ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



457 



Fig. 66. 



emergent polarized ray, whether it is shifted to one side of or remains 

 symmetrical to the long axis of the prism ; the proportion which the 

 length of the prism bears to its breadth ; and, lastly, the position of 

 the terminal faces, whether perpendicular or inclined to the long axis. 

 These requirements are fulfilled in different degrees by the following 

 methods of construction. 



1. The Nicol Prism.* — This (fig. 66), as is well known, is con- 

 structed from a rhombohedron of Iceland spar, the length of which 

 must be fully three times as great as the width. 

 The end faces are cut off in such a manner that 

 the angle of 72° which they originally form with 

 the lateral edge of the rhombohedron, is reduced 

 to 68°. The prism is then cut in two in a plane 

 perpendicular to the new end surfaces, the section 

 being carried obliquely from one obtuse corner 

 of the prism to the other, in the direction of its 

 length. The surfaces of this section, after having 

 been carefully polished, are cemented together again 

 by means of Canada balsam. A ray of light, on 

 entering the prism, is separated by the double re- 

 fraction of the calc-spar into an ordinary and an 

 extraordinary ray: the former undergoes total 

 reflection at the layer of balsam at an incidence 

 which allows the extraordinary ray to be trans- 

 mitted; the latter, therefore, passes through un- 

 changed. This principle of obtaining a single 

 polarized ray by means of total reflection of the 

 other is common to all the forms of prism now to 

 be described. 



Dr. Feussner gives a mathematical analysis of the paths taken by 

 the two polarized rays within the Nicol prism, and finds that the 

 emergent extraordinary ray can include an angular field of 29°, but 

 that this extreme value holds good only for rays incident upon that 

 portion of the end surface which is near to the obtuse corner, and 

 that from thence it gradually decreases until the field includes an 

 angle of only about half the previous amount. He finds, moreover, 

 that, although of course the ray emerges parallel to its direction of 

 incidence, yet that the zone of polarized light is shifted to one side 

 of the central line. Also that the great length of the Nicol — 3*28 

 times its breadth — is not only an inconvenience, but, owing to the 

 large pieces of spar thus required for its construction, prisms of any 

 but small size become very expensive. To this it may be added 

 that there is a considerable loss of light by reflection from the first 

 surface, owing to its inclined position in regard to the long axis of 

 the prism. 



It is with the view of obviating these defects that the modifica- 

 tions represented in figs. 67 to 71 have been devised. 



* Edin. New Phil. Journal, vi. (1828) p. 83. 



