Question in Electro- Optics. 383 



The plates are therefore parallel-surfaced, and of accurately- 

 equal thickness, and are silvered on the back as mirrors ; and 

 in their working positions they are almost exactly vertical 

 and parallel, and at 45° to the light. A pencil of light LB, 

 which passes through a vertical slit in front of the lamp, is 

 incident on the first plate at B, and is divided, in the manner 

 shown in the diagram, into two pencils BDCG and BFHG ; 

 and from G the light proceeds anew as one pencil, and passes 

 through a narrow circular diaphragm *, which is fixed at E 

 in front of the observer's eve. The result of the arrangement 

 is, that when the pieces are properly placed, the bright 

 vertical slit L, as seen from E in the direction EG, is crossed 

 by a set of interference-fringes. These are well defined in 

 position by reference to a constant black line, the image of a 

 fine wire which is fixed across the slit L. It may be assumed 

 without argument, that any small increase or decrease of 

 velocity of one of the pencils BF, CG, will produce a positive 

 or negative displacement of the fringes, at the rate of one 

 fringe-width of displacement for every wave-length of rela- 

 tive retardation. As far as the assumption is required, it is 

 easily verified by the introduction of thin plates of glass into 

 the course of the light, anywhere between the two thick 

 plates ; and I find in this way, definitely, that (as the pieces 

 actually stand in the diagram and in all the experiments) 

 an ascent of the fringes indicates a relative retardation of the 

 pencil BF. 



There are two essential pieces that remain to be noticed, of 

 which the first is the electro-optic cell. It is shown in the 

 diagram how the laterally separated component pencils pass 

 through the cell, BF through the electric field, and CG 

 through the space electrically screened by the second con- 

 ductor, this conductor being always to earth. The last piece 

 is a ^icol's prism N, which is placed in the path of either of 

 the single pencils GE, LB, with its principal section laid (1) 

 horizontally and (2) vertically. The design of the apparatus 

 will now be apparent, which is, to give the means of detecting 

 electrically generated changes of velocity of the light BF in 

 two successive cases, when the plane of polarization is (1) 

 perpendicular to the lines of force and (2) parallel to the lines 

 of force. But in actual experiment there is a difficulty en- 

 countered at once, which appears at first sight to be insur- 

 mountable. 



Disturhance of the Fringes. — Suppose all the pieces placed as 

 in the diagram, the cell nearly filled with carbon disulphide, 

 the second internal conductor put permanently to earth, and 

 * Or otherwise, through a telescope. 



