78 



Prof. D. B. Brace on Negative Results of 



in the article referred to for 

 measuring- the velocity of 

 light can be readily adapted 

 to the problem before us. 

 The first depends on the 

 Faraday " effect/'' the second 

 on the Kerr electrostatic 

 " effect " *. The second 

 method, in conjunction with 

 a half- shade elliptical pola- 

 rizer f, which I devised several 



years ago, 



has 



given pre- 



liminary results indicating a 

 superiority over those de- 

 scribed above, quite beyond 

 my expectations, in the 

 arrangement as originally 

 planned for determining the 

 velocity of light a number of 

 years ago. 



In fig. 1, k, k' are two con- 

 densers containing, say, the 

 dielectric, carbon disulphide 

 or nitrobenzol, giving the 

 Kerr " effect/' and placed 

 with their azimuths so as to 

 give a " crossed " system. 

 p, c, a and p r , c', a' are the 

 polarizer, elliptical half-shade 

 compensator, and analyser of 

 the two respective optical 

 systems placed in juxtapo- 

 sition, g is the spark-gap 

 and m a half-silvered mirror 

 system for sending identical 

 beams in opposite directions 

 through the optical systems 

 p' k k' c' a' and p k' k e a. 



A and A! represent the 

 azimuths of the various ele- 

 ments as seen from the one 

 side or the other. Thus at 

 A, p y and % are the azimuths 

 of the po]arizer and analyser, 



k and k' the traces of the 



i 



* Abraham and Lemoine, J. de Phi/s. 1899, p. 366, used an analogous 

 arrangement to determine the retardation of the Kerr and also of the 

 Faraday " effect." f Pays. Rev. vol. xviii. p. 70, also vol. xix. p. 218. 



