Motion of the Medium on the Velocity of Light. 379 



Fresnel's statement amounts then to saying that the ether 

 within a moving body remains stationary with the exception 

 of the portions which are condensed around the particles. If 

 this condensed atmosphere be insisted upon, every particle 

 with its atmosphere may be regarded as a single body, and 

 then the statement is, simply, that the ether is entirely unaf- 

 fected by the motion of the matter which it permeates. 



It will be recalled that Fizeau* divided a pencil of light, 

 issuing from a slit placed in the focus of a lens, into two par- 

 allel beams. These passed through two parallel tubes and then 

 fell upon a second lens and were re-united at its focus where 

 they fell upon a plane mirror. Here the rays crossed and were 

 returned each through the other tube, and would again be 

 brought to a focus by the first lens, on the slit, but for a plane 

 parallel glass which reflected part of the light to a point where 

 it could be examined by a lens. 



At this point vertical interference fringes would be formed, 

 the bright central fringe corresponding to equal paths. If now 

 the medium is put in motion in opposite directions in the two 

 tubes, and the velocity of light is affected by this motion, the 

 two pencils will be affected in opposite ways, one being retarded 

 and the other accelerated ; hence the central fringe would be 

 displaced and a simple calculation would show whether this 

 displacement corresponds with the acceleration required by 

 theory or not. 



Notwithstanding the ingenuity displayed in this remarkable 

 contrivance, which is apparently so admirably adopted for 

 eliminating accidental displacement of the fringes by extraneous 

 causes, there seems to be a general doubt concerning the results 

 obtained, or at any rate the interpretation of these results given 

 by Fizeau. 



never be 1. The above expression, however gives this result when the particles 



w 2 1 1 



are in contact — for then b=0 and x= — r—+ -j=l. 



ri 1 ri 1 



Resuming equation (1) and putting a + b=l we find (n— l)l=(fi— l)a. But for 

 the same substance [m and a are probably constant or nearly so; hence (n—l)l 

 is constant. 



But Clausius has shown that 1=k a, where k is a constant, <x the density of 



P 

 the molecule; p, that of the substance; and a, the diameter of the "sphere 

 of action." <x and a are probably nearly constant, hence we have finally 



■ =constant. 



P 



Curiously enough, there seems to be a tendency towards constancy in the 

 product (n—l)l for different substances. In the case of 25 gases and vapors 

 whose index of refraction and "free path" are both known, the average differ- 

 ence from the mean value of (n— 1)1 was less than 20 per cent, though the factors 

 varied in the proportion of one to thirteen; and if from this list the last nine 

 vapors (about which there is some uncertainty) are excluded, the average differ- 

 ence is reduced to 1 per cent. 



* Ann. de Ch. et de Ph., Ill, Mi, p. 385, 1859. 



