,894.] 



Fundamental Question in Electro-Optics. 



263 



From what I know of the apparatus and its performance I am sure 

 that no regular and abrupt retardation or acceleration amounting to 

 much as the hundredth part of an average wave-length could have 

 escaped observation in the experiments. It will be remembered also 

 that the jump of the fringes at high potential extended to four-fifths 

 }f the fringe width. With reference, therefore, to the dielectric CS 2 , 

 md the two principal vibrations parallel and perpendicular to the 

 line of force, it appears that the regular effect of the electric strain 

 ipon one of the vibrations is a positive retardation, while upon the 

 >ther vibration there is very probably no effect whatever, and cer- 

 linly no effect as large as the eightieth part of the former. 



Second Positive Dielectric : A. paraffin oil, specific gravity 0'845. This 

 liquid was far inferior to CS 2 electrically, and also as an optical 

 ledium. The method of experiment finally adopted as the best was 

 little different from that with CL 2 . The prime conductor had its 

 ipacity enlarged by connexion with a Leyden jar ; the machine was 

 cept working at a constant rate, and the prime conductor was 

 rtially discharged, at short and regular intervals, by sparks upon 

 knob of the first internal conductor, which was of course dis- 

 charged in each interval. The phenomenon looked for was a quick 

 action of the fringes at the instant of the spark ; that is, at the 

 istant of electric charging of the liquid. 



(1.) Plane of polarisation of the pencil BF (through the electric 

 ield) vertical : Rise of fringes indicates relative retardation of this 

 jncil. At the instant of the spark there was a quick upward jump 

 af the fringes through something like one-fifth of the fringe width, 

 enerally followed by a set of large and comparatively slow disturb- 

 ice-movements. In most cases also, immediately after the spark, 

 observer was able to detect the contrary jump quite clearly by 

 tying his finger on the knob of the first conductor. Through a long 

 of observations, taken at different potentials, the upward jump of 

 le fringes at the instant of charging was obtained with perfect 

 3gularity ; and amplitude excepted the effect was not inferior to 

 lat in CS 2 . 



(2.) Plane of polarisation of the pencil BF horizontal : Rise of 



ringes indicates relative retardation of BF. Many observations 



rere taken at different potentials, high and low. There were sluggish 



id irregular disturbance movements, great and small, but no trace 



a regular jump of the fringes in one direction or the other at the 



istant of the spark. There could be no doubt as to the true meaning 



these results. In this positive dielectric, as in CS 2 , the vibration 



long the line of force is retarded by electric strain, and the perpen- 



licular vibration is unaffected. 



First Negative Dielectric : Oil of Colza. This liquid also was far 

 inferior to CS 2 , especially as an optical medium. The method of 



