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    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:telescope:476924c4aafe",
    "title": "TELESCOPE",
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    "verified_text": "for the purpose of outlining the treatment adopted in this survey of progress since rg1o astronom- ical telescopes may be classified as follows:— j. reflectors. i]. refractors: (a2) visual, (0) photographic; (1) moderate fields, (2) wide fields. all of these forms belong properly under the head of telescopes except the last one (ii. b 2), which belongs more properly to such a subject as the camera or photography. comparative merits —the respective merits of the several kinds of telescopes for astronomical observations of one kind or another are determined by both theoretical and practical con- siderations. the theoretical considerations relate to colour per- formance and field, and are fundamental. the reflector has no chromatic aberration whatever but has only a limited field of critical definition, while a good refractor has a larger field but is never entirely free from secondary spectrum. other considera- tions of rather a fundamental nature relate to the superior light efhciency of the reflector at large aperture and the relatively greater jatitude allowable in the selection of the glass from which tuey are made. these important factors will always weigh against each other very considerably as they do now and have done in the past. practical considerations relate to such matters as attainable perfection of surface, sensitiveness of the telescope to temperature changes, convenience of handling and a multitude of things whose incidence on telescopic performance will vary as improve- ments in technical processes bring one or another of them under control. from the maker’s point of view a mirror has the impor- tant advantage that it has only a single surface to be figured, while a lens has at least four, but this is more or jess offset by the necessity of working the one surface to an extremely high degree of accuracy. there seems at present to be substantial ground for the hope that the provision of mirror material of low expan- sion coefficient and good working quality will contribute mate- rially to the attainment of accurate surface and that the disad- vantage just mentioned will be lessened. the distinction between the two types of refractors, visual and photographic, relates primarily to colour correction, the one being adapted to the region of the spectrum to which the eye is most sensitive, and the other to the blue-violet region. there is, however, a secondary consideration, which is, in effect, equally important, namely, that the eye can utilise only a small telescopic field, while the photographic plate can take full advantage of a large one, so that lenses intended for photography receive careful treatment for field (see photography). 754 i. reflectors the trend of telescope building since 1910 has been decidedly toward the construction of retlectors. the movement in this di- rection had sect in at the opening of the century, and had resulted by 1910 in the construction of the s-ft. reflector of the mount wilson observatory. since then the 100-in. mount wilson and the 72-in. victoria reflectors have been built (sec plate), while at least two of 60-in. aperture are reported to be nearing completion. _ the mount wilson telescope.—the diameter of the mirror is 100-4 in. (255 cm.), the focal length 42-3 ft. (12-89 metres). the tube of the telescope is swung in a split polar axis, as described under “ b” in the resume of types of equatorial mounting in vol. 26, p. 565, the cross pivots on which the tube rotates serving as the declination axis. the great weight of the telescope (100 tons) ts carried by large drums situated at the upper and lower ends of the polar axis, which float sg a of mercury, while the actual direction of the axis is deter- mined by self-aligning spherical bearings near its two extremities. the telescope can be used either in the newtonian or cassegrain form, separate terminal sections of the tube being provided as car- riers for the auxiliary mirrors of each. these carriers, or ‘‘ cages’ as they are called, are very magsive, and are removed and exchanged with the aid of an electric crane (sce the upper left background of plate). when it is necessary to remove the mirror for silvering, which 1s about twice a year, it is lowered by means of a large hydraulic jack into a laboratory just beneath the floor. the two large tubes lying parallel to the telescope contain weights that can be moved longi- tudinally to perfect the balance. the hour and declination circles are read through the small telescopes scen on the desk. the instru- ment is provided with quick and slow motion devices operated elec- trically, and essentially all movements of it and of the dome are accomplished by motors under independent control by the observer at the eye end and the assistant at the desk. forty motors and 14 m. of wire are used in the dome. the mirror is supported by a counter- poise system similar to that of the 60-in. telescope (see 26.569), and there is provision for making the same optical dispositions as are described in conjunction with that telescope. in order to minimise the effect on the mirror of changes in the temperature of the air, it is surrounded by an insulating sheath of cork-board. means were originally provided for keeping the mirror cell jacketed with brine, which was automatically held at a constant temperature, but they were eventually discarded and reliance was placed on the cork insula- tion as well as on the protective covering of the dome. access to the newtonian focus, at the upper end of the tube, is by means of an adjustable platform, which is swung in the dome open- ing and is reached by means of the ladder in the right foreground (see plate). a ring section of the floor, extending from the dome in- ward to the circular rail, forming part of the dome and rotating with it serves to stiffen the dome and contributes to the smoothness of its operation. the dome and walls are double sheathed for the purpose of reducing the diurnal range of the interior temperature. the victoria telescope.