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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:ligao:f578c27124d3",
    "title": "LIGAO",
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    "verified_text": "ligao, a town near the centre of the province of albay, luzon, philippine islands, close to the left bank of a tributary of the bicol river, and on the main road through the valley. pop. (1903) 17,687. east of the town rises mayon, an active volcano, and the rich volcanic soil in this region produces hemp, rice and coco-nuts. agriculture is the sole occupation of the inhabitants. their language is bicol. light. _introduction._--§ 1. \"light\" may be defined subjectively as the sense-impression formed by the eye. this is the most familiar connotation of the term, and suffices for the discussion of optical subjects which do not require an objective definition, and, in particular, for the treatment of physiological optics and vision. the objective definition, or the \"nature of light,\" is the _ultima thule_ of optical research. \"emission theories,\" based on the supposition that light was a stream of corpuscles, were at first accepted. these gave place during the opening decades of the 19th century to the \"undulatory or wave theory,\" which may be regarded as culminating in the \"elastic solid theory\"--so named from the lines along which the mathematical investigation proceeded--and according to which light is a transverse vibratory motion propagated longitudinally though the aether. the mathematical researches of james clerk maxwell have led to the rejection of this theory, and it is now held that light is identical with electromagnetic disturbances, such as are generated by oscillating electric currents or moving magnets. beyond this point we cannot go at present. to quote arthur schuster (_theory of optics_, 1904), \"so long as the character of the displacements which constitute the waves remains undefined we cannot pretend to have established a theory of light.\" it will thus be seen that optical and electrical phenomena are co-ordinated as a phase of the physics of the \"aether,\" and that the investigation of these sciences culminates in the derivation of the properties of this conceptual medium, the existence of which was called into being as an instrument of research.[1] the methods of the elastic-solid theory can still be used with advantage in treating many optical phenomena, more especially so long as we remain ignorant of fundamental matters concerning the origin of electric and magnetic strains and stresses; in addition, the treatment is more intelligible, the researches on the electromagnetic theory leading in many cases to the derivation of differential equations which express quantitative relations between diverse phenomena, although no precise meaning can be attached to the symbols employed. the school following clerk maxwell and heinrich hertz has certainly laid the foundations of a complete theory of light and electricity, but the methods must be adopted with caution, lest one be constrained to say with ludwig boltzmann as in the introduction to his _vorlesungen uber maxwell's theorie der elektricitat und des lichtes_:-- \"so soll ich denn mit saurem schweiss euch lehren, was ich selbst nicht weiss.\" goethe, _faust_. the essential distinctions between optical and electromagnetic phenomena may be traced to differences in the lengths of light-waves and of electromagnetic waves. the aether can probably transmit waves of any wave-length, the velocity of longitudinal propagation being about 3.10^10 cms. per second. the shortest waves, discovered by schumann and accurately measured by lyman, have a wave-length of 0.0001 mm.; the ultra-violet, recognized by their action on the photographic plate or by their promoting fluorescence, have a wave-length of 0.0002 mm.; the eye recognizes vibrations of a wave-length ranging from about 0.0004 mm. (violet) to about 0.0007 (red); the infra-red rays, recognized by their heating power or by their action on phosphorescent bodies, have a wave-length of 0.001 mm.; and the longest waves present in the radiations of a luminous source are the residual rays (\"_rest-strahlen_\") obtained by repeated reflections from quartz (.0085 mm.), from fluorite (0.056 mm.), and from sylvite (0.06 mm.). the research-field of optics includes the investigation of the rays which we have just enumerated. a delimitation may then be made, inasmuch as luminous sources yield no other radiations, and also since the next series of waves, the electromagnetic waves, have a minimum wave-length of 6 mm. § 2. the commonest subjective phenomena of light are colour and visibility, i.e. why are some bodies visible and others not, or, in other words, what is the physical significance of the words \"transparency,\" \"colour\" and \"visibility.\" what is ordinarily understood by a _transparent_ substance is one which transmits all the rays of white light without appreciable absorption--that some absorption does occur is perceived when the substance is viewed through a sufficient thickness. _colour_ is due to the absorption of certain rays of the spectrum, the unabsorbed rays being transmitted to the eye, where they occasion the sensation of colour (see colour; absorption of light). transparent bodies are seen partly by reflected and partly by transmitted light, and opaque bodies by absorption. refraction also influences visibility. objects immersed in a liquid of the same refractive index and dispersion would be invisible; for example, a glass rod can hardly be seen when immersed in canada balsam; other instances occur in the petrological examination of rock-sections under the microscope. in a complex rock-section the boldness with which the constituents stand out are measures of the difference between their refractive indices and the refractive index of the mounting medium, and the more nearly the indices coincide the less defined become the boundaries, while the interior of the mineral may be most advantageously explored. lord rayleigh has shown that transparent objects can only be seen when non-uniformly illuminated, the differences in the refractive indices of the substance and the surrounding medium becoming inoperative when the illumination