1894.] Fundamental Question in Electro- Optics. 259 



of the phenomenon, and its partial obscuration in many cases by dis- 

 turbance. 



Returning, therefore, to the first form of experiment, that with the 

 nicol N as eye-piece. When the principal section of N was horizontal, 

 and the vibration directed therefore along the line of force, there was 

 a perfectly regular jump of the fringes downwards at the instant of 

 discharge ; and at high potential the effect was large and strikingly 

 distinct. When the principal section was vertical, there was nothing 

 regular of this kind seen in any of a large number of observations : 

 there were disturbance-movements at or about the instant of dis- 

 charge, as before and after, but nothing that could be accepted as a 

 regular jump of the fringes at that instant, always in one direction or 

 always in the other. The interpretation of these results is obvious. 

 I have already stated, as a matter of observation, that a rise of the 

 fringes indicates a relative retardation of the pencil BF which passes 

 through the electric field. From the downward jump of the fringes 

 in one of the two cases, we infer therefore that the pencil BF is in 

 that case relatively accelerated in consequence of discharge. But in 

 the present experiment, and with reference to the pencil BF in, rela- 

 tion to the pencil CGr, it is evident that relative acceleration and 

 absolute are equivalent ; because it is only in that division of the cell 

 through which the pencil BF passes that there is any sudden physical 

 change at the instant of discharge. It appears, therefore, that to 

 relieve the liquid of electric strain, is to relieve one of the vibrations 

 (that along the line of force) of an absolute retardation, leaving the 

 perpendicular vibration unaffected. 



In several of the later sets of these experiments with CS 2 as 

 dielectric, and with nicol N" as eye-piece, I got what appeared to be a 

 perfectly clean liquid. The potential also was made to vary regu- 

 larly and very slowly ; and from both causes the disturbance was 

 very much reduced. The effects then were these : Principal section 

 of N horizontal : a slow ascent of the fringes during the process of 

 charge, pretty regular, but often obscured and sometimes over- 

 powered by disturbance ; the contrary jump seen always at the in- 

 stant of discharge. Principal section of N vertical : irregular, and 

 generally very small oscillations of the fringes during the process of 

 charge ; but no regular motion in one direction or the other exclu- 

 sively, either during the process of charge, or at the instant of spark- 

 discharge from high potential. 



From all these experiments with CS 2 , it seems to follow that of 

 the two principal vibrations, the only one immediately and regularly 

 affected by electric strain is that along the line of force. This con- 

 clusion requires and well deserves to be verified ; and I proceed to 

 verify it by another method, or rather by the use of new means. 



The Second Experimental Arrangement. The optical instrument 



