240 FRESNEL. 



velocity of a ray determines the refraction which it must 

 undergo when it falls obliquely on the surface of a trans- 

 fusion, which it is difficult to explain. The assertion is clear, and the 

 whole subsequent argument agrees with that assertion, in regard to 

 the emission theory. Here, undoubtedly, the original velocity with 

 which a ray enters a new medium, when it is acted upon by the 

 attractions of a number of surrounding particles, will essentially deter- 

 mine the velocity with which it will continue to move under the in- 

 fluence of these attractions, and the path it will take. But on the wave 

 theory there appears nothing obviously and antecedently to show what 

 will be the case. 



The author proceeds, as if continuing the last topic, to quite another 

 point, viz : the experimental fact that light from the most different 

 sources, both terrestrial and celestial, moves with precisely the same 

 velocity through air or vacuum. He argues that this is a " mathe- 

 matical consequence" of the wave theory; because, in the parallel 

 case of sound, tones produced by the most different instruments are 

 propagated through the air with the same rafndity. It is certainly a 

 close analogy, but hardly a " mathematical consequence." The re- 

 mark which follows as to the consequence of molecular theory, in 

 rendering light from different sources unequally rapid in its flight 

 from their differences of attractive power, presents, no doubt, a formi- 

 dable difficulty to that theory, as being in contradiction to the experi- 

 mental result just mentioned. 



But when in reference to his own beautiful experiment on observing 

 the refractions of light when its velocity is respectively increased and 

 diminished by the whole velocity of the earth, he adds, " such rays 

 ought to be unequally refracted," I can only understand the mean- 

 ing by referring to the mention made of the emission theory in the 

 next line, and supposing that theory alone to be intended. On that 

 theory, it is true, such rays ought to be unequally refracted. 



Observation, however, gives a perfect equality of refraction in the 

 two cases, and thus far completely contradicts the idea of molecular 

 attractions. And when he adds, that the only way in which this 

 contradiction could be reconciled with emission, would be by invent- 

 ing the subsidiary gratuitous hypothesis that the stars emit an infinite 

 number of rays, endowed with all possible velocities, and that only 

 those of a certain velocity can affect our organs with the sense of 

 vision, this would obviously only be to add " cycle on epicycle," " to 

 save appearances," and would afford no real explanation. On the 

 other hand, with respect to the undulatory theory, it does not appear 



