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XXXIY. Experiments on a Fundamental Question in Electro- 

 Optics : Reduction of Relative Retardations to Absolute. 

 By John Kerr, LL.D., F.R.S., Free Church Training 

 College, Glasgoiv*. 



TO prepare the way, I begin by recalling these well-known 

 facts : that when light passes through an electro- 

 statically strained medium in a direction perpendicular to the 

 line of electric force, it undergoes a uni-axal double refraction, 

 the optic axis coinciding with the line of force : that with 

 reference to this action, dielectrics are divisible into two 

 classes, the positive! and the negative J, which are optically 

 related to each other in the same way as the positive class of 

 crystals to the negative : that the intensity of the action, or 

 the quantity of optical effect per unit thickness of the dielec- 

 tric, is measured by the product CF 2 , where C is a constant 

 which is characteristic of the medium, and F is the value of 

 the resultant electric force : that the effects are generally 

 observed and examined still, as they were discovered first, by 

 simple experiments with a pair of Nicol's prisms and a slip of 

 strained glass or other phase-difference compensator. 



In every such experiment, the effect specified by the com- 

 pensator is a difference of phases, or a relative retardation ; 

 and we may therefore view it as a resultant effect — that is 

 to say, as the resultant, or the difference, of electrically 

 generated absolute retardations of two component lights 

 whose planes of polarization are parallel and perpendicular to 

 the line of electric force. What, then, are the values of these 

 two absolute retardations in any given case ? What are the 

 two absolute components of any electrically generated relative 

 retardation ? Such is the question here proposed for solution 

 by experiment. 



As long ago as 1882, and several years following, I was 

 much occupied at intervals with this interesting question. 

 In the summer of 1885, in some experiments with the dielec- 

 tric CS 2 , I obtained results as decisive as could be desired. 

 Other dielectrics, both solid and liquid, were tried after- 

 wards, but only with partial success, the experimental diffi- 

 culties being — in some cases — too much for my methods and 

 time. To these cases I shall make no further reference, as I 

 will keep to the one line of experiment, and to those experi- 

 ments in particular in which the indications were quite 



* Communicated by the Author. 



t Carbon disulphide, the hydrocarbons, &c, 



\ Amyl oxide, the heavy oils, &c, 



