Twin Prisms for Polan 'meters. 399 



through a small angle, thereby attaining a similar result, but 

 admitting of adjustment in the angle. He proposed to use 

 square-ended prisms constructed like those described by the 

 author* in 1881, having the balsam-film in a principal plane 

 of section of the crystal, and having the end-faces also prin- 

 cipal planes of section. A similar arrangement has been used 

 by von Helmholtz. Righif has proposed a combination of 

 two Nicol prisms laid parallel to one another, with two pairs 

 of plates of parallel-worked glass set V-wise to divide, and 

 again reunite side by side the beams traversing the two 

 Nicols. Poynting % has suggested the use of a shallow cell 

 covering the polarized field and containing a sugar solution, 

 but into which a plate of glass is introduced, so as to diminish, 

 over half the field, the thickness of the interposed solution, 

 and so bring about a small angular difference of displacement 

 of the plane of polarization between the two halves of the 

 field. Another suggestion due to Poynting is to cover the 

 two halves of the field with a quartz plate, of any convenient 

 thickness, but reduced uniformly by a small amount, say 

 O'l millim. over one half. Lastly, Pickering § has suggested 

 the use of a double-image prism, arranged like the prism of a 

 dichroiscope, so as to give two contiguous but oppositely- 

 polarized images of a rectangular aperture placed beyond it. 

 These will appear equally illuminated only when the analyzer 

 is at 45 , or when the illumination of each is half the 

 maximum illumination of either. 



In the two forms of twin-prisms that are now exhibited, 

 the angle between the planes of polarization in the two halves 

 of the field of vision is fixed. In the one it is 90°, in the 

 other about 2° 30'. 



In 1886 the author described some forms of simple polari- 

 zing prisms cut for him by Mr. Ahrens on the plan adopted 

 by that clever constructor for the later forms of his triple 

 prisms, the principle of which consists in finding a pair of 

 characteristic planes within the crystal of iceland spar such 

 that they are at right angles with the terminal faces (which 

 are principal planes of section), and so oriented that they 

 pass through the line of intersection made by the terminal 

 faces with the natural faces of cleavage. Prisms made on 

 this plan, which is a mere modification of the plan suggested 



* S. P. Thompson, Hep. Brit. Assoc. 1881, ii. p. 563, and Phil. Mag-. 

 Nov. 1881, p. 349. 



t Repertorium der Physik, xxii. p. 321, 1886. 



X See Glazebrook's Physical Optics, p. 409. 



§ Proc. Anier. Acad. 'May, 1885. See also Zeitschrift fur Instru- 

 mentenkunde, vi. p. 281, 1886. 



