﻿Question in Electro- Optics. 145 



scope and compensator as effects of electrostatic stress. They 

 were evidently produced by mechanical disturbance of the 

 dielectric ; and effects of that kind are of no interest in electro- 

 optics, except as hindrances to exact observation. It may be 

 easily understood, therefore, that I omitted all reference to 

 Professor Quincke's experiments in my paper, not from any 

 positive intention, but because they did not occur to me as 

 contributing in any degree to the solution or illustration of 

 the question that I had in hand. 



In this connexion 1 may notice an old set of experiments 

 of my own upon the same question, which did not occur to 

 me as worth mentioning in my paper, though they had given 

 a clear and striking exhibition of the double refraction. The 

 dielectric was carbon disulphide, the electro-optic field was a 

 very obtuse and very thin prism which extended from end to 

 end of a large plate-cell, and the light (monochromatic and 

 unpolarized) entered and left the cell respectively through the 

 collimator and the telescope of an ordinary spectroscope, the 

 slit being parallel to the lines of force. As the mechanically 

 compressed prisms of glass acted in Fresnel's well-known 

 experiment, so the electrically strained prism of CS 2 acted 

 here. At high potentials the telescope gave two parallel 

 images of the slit, clearly, though not very widely, separated 

 from each other, and polarized in planes parallel and perpen- 

 dicular to the lines of force ; but the mechanical disturbance 

 of the dielectric rendered this result useless for my purpose, 

 as it kept the double image of the slit moving incessantly and 

 very irregularly, so that no probable inference could be drawn 

 as to the absolute retardations of the two component rays. I 

 hope to have something more to say about this method and its 

 results hereafter. 



There are two statements in Professor Quincke's letter which 

 require some qualification. The first is, that his methods 

 were identical with those followed by myself. This applies 

 truly to the kind of instrumental means employed, and to the 

 general conception of the arrangements, means and methods 

 of great delicacy, for which we are indebted to Professor 

 Jamin, by whom indeed they were put among the common- 

 places of the higher experimental optics. But the method 

 described in my paper includes something more : it deals 

 with the chief difficulty of the subject by detaching the double 

 refraction from the irregular and ever present effects of 

 mechanical disturbance ; and it brings out in this way the 

 fact — clearly enough, perhaps, for a first and imperfect proof — 

 that electric stress acts exclusively on the (Fresnel's) vibra- 

 tion which is directed along the line of force. 



Phil. Mag. S. 5. Vol. 38. No. 230. July 1894. L 



