196 Josiah Willard Gihhs. 



do not disturb the explanation of any of the other known 

 phenomena ; and in the third paper he deduces, in a very rigor- 

 ous manner, tlie general equations of monochromatic light in 

 media of every degree of transparency, arriving at equations 

 somewhat different from those of Maxwell in that they do not 

 contain explicitly the dielectric constant and conductivity as 

 measured electrically, thus avoiding certain difficulties (espe- 

 cially in regard to metallic reflection) which the theory as 

 originally stated had encountered ; and it is made clear that " a 

 point of view more in accordance with what we know of the 

 molecular constitution of bodies will give that part of the ordi- 

 nary theory which is verified by experiment, without including 

 that part which is in opposition to observed facts." Some 

 experiments of Professor C. S. Hastings in 1888 (which showed 

 that the double refraction in Iceland spar conformed to Huy- 

 ghens's law to a degree of precision far exceeding that of any 

 previous verification) again led Professor Gibbs to take up the 

 subject of optical theories in a paper which shows, in a remark- 

 ably simple manner, from elementary considerations, that this 

 result and also the general character of the facts of dispersion 

 are in strict accord with the electrical theory, while no one of 

 the elastic theories which had, at that time, been proposed 

 could be reconciled with these experimental results. A few 

 months later upon the publication of Sir William Thomson's 

 theory of an infinitely compressible ether, it became necessary 

 to supplement the comparison b}" taking account of this theory 

 also. It is not subject to the insuperable difficulties which 

 beset the other elastic theories, since its equations and surface 

 conditions for perfectly homogeneous and transparent media 

 are identical in form with those of the electrical theory, and 

 lead in an equally direct manner to Fresnel's construction for 

 doublv-refracting media, and to the proper values for the 

 intensities of the reflected and refracted light. But Gibbs 

 shows that, in the case of a fine-grained medium, Thomson's 

 theory does not lead to the known facts of dispersion without 

 unnatural and forced hypotheses, and that in the case of 

 metallic reflection it is subject to similar difficulties ; while, on 

 the other hand, "it may be said for the electrical theory that 

 it is not obliged to invent hypotheses, but only to applj^ the 

 laws furnislied by the science of electricity, and that it is diffi- 

 cult to account for the coincidences between the electrical and 

 optical properties of media unless we regard the motions of 

 light as electrical." Of all the arguments (from theoretical 

 grounds alone) for excluding all other theories of light except 

 the electrical, these papers furnish the simplest, most philo- 

 sophical, and most conclusive with which the present writer is 

 acquainted ; and it seems likely that the considerations advanced 



