E. B. Hunt on the Dispersion of Light. 365 



in the time or intensity of this action. The facts required this 

 cause to be something intrinsic in and peculiar to the several col- 

 ored rays : peculiarities of media alone being wholly insufficient 

 for an explanation. Newton ascribed dispersion to differences of 

 size between the different colored luminous particles; the red be- 

 ing greatest and the violet smallest. To this a difference of 

 identity or intensity of molecular action must be superadded, as no 

 mere difference of volume, could, except in a resisting medium, 

 produce a refractive difference. By M. de Courtivon and Mr. Mel- 

 ville, a difference of initial velocity was supposed: this, by sub- 

 jecting the colored particles to molecular action for unequal times, 

 would produce refractions varying as an inverse function of their 

 velocities. Astronomical facts wholly disproved the hypothesis. 

 The idea of heterogeneous particles, between which and the 

 molecules of bodies, exist elective attractions, like chemical elec- 

 tive affinities, has been more widely entertained. Assuming the 

 emission theory and excluding difference of chromatic velocities, 

 we are forced to the admission of heterogeneous particles, either 

 in the Newtonian mechanical sense or this elective affinity sense, 

 which two nearly merge into each other. The emission theory 

 hypothesis of heterogeneous particles, certainly represents the 

 facts of dispersion. 



But the theory itself, encumbered by the ideas of "fits," 

 "sides" of rays, "rotating polarities," &&, has no strong claims 

 on our admiration and acceptance. In truth, its explanatory pow- 

 ers, so far from embracing new optical discoveries as they have 

 been developed, seem exhausted on the simple facts to which 

 Newton applied them. Admitting the incorrectness of this the- 

 ory, it is still singular that it should have served the purposes of 

 explanation so well. We can scarcely avoid the belief, that in 

 spite of its defects, it contains some vein o( truth, and presents 

 some fragmentary picture of nature's phases. If the views pre- 

 sented in this article be correct, the feature of molecular attrac- 

 tion on light, as the cause of its refraction, will be retained from 

 Newton's theory, while that of emitted luminous particles is 

 abandoned. This will effect a compromise between the emission 

 and wave theories, by making the waves themselves, subject to 

 molecular attraction, and that unequally according to their nature. 

 The fact of dispersion has been the chief stumbling block in 

 the way of the wave theory. All attempts to explain it from 

 this point of view, had by general consent, been unsatisfactory 

 up to that commenced by Fresnel and completed by Cauchy. 

 A failure to explain is compatible with sound theoretical bases, 

 but the wave theory seemed to prove that dispersion could not 

 exist: a conflict with fact which has been a great obstacle to its 

 adoption. Mathematical investigations all indicated that the ve- 

 locity of wave transmissions in the same medium of any density, 



