254 VELOCITY OF LIGHT. 



This consideration greatly simplifies the theory of Foucault's 

 experiment, and makes it evident, I think, that the results of all 

 such experiments depend upon the value of U, and not upon that of V. 



The discussion of the experiment by following a single wave, and 

 taking account of its rotation, is a complicated process, and one in 

 which it is very easy to leave out of account some of the elements of 

 the problem. The principal objection to it, however, is its unreality. 

 If the dispersion is considerable, no wave which leaves the revolving 

 mirror will return to it. The individual disappears, only the group 

 has permanence. Prof. Schuster, in his communication of March 11 

 (p. 439), has nevertheless obtained by this method, as the quantity 

 determined by " the experiments hitherto performed," F 2 /(2 V U), 

 which, as he observes, is nearly equal to U. He would, I think, 

 have obtained U precisely, if for the angle between two successive 

 wave-planes of similar phase, instead of 2wA/ V, he had used the more 

 exact value 2w\/ U. 



By the kindness of Prof. Michelson, I am informed with respect to 

 his recent experiments on the velocity of light in bisulphide of carbon 

 that he would be inclined to place the maximum brilliancy of the 

 light between the spectral lines D and E, but nearer to D. If we 

 take the mean between D and E, we have 



U) 



' 



K denoting the velocity in vacuo (see p. 249 of this volume). The 

 number observed was 1*76, " with an uncertainty of two units in the 

 second place of decimals." This agrees best with the first formula. 

 The same would be true if we used values nearer to the line D. 



J. WILLARD GIBBS. 

 New Haven, Connecticut, April 1. [1886.] 







