THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1880. 



XXII. Measurements and Law in Electro-optics. By John 

 Kerr, LL.D., Mathematical Lecturer, Free Church Train- 

 ing College, Glasgow*. 



MY 'last communication to the Philosophical Magazine 

 contains an account of some electro-optic measurements 

 in connexion with carbon disulphide, cumol, carbon dichloride, 

 and several other liquid dielectrics. That set of measurements 

 was very imperfect, and of little interest except as a first trial. 

 In the leisure of my last summer holidays I resumed the in- 

 quiry with better means, and carried it forward for some weeks 

 with great care. I had intended to examine several dielec- 

 trics in succession, but found time only for the first of them, 

 which was carbon disulphide. These new experiments form 

 the subject of the following paper. 



1. The dioptric actions of dielectrics are generally of one 

 kind — pure double refractions with reference to line of electric 

 force as axis ; but they vary largely in intensity, and vary 

 even in sign, from one dielectric to another. Dielectrified 

 bodies are therefore optically equivalent to uniaxal crystals, 

 and exhibit like variations, both from strong to weak, and 

 from positive to negative. Our present dielectric (carbon di- 

 sulphide) is of the positive class ; and in regard to strength, it 

 holds, as yet, a place among dielectrics like that of Iceland 

 spar among crystals. For the particular facts and the proofs 

 I must refer to my papers already published")". 



I consider it probable in the highest degree that these optical 



* Communicated by the Author. 



t Philosophical Magazine, Nov. and Dec. 1875 j also Aug 1 , and Sept. 



J.O / J. 



Phil. Mag. S. 5. Vol. 9. No. 55. March 1880. N 



