The literature of this subject is scattered and
somewhat difficult of access, and moreover only a small part of it has
hitherto been translated into English; and it would appear therefore
that a brief abstract of the paper may not be without service to those
among the readers of _Nature_ who may be unacquainted with the
original memoirs, or who may not have the necessary references at
hand.
Following the order adopted by Dr. Feussner, the subject may be
divided into two parts:
I.--OLDER FORMS OF POLARIZING PRISMS.
In comparing the various forms of polarizing prisms, the main points
which need attention are--the angular extent of the field of view, the
direction of the 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:
[Illustration: Fig. 1., Fig. 2., and Fig. 3.]
1. _The Nicol Prism_ (_Edin. New Phil. Journal_, 1828, vi., 83).--This
(Fig. 1), as is well known, is constructed 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 deg.
Pages:
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124