March 27, 1884] 



NATURE 



515 



and moi-eover 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 amongst 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 Polarising Prisms 

 In comparing the various forms of polarising prisms, the main 

 points which need attention are :— the angular extent of the field 

 of view, the direction of the emergent polarised 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 difterent degrees by the following 

 methods of construction. 



Fig. I. ''C-^- 



1. The Nicol Prism [Edin. New Phil. Jottmal, 1S2S, 

 vi. 83). — This (Fig. i), 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" which 

 they originally form with the lateral edge of the rhombo- 

 hedron, is reduced to 68°. The jirism 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 uri-m, 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 unchan:jed. This principle of obtain- 

 ing a single polarised 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 polarised rays within the Nicol prism, and finds that 

 the emergent extraordinary r.ry 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 

 polarised 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 reflect! )n 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. 2 to 6 have been devised. 



Fig.5. 



2. Tlu Shortened Nicol /"ww/.— This arrangement of 

 the Nicol prism is constructed by Dr. Steeg and Renter 

 of Homburg v. d. H. For the sake of facility of manu- 

 facture, the end surfaces are cleavage planes, and the 

 oblique cut, instead of being perpendicular, makes with 

 these an angle of about 84°. liy this alteration the prism 

 becomes shorter, and is now only 2 '83 times its breadth ; but if 

 Canada balsam is still u<ed as the cement, the field will occupy 

 a very unsymmetrical position in regard to the lonj axis. If 

 balsam of copaiba is made use of, the index of refraction of 

 whi:h is I -50, a .symmetrical field of about 24° will be obtained. 

 A prism of this kind has also been designed by Prof. B. Hasert 

 of Eisenach (/'c^j. Anii.c\\\\. 189), but its performance appears 

 to be inferior to the above. 



3. The Nicol Prism imth Perpendicular £«*.— The ter- 

 minal surfaces in this prism are perpendicular to the long 

 axis, and the sectional cut makes with them an angle of about 

 75°. The length of the prism is 375 times its breadth, and if 

 the cement has an index of refraction of i '525, the field is 

 symmetrically disposed, and includes an angle of 27°. Prisms 

 of this kind have been manufactured by Dr. tJteeg, by Mr. C. D. 

 Ahrens, and others. 



4. The FaucauU Prism {Comptes Rendus, 1857, xlv. 238).— 

 This cons ruction differs from all those hitherto mentioned, in 

 that a film of air is employed between the tu-o cut surfaces as 

 the totally reflecting medium instead of a layer of cement. The 

 two halves of the prism are kept in position, without touching 

 each other, by means of the mounting. The length of the prism 

 is in this way much reduced, and amounts to only i'528 times 

 its breadth. The end surfaces are cleavage planes, and the 

 sectional cut makes with them an angle of 59°. The field, how- 

 ever, includes not more than about 8°, so that this prism can be 

 used only in the case of nearly parallel rays ; and in addition 

 to this the pictures which may be seen through it are to some 

 extent veiled and indistinct, owing to repeated internal re- 

 flection. 



5. The Harinack Prism {Ann. de Ch. et de Physique, ser. iv. 

 vii. 181). — This form of prism was devised in 1866 by MM. 

 Hartnack and Prazmowski ; the original memoir is a valuable 

 one; a translation of it, with some addition;, has lately been 

 published (Journ. of the R. Microscopical Soc, June, 18S3, 428). 

 It is considered by Dr. Feussner to be the most perfect prism 

 capable of being prepared from calcspar. The ends of the 

 prism are perpendicular to its length ; the section carried through 

 it is in a plane per|iendicular to the principal a\is of the crystal. 

 The cementing medium is linseed oil, the index of refraction of 

 which is I "485. This form of prism is certainly not so well 

 known in this country as it deserves to be : a very excellent one 

 supplied to the present writer by Dr. Steeg is of rectangular 

 form throughout, the terminal surfaces are 19 x 15 mm., and 

 the length 41 mm. The lateral shifting of the field is scarcely 

 perceptible, the prism is perfectly colourless and transparent, 

 and its performance \- fir superior to that of the ordinary Nicol. 

 The field of view afforded by this construction depends upon 



