Light, Heat, and Electricity, 211 



double what it was at zero Centigrade. Even their velocity 

 through glass is 55,000 times less than the speed of setherial 

 undulations, and the extreme slowness of change of temperature 

 in the •' conduction of heat '' (as contrasted with the rapidity 

 with which the vibrations of the sether exhaust themselves, be- 

 coming insensible almost instantaneously when the action of the 

 exciting cause ceases) marks distinctly the essential difference 

 between molecular and setherial vibrations. It appears to me, 

 therefore, a very crude hypothesis to im.agine a combination of 

 setherio-molecular vibrations as accounting for the very minute 

 difference in the retardation of doubly refracted rays in crystals. 



Among the known facts which remain unexplained by Fres- 

 neFs theory the following are prominent : — 



1st. That NO interference can take place between two rays 

 originally polarized in perpendicular planes, even v/hen they 

 have been brought into the same plane of polarization. 



2nd. The rings from thin plates (observed by Arago) when 

 viewed through Iceland spar with its principal section parallel 

 OR perpendicular to the plane of incidence — the result being that 

 the hght of both the transmitted and the reflected rings is wholly 

 polarized in the plane of incidence, the colours being comple- 

 mentary and their intensities perfectly equal. 



3rd. That, in common light, in order to explain the pheno- 

 mena of interference, it is necessary to admit a sudden transition 

 [per saltum) from one system of v/aves to another, in which the 

 vibrations are altogether different and have no apparent con- 

 nexion. 



4th. The case of double refraction in crystals of quartz ap- 

 pears to afford another example of the inadequacy of the theory 

 of a system of waves propagated through a single elastic medium, 

 inasmuch as it presents a total breach of continuity m the transi- 

 tion from the velocity of the one ray to that of the other. 



5th. The phenomena of "absorption,^\ which will, I believe, 

 be ultimately explained through the principle of interference. 



We know, from the aberration of the light of the fixed stars, 

 that "the sether'''' encompassing our earth does not participate 

 in its motion; and therefore it is only the " excess ^^ of gether, 

 associated with the molecules or atoms of matter, which is carried 

 along with it in its motion through space; and v/e can have no 

 doubt with respect to the sameness of the density and elasticity 

 of the sether throughout space, inasmuch as the velocity of the 

 propagation of light (which depends on them) is the same what- 

 ever be its origin. 



How^ then, can we explain such theoretical difficulties as I 

 have alluded to ? I believe it will be necessary to consider what 

 we term the sether as consisting of two media, each possessed of 



P2 