—the diameter of the mirror is 72-5 in. (184-15 cm.) the focal length 30-1 ft. (9-180 metres). the mounting is of the so-called british form, designated as type a (26-565). critical features of this telescope, other than those apparent in the illustration, are the polar axis bearings and the construction of the tube. the polar axis rotates in ball bearings of large size, unrelieved by supporting or flotation devices. this departure from usual practice in the construction of jarge telescopes has proved very suc- cessful. the lattice-work part of the tube is built as a unit, without removable end pieces such as those of the mount wilson telescope, provision being made for the direct interchange of the cassegrain mirror, the newtonian mirror and the focal plane apparatus without the removal of a terminal section of the tube. the telescope is elec- trically operated. comparisons of three large reflecting telescopes——the two telescopes just described and the earlier 60-in. reflector at mount wilson possess a high degree of excellence. incomparable photo- graphs of the moon, the nebulae and other objects are the prod- uct of the mount wilson instruments, while the power of the victoria telescope for spectroscopic observation has fulfilled the most sanguine expectations of its projector. but these instru- ments suggest a comparative study of different types of tele- scopes. each of them differs in size and in pattern from the others and from the proof by trial of the many departures in de- sign is to be derived information essential to the determination of the steps by which further progress is likely to be made. the split or yoke form of polar axis used in the 1oo-in. tele- scope has the advantage that the very heavy tube is swung between pivots and does not overhang the polar axis. this construction makes for stability and centres the telescopes in the dome, advantages purchased at the cost of access to a con- telescope siderable area of the sky surrounding the north pole. the range of this telescope is still further reduced by interference of the large float at the upper end of the polar axis. in adopting this form of mounting the designers were no doubt intluenced by the circumstance that the mount wilson observatory already pos- sessed a powerful reflector capable of covering the entire sky, so that in the region inaccessible to the proposed telescope the loss would be merely that of the marginal efficiency of the larger over the smaller instrument. the excellent mechanical performance of the victoria telescope suggests that the limit of size for a mounting of its type has not been closely approached, and it seems likely that the design might be used for a very much larger instru- ment. the use of ball bearings in the polar axis of the victoria telescope has proved so successful that it is doubtful whether a return will be made to the system of counterweighted rolls, or to the rather cumbersome mercury floats and tanks. neither of the latter devices provides the ease of motion that is secured in the victoria telescope, nor does it appear to be superior in stabilising the position of the axis. with respect to the mechani- cal performance of the telescope, prof. plaskett writes: ‘ i can only repeat what i have said before, that it is a joy to operate and use the instrument.” fortunately, for the purpose of comparative study two of these great instruments are located at the same observatory. the 60 and roo-in. telescopes have now been in operation at mount wilson since 1920 under identical atmospheric conditions and in the hands of the same observers. the following comments are excerpted from a personal letter written by director w. s. adams of the mount wilson observatory in reply to an en- quiry relative to the performance of these two instruments:— in ease of control the 100-in. telescope is considerably superior to the 60-in., and the dome and shutter are vastly better. we could not improve upon the dome and shutter mechanism if we had them to do over again. one feature of the controls worth noting is the automatic speed change for right ascension and declination slow motions: this has proved almost invaluable in making settings around an instru- ment where the distances are so great. if the instrument were to be butlt at present i certainly think we should do away with mercury flotation. the northern float cuts off an important area of the sky from observation, and the mercury tanks give us considerable trouble with changes of temperature. large roller and ball bearings have developed greatly since the 100-in. telescope was designed, and i am sure they would now serve this purpose excellently. however, the driving of the 100-in. under normal conditions is very fine. probably our flotation system could have been made much better if we could have used a greater volume of mercury. it was built during the war, when mercury was almost unprocurable, and we got along