In a recent number of the _Zeitschrift fur Instrumentenkunde_ (iv., 42-50, February, 1884), Dr. K. Feussner of Karlsruhe has given a detailed description of a polarizing prism lately devised by him, which presents several points of novelty, and for which certain advantages are claimed. The paper also contains an account, although not an exhaustive one, of the various polarizing prisms which have from time to time been constructed by means of different combinations of Iceland spar. 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. which they originally form with the lateral edge of the rhombohedron is reduced to 68 deg.. 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 refraction 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 transmitted; the latter, therefore, passes through unchanged. 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.