is uniform on all sides. r. w. wood has performed experiments which confirm this view. the analysis of white light into the spectrum colours, and the reformation of the original light by transmitting the spectrum through a reversed prism, proved, to the satisfaction of newton and subsequent physicists until late in the 19th century, that the various coloured rays were present in white light, and that the action of the prism was merely to sort out the rays. this view, which suffices for the explanation of most phenomena, has now been given up, and the modern view is that the prism or grating really does _manufacture_ the colours, as was held previously to newton. it appears that white light is a sequence of irregular wave trains which are analysed into series of more regular trains by the prism or grating in a manner comparable with the analytical resolution presented by fourier's theorem. the modern view points to the _mathematical_ existence of waves of all wave-lengths in white light, the newtonian view to the _physical_ existence. strictly, the term \"monochromatic\" light is only applicable to light of a single wave-length (which can have no actual existence), but it is commonly used to denote light which cannot be analysed by the instruments at our disposal; for example, with low-power instruments the light emitted by sodium vapour would be regarded as homogeneous or monochromatic, but higher power instruments resolve this light into two components of different wave-lengths, each of which is of a higher degree of homogeneity, and it is not impossible that these rays may be capable of further analysis. § 3. _divisions of the subject._--in the early history of the science of light or optics a twofold division was adopted: _catoptrics_ (from gr. [greek: katoptron], a mirror), embracing the phenomena of reflection, i.e. the formation of images by mirrors; and _dioptrics_ (gr. [greek: dia], through), embracing the phenomena of refraction, i.e. the bending of a ray of light when passing obliquely through the surface dividing two media.[2] a third element, _chromatics_ (gr. [greek: chroma], colour), was subsequently introduced to include phenomena involving colour transformations, such as the iridescence of mother-of-pearl, feathers, soap-bubbles, oil floating on water, &c. this classification has been discarded (although the terms, particularly \"dioptric\" and \"chromatic,\" have survived as adjectives) in favour of a twofold division: geometrical optics and physical optics. _geometrical optics_ is a mathematical development (mainly effected by geometrical methods) of three laws assumed to be rigorously true: (1) the law of rectilinear propagation, viz. that light travels in straight lines or _rays_ in any homogeneous medium; (2) the law of reflection, viz. that the incident and reflected rays at any point of a surface are equally inclined to, and coplanar with, the normal to the surface at the point of incidence; and (3) the law of refraction, viz. that the incident and refracted rays at a surface dividing two media make angles with the normal to the surface at the point of incidence whose sines are in a ratio (termed the \"refractive index\") which is constant for every particular pair of media, and that the incident and refracted rays are coplanar with the normal. _physical optics_, on the other hand, has for its ultimate object the elucidation of the question: what is light? it investigates the nature of the rays themselves, and, in addition to determining the validity of the axioms of geometrical optics, embraces phenomena for the explanation of which an expansion of these assumptions is necessary. of the subordinate phases of the science, \"physiological optics\" is concerned with the phenomena of vision, with the eye as an optical instrument, with colour-perception, and with such allied subjects as the appearance of the eyes of a cat and the luminosity of the glow-worm and firefly; \"meteorological optics\" includes phenomena occasioned by the atmosphere, such as the rainbow, halo, corona, mirage, twinkling of stars and colour of the sky, and also the effects of atmospheric dust in promoting such brilliant sunsets as were seen after the eruption of krakatoa; \"magneto-optics\" investigates the effects of electricity and magnetism on optical properties; \"photo-chemistry,\" with its more practical development photography, is concerned with the influence of light in effecting chemical action; and the term \"applied optics\" may be used to denote, on the one hand, the experimental investigation of material for forming optical systems, e.g. the study of glasses with a view to the formation of a glass of specified optical properties (with which may be included such matters as the transparency of rock-salt for the infra-red and of quartz for the ultra-violet rays), and, on the other hand, the application of geometrical and physical investigations to the construction of optical instruments. § 4. _arrangement of the subject._--the following three divisions of this article deal with: (i.) the history of the science of light; (ii.) the nature of light; (iii.) the velocity of light; but a summary (which does not aim at scientific precision) may here be given to indicate to the reader the inter-relation of the various optical phenomena, those phenomena which are treated in separate articles being shown in larger type. the simplest subjective phenomena of light are colour and intensity, the measurement of the latter being named photometry. when light falls on a medium, it may be returned by reflection or it may suffer absorption; or it may be transmitted and undergo refraction, and, if the light be composite, dispersion; or, as in the case of oil films on water, brilliant colours are seen, an effect which is due to interference. again, if the rays be transmitted in two directions, as with certain crystals, \"double refraction\" (see refraction, double) takes place, and the emergent rays have undergone polarization. a shadow is cast by light falling on an opaque object, the complete theory of which involves the phenomenon of diffraction. some substances have the property of transforming luminous radiations, presenting the phenomena of",
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