with the irreducible minimum. we have not used the cooling system around the 100-in. mirror since the first few months, and about four years ago we took out the circulating pipes. the insulating cover has proved very efficient, and our automatic temperature records rarely show a range of over 20° c, inthe air surrounding the mirror. this insulation is about the most important single feature of the entire telescope, and { think it could be improved upon in future large instruments. our experience has shown that the number of nights when we get full benefit of the 100-in. over the 60-in. is surprisingly large. with seeing of 2 or better on a scale of 5 we regularly give exposure times with the spectrograph at the cassegrain focus one-third as long, and it is only when the secing is below 2 that the proportion becomes much worse. in very bad seeing, however, the 100-in. is nearly hope- less. in general, the performance of the 100-in. telescope depends very largely upon the figure of the mirror when the seeing is 2 or better. this is very susceptible to changes of temperature, and a general rise or fall of the mean temperatures of successive nights gives rise to astigmatism which is sometimes very persistent. our worst figure comes in winter after a cold wave, or a storm after which the temperature begins to rise rapidly. in summer, with settled weather, the figure is usually good with the occasional exception of the first hour or two after dark, when the edges are distorted. dr. hubble has given me the following table of the mean diameters of the faintest stars registered on plates with full aperture and exposures of 30 minutes:— seeing 2 to 3 on a scale of 5 mean diame- figure of mirror ters of smallest images average 2 2 «= «» wo mb wa! se -s 1-5 in. very good . : : . a # ; i-o in. 2exceptional plates . . . « « 0-6 in. telesco pe plate i, the mt. wilson 100-in. reflecting telescope plate ii. telescope - 4 7 sze eee” ee telescope in addition to speed and resolution the superiority of the 100-in. is very marked for fine details in nebulosity or other surfaces, and especially for narrow linear features, such as are found in planetaries or network nebulae. thus the full advantage of size is secured in the light gathering power of the larger telescope; and the two exceptional plates mentioned by adams indicate most excellent defining power, for while it is true that the theoretical resolving power for a mirror of that size, o-o5 in. is not reached, it is probably more closely approached by this than by any other large mirror in use. further, the diameters were measured on photographs taken with comparatively long exposures, and the images were consequently subject to the usual unsettling influences of photo- graphic observation. it seems safe to state that no existing re- fractor, nor any that could be made, could compete with the 100-in. telescope in the matter of light delivery. in respect to definition, the superiority probably does not exist. it is perhaps fair to say that on photographs taken with refractors of 30 or 36- in. aperture images of 0-6 in. are to be expected under “ good ” conditions of observation, and slightly larger ones under aver- age conditions. furthermore, the figure of a refractor can always be depended upon. with exceptionally steady atmosphere star disks, as seen with the eye through such an instrument as the 36-in. refractor of the lick observatory, are reduced approxi- mately to their theoretical size of o-13. thus in the matter either of ultimate or of average definition the refractor still maintains the lead; but if the figure of the mount wilson telescope could be stabilised it would, under aver- age conditions of observation, compare favourably in respect to definition with the best refractors. causes of instability of figure.—the principal factor contribut- ing to instability of figure is ununiformity of thermal distribution through the material of the mirror, resulting from ordinary changes in temperature of the surrounding air. it has long been realised that this effect might be reduced through the employ- ment of a mirror material of low expansion coefficient and “ py- rex ’’ mirrors of moderate size, which are in use at mount wilson, are said to suffer only about 7's the focal disturbance of those of glass. importance of fused quartz—fused quartz, the thermal ex- pansion of which is almost negligible, has been suggested as a material for telescope mirrors, in fact experiments looking toward the production of that material in disks sufficiently large for the purpose were undertaken by prof. elihu thomson in the opening years of the century. at that time the material was rather a curiosity, and was obtainable only in small pieces. in the hands of thomson the extremely diflicult technique of quartz fusing has been sufficiently mastered to permit the production of disks approximately a foot {30 cm.) in diameter. a mirror of this material would be immune from the disturbing effects of tem- perature and would retain excellence of surface under essentially all conditions of observation. dr. thomson, in some very illumi- nating letters relating to fused quartz, expresses the opinion that with proper mechanical facilities, disks of a quality suitable for mirrors can be produced in whatever size may be desired. with respect to the working qualities of the material he says:— i find the work of fused quartz for optical surfaces is peculiarly satisfying in that its freedom from expansion makes it unnecessary to look after the temperature of the workroom or take precautions as to its unequal