212 Dr. H. Hudson on Wave-Theories of 



equal and enormous self- repulsion or elasticity and both existing 

 in equal quantities throughout space^ whose vibrations take place 

 in perpendicular planes, the two media being mutually indiffer- 

 ent, neither attracting nor repelling. 



Common light, therefore, will consist of waves (equal in every 

 respect and undistinguishable by our organs of vision) in each 

 medium, the vibrations being in a plane perpendicular to the 

 direction of the wavers progress. But the reflected ray in the 

 one medium will, I conceive, present the qualities of the refracted 

 ray in the other ; and it is manifestly impossible, no matter how 

 perfectly similar they may appear to be, that any interference 

 can take place between waves thus propagated independently in 

 different media. 



By this slight modification of the physical conception on which 

 the wave-theory is founded I believe every difficulty will be re- 

 moved, and the cause of the equal quantities of polarized light 

 obtained by reflection and refraction is obvious. 



I think we have a proof of the existence of two distinct media 

 in the aether in the fact that the "ordinary''' and "extraordinary^^ 

 rays (when produced by two similar plates of the same crystal in 

 rectangular positions) will inteifere, whereas the two rays of the 

 same denomination (even v/hen brought into the same plane) are 

 incapable of interference. The reason is obvious if the two or- 

 dinary rays (thus produced) are in diff'erent media, while the 

 ordinary ray from one plate and the extraordinary ray from the 

 other arc in the same medium. As a further test of the hypo- 

 thesis, I beg to suggest an attempt, experimentally, to make the 

 "ordinary refracted ^^ ray (through Iceland spar) show any indi- 

 catioDs of interference with the apparently similarly polarized 

 ray obtained by total reflection. As I have already said, I believe 

 they are produced by vibrations in different media, and must 

 uniformly increase the illumination (instead of producing dark- 

 ness or fringes), no matter how either ray may be retarded in 

 its path. 



With regard to calorific waves, I believe that they are due to 

 those vibrations of the aether which take place in the direction 

 of the wave'' s progress, and, as such, are totally distinct from lu- 

 miniferous undulations. 



In reference to lum.iniferous (or transversal) vibrations, Fres- 

 nel has shown that, "in such a medium as the aether, the force 

 which resists the approach of two strata is much greater than that 

 which opposes their sliding on one another.''^ Poisson in his 

 researches (1830) points out that, according to his theory, there 

 will be two waves, the vibrations in one being " in the direction 

 of propagation and attended with dilatations proportional to the 

 absolute velocity of the molecules '' (this I therefore look upon 



