388 Dr. John Kerr on a Fundamental 



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 there tore, 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 3 

 as dielectric, and with nicoi X as eyepiece, I got what 

 appeared to be a perfectly clean liquid. The potential also 

 was made to vary regularly 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 overpowered by 

 disturbance ; the contrary jump seen always at the instant 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 exclusively, 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 conclusion 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 instru- 

 ment here used is known as Jamin's Interference Refractor 

 for Polarized Light. For a description of it 1 might refer to 

 a paper already published * ; but I think I ought rather to 

 describe the apparatus here again. The essential pieces are 

 shown in horizontal section in the following diagram. 



rig. 4. 



Rand S are large blocks of Iceland spar, of equal thickness, 

 their principal sections horizontal, and their faces parallel. A 

 pencil of light from a vertical slit L, passes through a Foucault's 

 prism H, and is polarized by it at 45° to the vertical, and 

 then enters the rhomb R. The two pencils emergent from R 



* "On the Birefringent Action of Strained Glass." Phil Mag. for 

 Oct. 1888. 



