LIGHT. 



465 



to greater distances, through the lessened attraction? 

 Yet these particles, with this amazing motion, will 

 not drive before them or remove the least and slight- 

 est dust they meet with, and the sun appears to con- 

 tinue of his ancient dimensions, and his attendants 

 move in their ancient orbits." He therefore con- 

 jectures tnat all the phenomena of light may be more 

 properly solved, by supposing all space filled with a 

 subtile elastic fluid, not visible when at rest, but 

 which, by its vibrations, affects that fine sense in the 

 eye, as those of the air affect the grosser organs of 

 the ear ; and even that different degrees of vibration 

 of this medium may cause the appearances of differ- 

 ent colours. And the celebrated Euler has main- 

 tained the same hypothesis, urging some further ob- 

 jections to the materiality of light, besides those of 

 doctor Franklin above alluded to. 



Newton first discovered that certain bodies exer- 

 cise on light a peculiar attractive force. When a ray 

 passes obliquely from air into any transparent liquid 

 or solid surface, it undergoes, at its entrance, an 

 angular flexure, which is called 'refraction. The 

 variation of this departure from the rectilineal path 

 for any particular substance, depends on the obliquity 

 of the ray to the refracting surface ; so that the sine 

 of the angle of refraction is to that of the angle of 

 incidence in a constant ratio. Newton, having found 

 that unctuous or inflammable bodies occasioned a 

 greater deviation in the luminous rays than their 

 attractive mass, or density, gave reason to expect, 

 conjectured, that both the diamond and water con- 

 tained combustible matter a conjecture which was 

 verified by subsequent discovery. Doctor Wollaston 

 invented a very ingenious apparatus, in which, by 

 means of a rectangular prism of flint glass, the index 

 of refraction of each substance is read off at once by 

 a vernier, the three sides of a movable triangle per- 

 forming the operations of reduction in a very com- 

 pendious manner. (Phil. Trans., 1802.) But trans- 

 parent media occasion not merely a certain flexure 

 of the white sunbeam, called the mean refraction : 

 they likewise decompose it into its constituent 

 colours. This effect is called dispersion. Now, the 

 mean refractive and dispersive powers of bodies are 

 not proportional to each other. In some refracting 

 media, the mean angle of refraction is smaller, 

 whilst the angle of dispersion is larger. From the 

 refractive power of bodies, we may, in many cases, 

 infer their chemical constitution. For discovering 

 the purity of essential oils, an examination with doc- 

 tor Wollaston' s instrument is of great utility, on 

 account of the smallness of the quantity requisite for 

 trial. This idea of doctor Wollaston has been hap- 

 pily prosecuted by M. Biot with regard to gaseous 

 compounds ; and we now have accurate tables of the 

 refractive power of all transparent gaseous, liquid, 

 and solid bodies. Carburet of sulphur exceeds all 

 fluid substances in refractive power, surpassing even 

 flint glass, topaz, and tourmalin ; and in dispersive 

 power, it exceeds every fluid substance, except oil of 

 cassia. Rays of light, in traversing the greater 

 number of crystallized bodies, are commonly split 

 into two pencils ; one of which, called the ordinary 

 ray, follows the common laws of refraction, agree- 

 ably to the tables alluded to, whilst the other, called 

 the extraordinary ray, obeys very different Jaws. 

 This phenomenon is produced in all transparent crys- 

 tals, whose primitive form is neither a cube nor a 

 regular octahedron. The division of the beam is 

 greater or less, according to the nature of the crys- 

 tal, and the direction in which it is cut ; but, of all 

 known substances, that which produces this pheno- 

 menon in the most striking manner, is the crystallized 

 carbonate of lime, called Iceland spar. If the white 

 Kimhf am, admitted through a small hole of a window- 



shutter into a darkened room, be made to pass 

 through a triangular prism of glass, it will be divided 

 into a number of splendid colours, which may be 

 thrown upon a sheet of paper. Newton ascertained 

 that if this coloured image, or spectrum, as it is 

 called, be divided into 3(30 parts, the red will occupy 

 forty-five, the orange twenty-seven, the yellow forty- 

 eight, the green sixty, the blue sixty, the indigo 

 forty, and the violet eighty. The red rays, being 

 least bent by the prism from the direction of the 

 white beam, are said to be least refracted, or the 

 least refrangible, while the violet rays, being always 

 at the other extremity of the spectrum, are called 

 the most refrangible. If these differently coloured 

 rays of light be now concentrated on one spot, by a 

 lens, they will reproduce colourless light. Newton 

 ascribes the different colours of bodies to their power 

 of absorbing all the primitive colours, except the 

 peculiar one which they reflect, and of which colour 

 they therefore appear to our eye. The different 

 coloured rays possess very different powers of illu- 

 mination. The lightest green, or deepest yellow, 

 which are near the centre, throw more light on 

 a printed page than any of the rays towards either 

 side of the spectrum. The rays of the prismatic 

 spectrum differ from one another also in their heat- 

 ing power, as was first noticed by Herschel. In view- 

 ing the sun, by means of large telescopes, through 

 differently coloured darkening glasses, he some- 

 times experienced a strong heat, attended with very 

 little light, and, at other times, he had a strong light 

 with a little heat. This observation led to his well 

 known researches upon this subject, from which he 

 concluded that the maximum heat is just without the 

 spectrum, beyond the red ray. Others have found 

 the greatest heat in the red ray itself ; but the recent 

 observations of M. Seebeck have shown that the 

 point of greatest heat was variable, according to the 

 kind of prism which was employed for refracting the 

 rays. When a prism of fine flint glass is used, the 

 greatest heat is constantly beyond the red ; when a 

 prism of crown glass, the greatest heat is in the red 

 itself. It has long been known, that the solar light 

 is capable of producing powerful chemical changes. 

 One of the most striking instances of it is its 

 power of darkening the white chloride of silver an 

 effect which takes place slowly in the diffused light 

 of day, but in the course of two or three minutes by 

 exposure to the sunbeam. This effect was formerly 

 attributed to the influence of the luminous rays ; but 

 it appears, from the observations of Ritter and 

 Wollaston, that it is owing to the presence of certain 

 rays, that excite neither heat nor light, and which, 

 from their peculiar agency, are termed chemical 

 rays. It is found that the greatest chemical action 

 is excited just beyond the violet ray of the prismatic 

 spectrum, and that the spot next in energy is occu- 

 pied by the violet ray itself, and that the property 

 gradually diminishes as we advance to the green, 

 beyond which it seems wholly wanting. The sun- 

 beams, in traversing a coloured glass, produce simi- 

 lar effects to those caused by the differently coloured 

 portions of the spectrum. Thus the chloride of 

 silver acquires a black tint behind a blue or violet 

 glass, but does not blacken behind a red or orange 

 glass ; on the other hand, it becomes red behind a 

 red glass, and that much more quickly than even in 

 the solar spectrum. Light produced by coal and oil 

 gases, or by olefiant gas, even when concentrated so 

 as to produce a sensible degree of heat, was found, 

 by Mr Brande, to occasion no change in the colour 

 of muriate of silver, nor in mixtures of chlorine and 

 hydrogen ; while the light emitted by electrized 

 charcoal speedily affected the muriate, and caused 

 these gases to unite, and sometimes with explosion. 

 t 



