382 



Dr. John Kerr on a Fundamental 



arranged face to face in one block. The inner rectangle 

 represents a tunnel (5} by 3J) which passes right through 

 the block. Inside are shown the conductors with supporting 

 frame, the shaded pieces being of brass and the unshaded 

 of plate glass. The lengths of the conductors, at right angles 

 to the plane of the figure (and parallel to the light), are 6 J 

 and 1\ inches, the thickness of the cell being nearly 8 inches. 

 By means of wires which pass through the wall of the cell, 

 the internal conductors are connected 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 2 , 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 deficiency of insu- 

 lation) 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. 



P Cond 



Fig 2. 



Two half-inch plates of glass are represented in section by 

 the rectangles PQ, RS. Their function is the same as that 

 of the two plates in Jamin's Interference Befractometer *. 



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



