Light traverses Transparent Media. 341 



the infallibility of the undulationists. They tell us, it is true, 

 that the image of the sun's disk, produced by a lens of short 

 focal length, may be used instead of a luminous body, in all 

 experiments in which it is wished to produce a series of waves 

 diverging from a point. And the measurements hitherto taken, 

 which have been supposed by some to demonstrate the undu- 

 latory theory, have been made with common lenses, without 

 its being imagined there was any need of achromatic, apla- 

 natic glasses for these delicate purposes. 



Without attempting to clear the foregoing results of the 

 effects of aberration, we may still convince ourselves by a 

 simpler procedure, that their discordance with the hypothesis, 

 — that the velocity of light in passing through transparent 

 bodies is inversely as their refractive indices, — is more ap- 

 parent than real. 



The experiments appear to show that the velocity (w) of 

 the light in the glass was slower as the incidence was in- 



v 

 creased, making the fraction — - of larger value. Then the 



light must also have moved more slowly in the lower glass 

 than in the upper one, on account of its greater incidence 

 upon it. 



Let us, therefore, instead of 



i = ± + R = li + v< 



w v iso 



take the w in the denominator on the second side equal to 

 z w, and we have 



t = -I + R = A " + J>l 



V IV V zw 



iL.D, 



and z = 



In applying this to the experiments of the 9th of May, let 

 us assume, as the experiments warrant us in doing, that the 

 velocity in the glass at a perpendicular incidence is to the ve- 

 locity in air in the inverse ratio of their refractive indices. 

 We find for z the following : 



z = -999912 



and from this = 1*49213 



zw 



which is nearly, as we supposed it, at a perpendicular inci- 

 dence, namely, 1*492. 



