THE POLARIMETER 



475 



and emerges as plane polarized light vibrating in a direction perpendicular 

 to the principal plane of the nicoL* 



In the natural rhomb of Iceland spar the angles BAD and BCD are 

 71°. As constructed b}' Nicol, these angles were cut down to 68° so as to 

 obtain such an angle of incidence as to eliminate the ordinary ray. This 

 construction was afterwards altered b\' Nicol himself"* and b}' many phj'sicists. 



Thus, Hartnack and Prazmowski' first suggested that the prism should 

 be sawn from a large crystal of spar, and that to it a rectangular section 

 should be given. Later developments are due to Soleil,® Thompson,* 

 Glan,^", Lippich,^^ Glazebrook^^ and Feurstner,^^ the two last mentioned 

 having given very complete mathematical analyses of the passage of light 

 through the prism. Of the various suggestions, that independently made 

 b}^ Thompson and by Glan and a form due to Lippich are now used. The 

 Thompson-Glan combination consists of a right prism with vertical end 

 faces, so cut that the optic axis is parallel to the plane of section. The 

 Lippich prism is cut so that the optic axis is perpendicular to the axis of 

 length, but has no relation to the plane of section, though usualh' perpendi- 

 cular thereto. Prisms of the above construction are shown in perspective 

 view in Fig. 287. With these prisms a wider pencil of light can be admitted, 

 combined with total extinction of the ordinary ray than can be obtained 

 in the original form ; at the same time loss of light by surface reflection 

 also disappears. 



P A 



Fig. 287 



Fig. 2f 



Passage of Light through two Nicols. — Let there be two nicols, P and A, 

 Fig. 288, with monochromatic light passing in the direction indicated. The 

 prism next the source of light is called the polarizer, and the one that receives 

 the polarized ray is the analyser. Let the prisms be so arranged that their 

 principal planes are parallel. Then the emergent ray of light from P will 

 fall on ^ in a direction parallel to the optic axis, and the extraordinar}^ ray 

 will emerge with its direction substantially unchanged and the quantity of 

 light passmg will be a maximum. Let A now be rotated through a right 

 angle so that its principal plane is perpendicular to that of P. The vibrations 

 of the extraordinary ray now are perpendicular to the optic axis of A, and 

 no light passes, and the eye of an observer looking through A towards P 

 receives the impression of total darkness. These two positions are referred 

 to as parallel and crossed nicols respective^. 



Now between the nicols P and A set as crossed nicols let an optically 

 active material be introduced whereby the plane of rotation of the light 

 emergent from P is rotated. Light will now reach the eye of an observer, 

 and to again obtain the position of total darkness the analyser A must be 

 rotated through an angle equal in magnitude and opposite in sign to that 

 through which the plane was rotated by the optically active material. By 



• It is evident that that part of the prism remote from the face at which light enters could be substituted 

 by a glass prism of suitable refractive index. For prisms constructed of a combination of glass and Iceland spar 

 see Jamin* and Sang..*- 