heating. the grinding proceeds with the utmost satisfaction, without danser of scratches, which is always a trouble with glass, and a very fine surface just before polishing is attainable with comparative ease, since the property of fused quartz, which insures stability of surface, also facilitates the shaping of the mirror, the use of this material may be expected to result not only in a more quiescent surface but also in mirrors of greater perfection. on the pro- duction of fused quartz, or something equivalent to it, more than on anything else, depends the full utilisation of the advantages of the reflecting form of telescope. theoretical considerations —two attempts of a theoretical na- ture have been made to overcome the field limitations of instru- ments of this class. where the optical system consists of a single fee mirror the shape of the surface is uniquely determined as that of a paraboloid of revolution. in the cassegrain and gregorian forms there are, however, two reflecting surfaces, and it is possible, within limits, to balance departure from the paraboloidal form of the primary by reciprocal alterations in the surface of the secondary mirror. the problem whether it is possible to do this to the advantage of the field has been attacked by schwarzschild and by sampson. the investigation of the former is, in the present state of the art of glass working, hardly more than of theoretical interest. sampson, besides modifying the reflecting surfaces, brings into the system a number of glass lenses, all of the same material, which without introducing sensible chromatic aberra- tion provide a ficld comparable with that of a good refractor. the suggestion seems promising for the future, but noinstrument of the proposed type seems as yet to have been constructed. ii. refractors the production of refractors has not been entirely neglected, and much more would undoubtedly have been accomplished in this direction but for the war, which proved especially disturb- ing to the optical industry (see opricat grass). the instru- ments include a few intended for general observational purposes, but a very significant departure has been made in the direction of photographic telescopes. among the pioneers in large instru- ments of this class is a 30-in. (76 cm.) thaw telescope of the allegheny observatory, which has proved extremely efficient in the determination of stellar parallax, in the study of star clus- ters, and in other investigations which require, or derive advan- tage from, a larger field than is provided by a reflecting telescope. as a result, possibly, of the success of that instrument the con- struction of a number of others has been undertaken, notably one of 24-in. (61 cm.) aperture for the south african station of the yale observatory, and one of 41-in. (104 cm.) aperture and 35 ft. (10-66 metres) focal length (which will be larger than any re- fractor now in existence) for the nikolayev observatory in south russia. a 24-in. (61 cm.) instrument of 35 ft. (10-66 metres) focus is under construction for the national observatory near santia- go, chile. at johannesburg there has recently been installed a 26-in. (66 cm.) visually corrected telescope, and one of approxi- mately the same aperture is being built for the south african station of the detroit observatory. lens design the most noteworthy advances in design have been made in relation to lenses of wide field. a number of new lenses have ap- peared since the war, the stimulus for this development being provided perhaps by the necessities of aeroplane photography. a lens calculated by f. e. ross for work of that nature has, on modification for astronomical requirements, proved very suc- cessful, notably in the extremely exacting observations made during a solar eclipse for the measurement of the gravitational displacement of light. among the excellent wide-angle wide- aperture lenses constructed in recent years are the 16-in. lens of the arequipa station of the harvard observatory, and the 1o-in. at mount wilson, both of the cooke type, while a number of lenses of the petzval type are in successful operation in europe and america. improvements in material and methods of manufacture certain glasses having optical characteristics that would lend themselves to the reduction of secondary spectrum are barred from use by their chemical instability, or by the possession of detrimental physical properties. it seems improbable that any marked departure from the existing run of optical properties (refractive index, etc.) will be effected without a serious sacrifice of durability. however, an important english firm of glass makers is reported to have improved the transparency of optical flint glass, and to have produced successfully a large disk of dense barium crown (see optical glass). the refractive char- acteristics of glass of the latter type make it especially valuable for the suppression of coma, and its availability holds out hope for marked improvement in the optical performance of refrac- tors. deterioration of the surface of glass under the action of 756 air and the weather is occasionally noticed in the most valuable telescopic objectives. experiments directed toward the pre- vention of such impairment have been undertaken by the brit- ish scientific instrument research assn., apparently with some prospect of success. testing of lenses ——among improvements in facilities for testing of lenses may be noted an adaptation to this purpose, by twyman, of the michelson interferometer. the instrument ts applicable to the testing of small lenses, rather than of large ones, the present limit of aperture being about 53-in., but within its range tests of extreme accuracy can quickly and conveniently be made. lenses and mirrors of large size are still tested by long- established methods. the telescope-intcrferometer.