254 



Dr. J. Kerr. Experiments on a 



[Mar. 15, 



nected with prime conductor and earth, as indicated in the figure. 

 It is understood, of course, that the surfaces of the two conductors 

 are well planed and polished, all corners and edges rounded off, and 

 the two fronting faces accurately parallel. The cell is closed, in the 

 usual way, by panes of plate glass laid against the ends, and the 

 whole block is kept together by a screw-press. Two borings in one 

 of the plates provide for the filling and emptying. When the cell is 

 put in order and charged with CS a , and examined according to the 

 old method (with a pair of crossed nicols), it gives a very pure 

 double refraction, and acts well in all respects, except that (from de- 

 ficiency of insulation) the largest effect is less than might be expected, 

 hardly amounting to one average wave-length of relative retardation. 

 But this defect is of no great consequence. 



The First Experimental Arrangement is shown in the next diagram, 

 in horizontal section through the lamp L and the observer's eye E, 

 but without strict regard to scale. 



Co net 



Two J-in. plates of glass are represented in section by the rect- 

 angles PQ, RS. Their function is the same as that of the two plates 

 in Jamin's interference refractometer.* 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 cir- 

 cular diaphragm,| which is fixed at E in front of the observer's eye. 



* Preston's ' Theory of Light,' p. 157. 

 t Or, otherwise, through a telescope. 



