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CHAPTER XXIV 



long edges should be of length about two inches, or a little more, is cut by 

 two planes perpendicular to those edges, so as to form a right prism as in 

 Fig. 291. This prism is divided by a plane parallel to those edges, and making 

 a small angle with the longer diagonal of the base. One of the two parts 

 into which the prism is divided is then reversed, so as to place the base up- 

 wards, and the two parts are cemented together as in Fig. 292, with the sur- 

 faces of section in contact and the ends of the prism thus formed are then 

 ground and polished." 



Cornu Half-Shadow Device.^^ — Cornu applied Jellett's principle thus. An 

 ordinary nicol prism is divided into two parts following the plane of the 

 lesser diagonals. Each face of cleavage is then ground down 2^ degrees, 

 after which the two parts are cemented together. A prism with a half- 

 shadow angle of 5 degrees is thus obtained. 



Fig. 291 



Fig. 292 



Schmidt and Haensch Prisin.^^ — The German firm of Schmidt and Haensch 

 have employed a prism made thus. The prism of calc spar is divided into 

 two parts by a plane perpendicular to the principal section. One half only 

 is then treated as in Cornu's method, after which the three pieces are united 

 and arranged so that the incident light falls on the undivided half. 



The Laurent Half Shadoiv Device.^^ — ^The Laurent half shadow polariscope 

 obtains its end point in a manner quite different from the instrument described 

 above. Between the polarizing and analysing nicol of ordinary construction, 

 and close to the former, is interposed a thin plate of active quartz, which is 

 cut parallel to the optical axis of the crystal. A beam of light entering such 

 a plate perpendicular to its surface is doubly refracted into two beams, with 

 vibration planes parallel, and perpendicular to the optical axis. In such a 

 system that ray which vibrates perpendicular to the optical axis has its 

 speed of vibration increased, and the thickness of the plate of quartz is so 

 taken that that ray vibrating perpendicular to the optical axis has gained 

 half a wave length on the wave vibrating parallel to the optical axis at the 

 moment they emerge from the quartz plate. In Fig. 29^ let the circle 



