on the Undulatory Hypothesis of Light. 467 



merion of double refraction has been selected for present consi- 

 deration because the theory I propose to give of it will be a good 

 exemplification of the principles maintained in that communica- 

 tion. 



In the article just mentioned the ultimate properties of matter 

 and force are deduced, conformably with a rule of philosophizing 

 laid down by Newton, from the indications of sensation and ex- 

 perience. Being so deduced they are perfectly comprehensible, 

 and, if Newton's rule be good, may be regarded as necessary 

 foundations of physical science. The hypotheses of the follow- 

 ing theory, being framed in accordance with these fundamental 

 conceptions, will be treated as true hypotheses, from which by 

 right and sufficient reasoning the explanations of the phenomena 

 necessarily result. Those physicists, however, who are not pre- 

 pared to admit the validity of the a priori reasoning, cannot 

 refuse to accept as mere hypotheses the ultimate properties which 

 were reached by it, and as capable of being proved to be true by 

 a sufficient amount of accordance with experiment of results de- 

 duced from them mathematically. I shall be content if their 

 truth be considered to rest solely on this kind of evidence. 



Now, in the first place, since on these principles the aether is 

 to be regarded as a continuous fluid which presses proportionally 

 to its density, and as experiments indicate that light has the same 

 properties and is subject to the same laws under very different 

 circumstances of original generation, those movements of the 

 aether are admissible as exponents of phenomena of light, which 

 can be mathematically shown to exist in such a medium inde- 

 pendently of the particular nature of the disturbance. The in- 

 vestigation of such movements is therefore a necessary prelimi- 

 nary. This requisite I consider to be fully satisfied by the ma- 

 thematical theory of the vibrations of an elastic fluid contained 

 in the Numbers of the Philosophical Magazine for June, Au- 

 gust, and October 1862. 



Again, because, according to the same principles, the aether is 

 a uniform fluid, and all the natural forces are modes of its action, 

 its density must be the same within as without the refracting 

 medium, there being no active force resident in the atoms of the 

 medium to produce any alteration of the density. Also, because 

 our principles led to the conclusion that the constituents of vi- 

 sible and tangible substances are inert spherical atoms of con- 

 stant magnitude, the modifications which vibratory movements 

 of the aether undergo when they pass from vacuum into a re- 

 fracting medium, can only be due to the obstacles presented by 

 the atoms to the free motion of the aether ; and the effect of 

 these obstacles will depend on the form, magnitudes, and ar- 

 rangement of the atoms, on the number in a given space, and on 



