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X. — New Polarizing Prism. By C. D. Ahrens. 



CBecid Uth April, 1886.) 



I ONCE more trespass upon the time of the meeting by bringing to 

 the notice of the Fellows a new polarizing prism, and I do so 

 because it has been much commended by some of the leading 

 physicists. 



The prism, although constructed on the same general principle 

 as the Nicol prism, has the great advantage over the latter of being 

 much shorter, while giving about the same angular field. It may 

 be described as consisting of two Nicol prisms placed side by side, 

 the plane of junction between the „ gj 



two being abolished by making 

 the middle portion out of one 

 wedge of calc-spar. The actual 

 mode of construction will be seen 

 from fig. 81. 



A rectangular parallelopipe- 

 don is cut from a natural crystal 

 of calc-spar in such a direction 

 that the optic axis lies at right 

 angles to its length, the propor- 

 tion of the length to the breadth 

 being as 1 : 1 • 8. This block is 

 then divided into three wedges by 

 cuts made in the dkections A B 

 and A C, the acute edges of the 

 wedges being at right angles to 

 the optic axis. The planes of 

 section are next pohshed and 

 cemented together with balsam, 

 so as to make up again the original parallelopipedon. Finally, the 

 ends are polished, the bottom end in the figui-e being carefully 

 ground away until the edge A of the middle prism just appears as 

 a fine hne. 



If, now, a beam of common light enters normally the face B C, 

 that component of it which forms the ordinary ray is, when it 

 reaches the balsam film, totally reflected towards the sides of the 

 prism, while the extraordinary component passes on and emerges 

 as a plane-polarized beam. The same result occurs in the case of 

 all rays incident within a certain range of the normal ; in fact, the 

 prism acts precisely like a Nicol prism, but the totally reflecteJ 

 rays pass ofi" towards both sides of the prism, and not towards one 

 only. 