—the outstanding achievement of observational astronomy during the opening quarter of the present century was the measurement of the diameters of giant stars with interference apparatus attached to the 1oo-in. mount wilson telescope. the writer is greatly indebted for information to messrs. w. s. adams and f. g. pease of mount wilson, j. s. plaskett of victoria, b.c., elihu thomson of lynn, mass., and f. twyman of london. bibliography.—the troo-in. mount wilson telescope—f. g. pease, ‘‘ mount wilson’s 100-in. reflector,’”’ scient fic american, vol. 117, p. 100 (1917); g. e. hale, speech at royal astronomical society, observatory, vol. 41, p. 429 (1918); publications of the as- tronomical soc. of the pacific, vol. 31 (1919); adolfo staht lectures in astronomy, published by astronomical society of the pacific (1919). the 72-in. victoria reflector—j. s. plaskett, publications domzn., obs. victoria, b. c. (1919). cassegrain reflector—r. a. sampson, ‘on a cassegrain reflector with corrected field,” phil. trans. roy. soc., vol. 213, p.27 (1914). improvements in optica! technology — f. twyman, “ the vitality of the british optical inctustry,”’ journ. sci. instruments, vol. 2, p. 369 (1925). properties of fused quartz — e. thomson, “ silica glass or fused quartz,” gen. electric review, vol. 26, p. 68 (1923), and ‘‘ the mechanical, thermal and optical properties of fused silica,” journ. franklin inst., vol. 200, p. 31 (1925). (w. h. wr.) television.—although, strictly speaking, the word televi- sion might be held to cover any means of secing distant ob- jects, such as ordinary telescodic vision, the term is in prac- tice restricted to the seeing of distant objects through the in- termediary of long electromagnetic waves, passing either through empty space or along wires. since such waves, in their passage over the surface, can follow the curvature of the earth, obstacles which would cut off the direct passage of light from the object to the observer can be disregarded. again, the long waves are not appreciably scattered by dust or liquid droplets in the atmosphere, and so do not suffer absorption in the sense that ordinary light does. further, the attenuation of the energy in passing over long distances is less marked than it is with ordinary light, owing to the fact that the long waves do not spread out with spherical symmetry around the source, but tend, on account of the existence of the upper atmospheric ionised layer, to hug the surface of the earth. in addition, the diminution of the energy is of little moment, since modern wireless technique enables us to amplify the signals to any extent desired. even in cases, then, where light could pass by a direct path from the distant object to the observer, the utili- sation of hertzian waves as a vehicle for the transmission of vision offers certain advantages. mode of transmission.—for television we must have a trans- mitting apparatus, by means of which the light from the ob- ject is transformed into electric impulses. these electric 1m- pulses must then be amplified and made to modulate a carrier wave, as in the case of wireless telephony. at the receiving end the modulated wave must be transformed into current i1m- pulses, which can then be used to modify or steer a ight source so as to make it imitate in intensity the light falling on the transmitting system. it is clear that in the transmitter any light-sensitive cell such as is used in the telegraphic transmission of photographs, i.e., a selenium cell (see selenium) or a photoelectric cell, can be used to convert a light of fluctuating intensity into a corre- television sponding current impulse. the amplification of the current and the modulation of the carrier wave can be carried out with the help of electronic relay tubes (“‘ valves ”’) by the or- dinary method of wireless telephony. the invention of these relays is, of course, one of the things that make television a practical project. at the receiving end the modulated wave produces current impulses which can be made to govern the intensity of a light beam in various ways: for example, the de- flection of the string in a galvanometer of einthoven type might move a very light and small disk so as partly to obscure or release a light beam, or the amplified current might be passed through a glow discharge lamp of the neon type, which responds without appreciable lag. in this way fluctuatiors of light falling on the light sensitive transmitting cell could be made to re- produce themselves at the receiving end, by the intermediacy of hertzian waves. an imaginable way of adapting these devices to send a whole picture in tone would be to imitate the human eye. the eye consists essentially of a large number of light-sensitive cells, each one of which reports to the brain by its own nerve fibre the strength (and colour) of the light falling upon it. an arti- ficial retina would be made up of a large number of photoelec- tric cells, each one with a corresponding light-governing de- vice at the receiving end. the relative positions of the light- governing devices to one another at the receiving end would, of course, have to correspond to the relative positions of the transmitting cells to one another at the transmitting end. each cell would then require either its own wire, if wires were used to transmit the impulse, or its own wave-length, if hertzian waves were used: for this reason, and on account of the extreme minuteness of the cells required, the method is impracticable. possible methods.