CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, 

 HARVARD UNIVERSITY. 



DISPERSION IN ELECTRIC DOUBLE REFRACTION. 

 By Howard Lane Blackwell. 



Presented by John Trowbridge, April 8, 1906. Received December 31, 1905. 



The object in the following paper is to present a study of dispersion 

 in the electric double refraction of carbon bisulphide; for though as 

 many as fourteen observers have experimented with electric double 

 refraction since its discovery thirty years ago, even the existence of 

 dispersion in the phenomenon has been unnoticed by all except two. 

 Only one of them made any suggestion as to what function of the 

 wave-length represents the double refraction, and the function which 

 he suggested turns out not to hold even in the visible spectrum. The 

 departure from it is important both in the direction of electro-magnetic 

 waves and towards the ultra-violet. 



When a condenser is charged, the dielectric becomes doubly refract- 

 ing. Under the influence of the electric field it acquires the optical 

 property of a uniaxial crystal. Electric double refraction has been 

 induced in glass, resin, and other transparent solids, and in over one 

 hundred insulating liquids. Even conducting liquids, — water, nitro- 

 benzol, and others, — are capable of exhibiting this effect, though only 

 for an instant. Solid insulators, like glass, require a number of seconds 

 after the field is established to acquire the new property most strongly. 

 In liquids, however, the change is sensibly instantaneous. Moreover, 

 just as natural uniaxial crystals fall into two classes, positive and nega- 

 tive, so substances rendered doubly refracting by the electric field 

 behave some of them like positive, some like negative crystals. In 

 other words, the velocity of the extraordinary wave is increased over 

 that of the ordinary in some substances and decreased in others. Car- 

 bon bisulphide becomes positively uniaxial and exhibits electric double 

 refraction more strongly than almost any other liquid tried. 



The first investigation in this general field was an unsuccessful 

 attempt by Faraday in 1834 to discover some effect of electrostatic 



