Birefrinyent Action of Strained Glass. 323 



merits : it is represented in the adjacent diagram, in horizontal 

 section through the source of light L and the observer's 

 eye E. 



Fi-. 1. 



A 



IT D 



A and B are two blocks of Iceland spar, of precisely equal 

 thickness, placed with their faces parallel, and principal 

 sections horizontal. A pencil * of lamp-light, through a 

 vertical slit at L, falls first on a Foucault's prism, and is 

 polarized by it in a plane at 45° to the vertical ; it is then 

 doubly refracted in the spar A, and the emergent pencils pass 

 immediately through a half-wave plate P, which is so placed 

 as to interchange the two planes of polarization. Ordinary 

 ray and extraordinary in the crystal A are thus extraordinary 

 and ordinary in the crystal B ; and the birefringent action of 

 A is neutralized by that of B. The light enters A and leaves 

 B as a single pencil ; but in the space between P and B it 

 passes as a couple of pencils V and H, about 14 millimetres 

 apart, and polarized in planes respectively vertical and hori- 

 zontal. The pencil emitted from B is received at E through 

 a NicoFs prism, which is placed as for extinction with the 

 Foucault at L. When the principal sections of A and B are 

 very nearly parallel, the slit L is seen crossed by a set of 

 fringes, which are due to differences of retardation of corre- 

 sponding rays of the pencils V and H. By fine screw- 

 movements of the spar B, the fringes are widened or narrowed 

 at pleasure, and raised or lowered over a fine wire of reference 

 which crosses the slit L. 



At D are shown, in section, the two plates of a Jamin's 

 glass compensator. These are slips of parallel -surfaced glass, 

 of equal thickness, attached to the horizontal axis of a small 

 piece resembling a Wollaston's goniometer, and bearing a 

 finely divided circle with vernier. The plates are inclined 

 to each other at a small and constant angle which is chosen 

 by the observer ; and when either plate is vertical, it is tra- 

 versed normally by the corresponding pencil V or H. When 

 the compensator is turned through any angle, the retardations 

 of the two pencils in the plates D are changed unequally, 

 and the fringes rise or fall definitely over the wire of 

 reference. All the preceding pieces are established on a 

 solid optical bench. 



* Rather a system of pencils ; and similarly afterwards. 

 Y2 