—the only hopeful methods suggested are based upon the persistence of vision, and demand the use of one cell only, on which falls i sequence light gathered from different parts of the picture to be transmitted. at a given instant light from a given smal! patch of the picture is focused on to the light sensitive cell, and the picture is ‘ explored ” or “scanned,” z.e., there is some moving element in the focusing system which selects successively and systematically different parts of the picture until the whole has been covered. at any one instant the strength of the governed beam at the receiv- ing end—the recording beam—corresponds to the intensity of light proceeding from a small patch of the picture at the trans- mitting end. if now the recording beam can be moved in ex- act synchronism with the scanning beam at the transmitting end, so that not only the intensities but also the positions of the two beams exactly correspond, the picture will be traced out at the receiving end. if the whole picture can be scanned and transmitted in something jess than a tenth of a second, and the receiving mechanism can respond sufficiently quickly, then, owing to the persistence of vision (28.130) the impression of the initially recorded part of the picture will not have faded for the observer by the time the last parts are being traced out, and the picture will be seen as a whole. if the object to be seen is moving, and pictures be sent in rapid and uninterrupted succession, the scanning beam returning to its initial position and starting again as soon as it has covered the whole picture, then a moving picture will be seen as in the cinematograph. accurate synchronisation.—it will be clear that this method demands not only the mechanism already .contemplated for modulating the carrier waves so as to produce a correspond- ence in intensity between the scanning beam and the record- ing beam, but also a mechanism for producing exact synchro- nism, in the sense not only that the two beams must move at exactly the same speed, but also that they must always be, at a. given instant, at corresponding positions of the picture. if, for example, the picture be scanned in horizontal lines—or, rather, narrow horizontal bands touching each other—then obviously the recording beam must begin a line at exactly the same instant that the scanning beam begins a line, and similar considerations hold if the picture be scanned in any other way, for example in a close spiral. . telpherage number of signals necessary.—further, a very large number of separately detectable signals, corresponding each to a small element of the picture, must be sent per second. the ordinary newspaper half-tone reproductions contain some 50 dots to the linear inch, which gives 1o,ooo dots for a picture two in. square. it must be remembered that, owing to the fact that the image reproduced by a mechanism such as that contem- plated above will consist of contiguous patches in tone which may vary in continuous gradation from black to white, where- as the half-tone block consists of isolated dots which are either black or white, the representation will be very much better for a given number of elements in the television picture than in the half-tone block. it is possible that quite good repre- sentation would be given with half the number of elements to the linear inch, 7.e., only 2,500 to the two in. square. taking this as a conservative estimate, and ro pictures per sec. as necessary for the cinematographic representation, also a con- servative estimate, it will be seen that 25,000 distinguishable signals per sec. must be obtained. feasibility—the carrier wave imposes a limit upon the number that can be sent, for in the modulation a certain num- ber of oscillations are necessary for the building up and dying down of the signals. taking a wave length of rt50 metres, which is about the least at present used in broadcasting, there are 2,000,000 vibrations per sec., and if 50 waves are necessary for effective modulation of one signal, it follows that a limit of some 40,000 signals per sec. is imposed. it may be taken, then, that with the present technique of broadcast- - ing a picture of some two in. square could be produced, if the transmitting and receiving mechanism could deal with the signals at the rate just specified. method proposed.—various patents have been taken out for meth- ods by which the scheme just outlined is to be attained practically. one of the most elaborate isthat of the western electric company, described in british patents nos. 228961 and 230401, the picture . to be transmitted is scanned by a small and very light mirror, which moves so that, if the whole picture be imagined to be covered by a narrow spiral band, successive turns of which touch one another, every point of the spiral is successively thrown on a photoelectric cell. the movement of the mirror is secured by mounting it at the junction of two fine wires crossing one another at right angles in a magnetic field, and passing damped oscillatory dis- charges, with the required phase of dierence, through both wires. the fluctuations of current produced in the cell are amplified and impose on a carrter wave which at the receiving end controls the speed of the cathode stream in a braun tube. the tube is provided with a translucent fluorescent screen, and the intensity of the small patch of light produced on