THE VELOCITY OF LIGHT. 221 



these platinum wires exactly covered the second reflection 

 from the other platinum wire, so that he got a double 

 intensity of light. This was a very ingenious application 

 of optical principles to increase the intensity of the light. 

 In his communication to the Academy of Sciences in 1850, 

 M. Foucault was simply measuring the relative velocities 

 of light through air and throiigh water, and he did so by 

 interposing between two of the mirrors a tube half full of 

 water, so that part of the ray of light went through the 

 water and part went through the air, and he found that the 

 light which was passing through the air was deflected from 

 its natural position more than the light which passed 

 through the water, in the ratio of four to three, showing 

 that the velocity of light in air is of the velocity of light 

 in water ; exactly, in fact, in proportion to the refractive 

 indices of the two media. This brilliant result completely 

 settled the question of the two theories of light as they 

 were at that time enunciated, and the undulatory theory 

 has acquired, I will not say an absolute certainty, but we 

 are much more nearly certain than we could possibly have 

 been without such an experiment, and the corpuscular 

 theory of light enunciated by Newton is absolutely un- 

 tenable. 



Exactly on the same day that Foucault communicated 

 these brilliant researches to the Academy of Sciences, 

 Fizeau also laid before the Academy a description of the 

 same apparatus which he had mounted for the same 

 purpose. But there had not been sufficient sunlight since 

 it had been mounted in order to give him a definite result, 

 and consequently he could not give the results at that 

 time. In the year 1862 Foucault redetermined the absolute 

 velocity of light in the same manner by employing the 

 same apparatus which I have just described, and on this 

 occasion he increased the distance which he employed, by 

 interposing more mirrors before the light was finally 

 reflected back again, so that he was able to get a distance 

 five times as great as the one he had employed before ; and 

 in these experiments he employed the surprisingly small 

 distance of nearly four metres at first, and in the second 

 instance a distance of only twenty metres, and yet he was 

 able to measure accurately that small interval of time 

 which was taken by the light passing over that distance. 



