CHAP. V., 7.] 



OPTICS. M. CAUCHY HITTER. 



123 



the undulatory theory. That theory, in its simple 

 form, enables us indeed to explain clearly enough 

 the refraction of light owing to a change of velocity 

 in the wave as it passes from one medium to an- 

 other ; but it assigns no reason why that change of 

 velocity should be different for light of various co- 

 lours, in other words, of different wave-lengths. The 

 corpuscular theory, on the other hand, furnishes at 

 least a plausible explanation, by assuming a variable 

 attraction between refracting media and the mole- 

 cules composing the different rays. 



(560.) When, however, undulationists were pressed on 

 Depends ^ Q su kj ec t ft was eag y ^ o see t ne direction in which 



>n the n- > ' . . , 



ate dis- a * least a plausible explanation might be sought. In 

 ;ances of the usual form of equation for vibrations in air, 

 gj ven by Lagrange, the integration is effected by as- 

 suming the intervals between the particles evanescent 

 compared with the length of a wave. This is per- 

 fectly true in the case of sound, and all sounds ap- 

 pear in consequence to travel uniformly. But should 

 it fail in the case of light, that is, should the inter- 

 vals of the ethereal particles bear some sensible ratio 

 to that very small quantity, the length of a wave, 

 what would be the result ? M. Cauchy has made 

 out, by a very complex analysis, that in this case the 

 longer waves will travel most rapidly, and conse- 

 quently be least refracted. Several other writers, 

 especially Professor Kelland, obtained similar re- 

 sults ; and Mr Airy, by very simple, though only 

 approximate, considerations, showed the dependence 

 of refraction on the length of a wave. 1 M. Cauchy's 

 memoir appeared in 1835 at Prague, in a bulky and 

 abstruse form : the mathematical investigations are 

 very long and complex, the numerical verifications 



brat- 

 ides. 



scarcely less so. 2 The indices of refraction, observed 

 by Fraunhofer, for different lines of the spectrum, in 

 different kinds of glass, together with the correspond- 

 ing wave-lengths for these rays, formed the principal 

 data for comparison. But others have since been 

 obtained by Rudberg and Professor Powell, and care- 

 fully compared with M. Cauchy's theory by the lat- 

 ter, and the coincidence appears satisfactory. 3 But 

 in estimating the value of this coincidence, it is to 

 be observed that of 7 indices of refraction observed 

 for each substance, 3 must be used to ascertain the 

 constants in the formula, and only 4 remain to be 

 calculated. 



One difficulty, however, remains. If the rays of (561.) 

 light travel through space with variable velocities, the Non ; dis - 

 images of the stars would present tails of colour in- fight i n 

 consistent with observation. M. Cauchy eludes this free space, 

 difficulty by the following hypothesis respecting ethe- 

 real media : The existence of transversal vibrations 

 (according to him) requires that the law of force be- 

 tween particle and particle of ether must not be inter- 

 mediate between the inverse 2d and inverse 4th power 

 of their distance. In the former case the force is an 

 attractive, in the second a repulsive one. In the first, 

 the velocity of propagation depends on the length of 

 a wave, in the second it is independent of it. [The 

 first case, too, alone will be consistent with perma- 

 nent longitudinal vibrations.] Consequently if we 

 suppose that in free space the particles of ether are 

 arranged in close order, and exert a repulsive force 

 on each other, no dispersion results ; but in refract- 

 ing media, supposing the distance of the molecules 

 increased, and the mutual action attractive, then dis- 

 persion occurs. 4 



7. HITTER. Chemical Rays of the Spectrum. NIEPCE ; DAGUERRE ; Mr TALBOT. Art of Helio- 

 graphy or Photography Daguerreotype Calotype. Professor STOKES. Chemical Rays ren- 

 dered visible Fluorescence. 



(562.) A very curious chapter of the history of Light re- 



hemicai ma j ns ^ o be written, respecting the chemical ener- 



ction of . ,.,.. , , , r . i i , i 



ght. g ies which itis capable of exerting, or which at least are 



found in those parts of the solar ray s which are dispersed 

 by a prism. These are in part luminous and partly 

 invisible under ordinary circumstances, the latter pos- 

 sessing these chemical qualities in a still higher degree 

 than the others. Though not perhaps very closely 

 associated with the optical discussions of the previous 

 sections, it seems impossible to separate this part of 

 the subject from the rest, since the rays called Che- 

 mical may, as we have reason to think, be reflected, 

 refracted, polarized, absorbed, and made to interfere 

 like visible light ; and farther, because to the extreme 



limit of their sensible action they may, by certain 

 treatment, be made visible to the eye. The art of 

 photography, though belonging quite as much to che- 

 mistry as optics, being a means of inquiry into the 

 qualities of the solar radiations invaluable to the 

 natural philosopher, cannot by any means be excluded 

 from a sketch, however general, of the progress of 

 physical science. 



J. W. BITTER, Professor of Chemistry at Jena, (563.) 

 and well-known for his numerous contributions Ritter. 

 to the earlier progress of voltaic electricity, has 

 the merit of having first clearly pointed out in 1801 

 the separate existence of chemical rays in the spec- 

 trum which extend beyond the most refrangible or 



1 In all these cases the expression for refrangibility depends on the ratio of the sine of an arc to the arc itself; which are 

 again includes the ratio of Aa-, the distance of the molecules to A, the length of a wave. When Aa; becomes very small, the first 

 ratio becomes unity. 



2 A portion of these researches had, however, been printed in Paris (privately, I believe) in 1830. 



3 Powell on the Undulatory Theory. 1841. * Moigno, Repertoire d'O^tique, p. 128. 



