor which rotates a cylinder similar to that at the sending end, but carrying an unexposed photographic sensitive film. picture signals are received by a light valve, which controls the intensity of a spot of light falling on the sensitive film. length of travel of the scanning device, all pulses being recorded as of equal strength. pictures have been sent by this apparatus over very great distances, the maximum being from honolulu to new york (sce telephony). bibliography.—korn-glatzel, jfandbuch der photetelegraphie und telautographie (1911); thorne baker, the telegraphic trans- mission of photographs (1910). articles: committee on communica- tion, * recent advances in communication art,’ american in- stitute of electrical engineers journal, p. 1, 302 (dec. 1925); ives, horton, parker and clark, ‘* transmission of pictures over tele- phone lines,” bell system technical journal, p. 187 (april 1925); h. e. ives, ‘some photographic problems encountered in the trans- mission of pictures by electricity,”’ jour. optical society of amterica (march 1926); r. h. ranger, ‘ photo-radiograms,” proc. of the institute of radio engineers (april 1926). (het) phthisis: sce miners’ phthisis; tuberculosis.