Question in Electro- Optics, 389 



pass immediately through a half- wave plate P, so placed as to 

 interchange the two planes of polarization. Ordinary pencil 

 and extraordinary in the crystal R become thus extraordinary 

 and ordinary in S, and the birefringent action of E, is 

 neutralized by that of S. The light enters R and leaves S as 

 a single pencil ; but between P and S it passes as a couple of 

 pencils BF and CG, about 14 millim. apart, and polarized in 

 planes vertical and horizontal. The pencil emergent from S 

 is received at E through a Nicol's prism N, which is laid as 

 for extinction with the Foucault H. When all the pieces 

 have been properly placed, the slit L is seen crossed by a set 

 of interference-fringes ; and these are modified at pleasure by 

 fine screw-movements of the spar S. 



The electro-optic cell is not given in the diagram. It is 

 the same piece as that shown in the diagram of the First 

 Arrangement, and is placed here exactly as there, so that the 

 two laterally separated component pencils pass normally 

 through it, BF through the electric field, and CG behind the 

 second conductor. 



The only other optical piece employed in the experiments 

 is a Jamin's glass compensator *, which is placed immedi- 

 ately in front of the spar S : it enables the observer to specify 

 small differences of retardation of the pencils BF and CG. 



The results obtained formerly (with nicol N as eyepiece) 

 were fully verified with the new apparatus. The method 

 finally adopted as the best was so similar to the former, and 

 the effects also, that any long description of the experiments 

 would be superfluous. But to give a fair view of the results, 

 I will describe one day's work. 



Final Experiments with CS 2 . — The first internal conductor 

 connected permanently with prime conductor without Leyden 

 jar, the liquid quite clean, and the conditions of electric work 

 perfect all day. The observations were taken in five succes- 

 sive sets. 



First Set. — Plane of polarization of the pencil BF (through 

 the electric field) vertical, or perpendicular to line of force ; 

 rise of fringes indicates relative retardation of that pencil. 

 When the fringes were obtained in good form and position, 

 the machine was started, and kept working at a constant rate 

 throughout the experiment. As before, the first effect was 

 a large disturbance, the fringes being displaced and deformed, 

 and disappearing altogether at the second or third turn of the 

 plate ; but in a little time (30 or 40 turns) they reappeared 

 in good form and approximately constant position. For 

 distinctness of effect, the central fringe was brought back to 

 * Preston's ' Theory of Light,' p. 159. 



Phil. Mag. S. 5. Yol. 37. No. 227. April 1894. 2 D 