this by the cathode stream depends on the velocity of the stream, so that in this way the varying intensity of the light at the transmitting end is reproduced by an apparatus possessing no appreciable inertia. a movement of the cathode stream synchronous with the movement of the light beam at the transmitting end is pro- duced by a very elaborate device. an evacuated tube—the syn- chronising tube —contains a photoelectric surface, so arranged that the current from a subsidiary electrode depends upon the position at which a spot of lizht, of constant intensity, falls on the surface. the scanning mirror, besicles exploring the picture, reflects the light from a constant source, which is brought to a focus on the ai aauarastadeg surface: as the mirror describes its path this spot of ight moves and produces corresponding variations in the current from the synchronising tube. these variations are imposed on the carrier beam: at the transmitting end the cathode beam is moved by two pairs of condenser plates whose planes are set at rizht angles to one another, the carrier beam varying the potentials of the condensers by suitable devices, the frequency of the synchronising waves being so much less than that of the waves controlling the intensity, the two sets of waves can be imposed on the same carrier beam, and separated at the receiving end by a so-called wave filter. the baird system.—this method is described on account of its ingenuity, but it is doubtful if it is really a practicable one. at any rate, no actual transmission of pictures appears to have been realised by its aid. at the time of writing, the most promising system appears to be that of mr. j. l. baird, who has already suc- ceeded in producing a crude, but convincing, representation of moving objects through the agency of hertzian waves, and has so achieved a true ‘“‘ vision by wireless.” full details of the method have not yet been made public, as certain devices not disclosed in the patents (b. p. 220604, 230576, 235619, 236978) have apparently been employed, but the experiments have been openly exhibited and the general features of the method disclosed. the picture is scanned by a system of lenses carried by a revolving 757 disk. the lenses are arranged in a single turn of a spiral of archi- medes, the angular distance between two lenses being such that as soon as one passes right across the picture the succeeding one begins its passage, while the radial distance, which must be small, deter- mines the closeness of the lines (narrow strips) in which the picture is scanned. the disk is large, so that all the lenses are in the neigh- bourhood of the edge. between the picture and the lens disk pro- trudes the edge of a second disk, pierced by a very large number of close radial slots, and close to this disk is a fixed plate pierced by similar slots, so that as the second disk rotates light is transmitted and shut off in very rapid succession. [fa black part of the picture is being scanned, there is, of course, no alternation; for a brighter part of the picture the intensity, but not the period, of the alternations is governed by the luminosity of the part in question and the dura- tion of the alternations of given intensity by the length of the bright strip. the light so broken up falls, after passage through the scanning system, on toa light-sensitive cell of special construction; the alterna- tions of current so produced are amplified and imposed on a carrier wave. at the receiving end the modulated wave is made, by the intermediacy of a suitable relay system, to control the brightness of a glow lamp of the neon type, where, since we are dealing with a lumi- nous gas, and not a luminous solid, the intensity can follow verv rapid fluctuations. to reveal the picture a narrow beam from this lamp, falling on a screen, must be moved about in synchronism with the scanning beam at the transmitting end. to move the beam a disk provided with a spiral slot and a disk provided with a radial slot revolve at different speeds in front of one another, the first- named disk having much the higher speed. in this way the spot on the screen is moved in close parallel lines, and, by its changes of intensity, reproluces the picture. synchronism is secured with the help of an alternating current generator mounted on the same shaft as the scanning disk. the slow alternations so produced are superimposed on the carrier wave which is used for conveying the variations of intensity which pro- ‘duce the picture. at the receiving end the synchronising wave is separated from the light modulations by a wave filter and is then made to control the speed of the slit and the spiral disk, the speed ratio of which is fixed by a gearing. this mechanism secures syn- chronism but does not secure that the scanning and tracing beam shall each begin a line together. this last adjustment can be made by hand at the receiving end, and, once it is made, the synchronising mechanism suffices. in feb, 1926 mr. baird had succeeded, with quite crude appa- ratus constructed on the lines just laid down, in transmitting recognisable pictures of moving objects such as a living face. the opening and shutting of the mouth, the turning of the head, and such general features are clearly visible, but no finer details appear. it secms, however, to be possible that, with adequate financial support, the method may be developed so as to transmit by wireless waves a clear and detailed moving picture of any brightly illuminated object or scene. (be. n.daec: a,)",
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