340 Dr. Lodge on the Rotation of the Plane of Polarization 



exhibits no change in its sensitive tint during the discharge ; 

 similarly also Jellett, and other such reversible detectors, 

 would be useless for the purpose : the effect is an irreversible 

 one. Nevertheless some sort of measure of the effect can be 

 made by finding the position of the analyser whereat the 

 brightness of the field suffers no appreciable change at the 

 occurrence of the spark, because in one position the oscilla- 

 tion on one side will darken it just as much as that on the 

 other side brightens it. 



It seemed to me possible that if the effect could be rendered 

 pretty considerable a slight darkening of the field might be 

 obtained with some adjustments, because some arcs of the 

 sine curve have an average ordinate less than their middle 

 ordinate. But when a fairly bright field winks slightly it is 

 not easy to say whether it winks brighter or darker, and after 

 all I do not know that it much matters. 



The main interest in the experiment seemed to me to lie in 

 the evidence it afforded of practical instantaneity in the 

 development of the property in the substance under examina- 

 tion ; and in order to try oscillations of much greater fre- 

 quency than those I first used, I got my assistant, Mr. 

 Robinson, to make a long tube of carbon di sulphide with 

 which to repeat the experiment in a sensitive manner. 



Now, a most interesting experiment of Villari*, in which 

 he whirled a drum of heavy glass up to 200 revolutions per 

 second between the poles of a magnet, and perceived the 

 electro-optic effect to diminish from 100 revolutions per second 

 upwards, and ultimately nearly cease at high speeds (sa}" 180 

 per second), had led one to suppose that some distinct time 

 was necessary for the production of the effect — something 

 between g Jq ^^^ too second. But on referring to that most 

 useful summary of Electrical Science, Prof. Chrystal's article 

 in the Encycl. Brit., I found, along with a quotation of Yillari's 

 experiment, a statement that Professors Bichat and Blondlot f, 

 of Nancy, had by means of Leyden-jar discharge proved 

 that practically no time was necessary. I accordingly pro- 

 cured a copy of the volume of the Comptes Rendus from 

 London, and there found an all too brief account of a most 

 beautiful series of experiments, by which they considered it 

 proved that if any time is required, it is less than the ^^-^ 

 second. The skill of these French Professors in optics, and 

 their previous researches in connexion with the Faraday 



* Pogg. Ann. cxlix. 1873, p. 324. Translated from the Rendiconti 

 Istit. Lombardo, ser. 2, vol. iii. 



t Comptes Rendus, xciv. 1882, p. 1590. 



