7 12 THE POPULAR SCIENCE MONTHLY. 



This theoretical impossibility of dispersion has always been recog- 

 nized as one of the most formidable difficulties of the undulatory 

 theory. In order to obviate it, Cauchy, at the suggestion of his friend 

 Coriolis, entered upon a series of analytical investigations, in which be 

 succeeded in showing that the velocities with which the various colored 

 rays are propagated may vary according to the wave-lengths, if it be 

 assumed that the ethereal medium of propagation, instead of being 

 continuous, consists of particles separated by sensible distances. 



By means of a similar assumption, Fresnel has sought to remove 

 the difficulties presented by the phenomena of polarization. In ordi- 

 nary light, the different undulations are supposed to take place in dif- 

 ferent directions, all transverse to the course or line of propagation, 

 while in polarized light the vibrations, though still transverse to the 

 ray, are parallelized, so as to occur in the same plane. Soon after this 

 hypothesis had been expanded into an elaborate theory of polarization, 

 Poisson observed that, at any considerable distance from the source 

 of the light, all transverse vibrations in a continuous elastic medium 

 must become longitudinal. As in the case of dispersion, this objection 

 was met by the hypothesis ot the existence of " definite intervals " 

 between the ethereal particles. 



These are the considerations, succinctly stated, which theoretical 

 physics are supposed to bring to the support of the atomic theory. In 

 reference to the cogency of the argument founded upon them, it is to 

 be said, generally, that evidence of the discrete molecular arrangement 

 of matter is by no means proof of the alternation of unchangeable and 

 indivisible atoms with absolute spatial voids. But it is to be feared 

 that the argument in question is not only formally, but also materially, 

 fallacious. It is very questionable whether the assumption of definite 

 intervals between the particles of the luminiferous ether is competent 

 to relieve the undulatory theory of light from its embarrassments. 

 This subject, in one of its aspects, has been thoroughly discussed by E. 

 B. Hunt, in an article on the dispersion of light (iSilliman's Journal, 

 vol. vii., 2d series, p. 364, et seq.), and the suggestions there made ap- 

 pear to me worthy of serious attention. They are briefly these : 



M. Cauchy brings the phenomena of dispersion within the do- 

 minion of the undulatory theory, by deducing the differences in the 

 velocities of the several chromatic rays from the differences in the cor- 

 responding wave-lengths by means of the hypothesis of definite inter- 

 vals between the particles of the light-bearing medium. He takes it 

 for granted, therefore, that these chromatic rays are propagated with 

 different velocities. But is this the fact ? Astronomy affords the 

 means to answer this question. 



We experience the sensation of white light, when all the chromatic 

 rays of which it is composed strike the eye simultaneously. The light 

 proceeding from a luminous body will appear colorless, even if the 

 component rays move with unequal velocities, provided all the colored 



