Dr. J. Kerr. Experiments on a [Mar. 15, 



here used is known as Jamin'a Interference Refractor for polarised 

 light. For a description of it I might refer to a paper already pub- 

 lished ;* but I think I ought rather to describe the apparatus here 

 again. The essential pieces are shown in horizontal section in the 

 following diagram. 



r . , * . * 



rig.. . 



B and 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 B. Tbe two pencils emergent from B pass immediately 

 through a half-wave plate P, so placed as to interchange the two 

 planes of polarisation. Ordinary pencil and extra-ordinary in the 

 crystal B become thus extra- ordinary and ordinary in S, and the bi-re- 

 fringement action of B is neutralised by that of 8. The light enters 

 B and leaves S an a single pencil, but between P and S it passes as 

 a couple of pencils, KF and CG, about 14 mm. apart, and polarised 

 in planes vertical and horizontal. The pencil emergent from S is re- 

 ceived 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, 

 H n i.l these are modified at pleasure by fine screw movements of the 

 spar 8. 



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 

 Jarnin's Glass Compensator, t which is placed immediately in front of 

 the spar 8 ; it enables the observer to specify small differences of re- 

 tardation of tihe pencils BF and CG. 



The results obtained formerly (with nicol N as eye-piece) were 

 fully verified with the new apparatus. Tbe 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 tke results I will describe one day's work. 



" On the Bi-refringent Action of Strained Glass," ' Phil. Mag.' for October, 

 1888. 

 t Preston's ' Theory of Light,' p. 159. 



