5i6 



NATURE 



\_March 27, 1884 



the cementing; substance used, and also upon the inclination of 

 the sectional cut in regard to the ends of the prism ; it may vary 

 from 20" to 41". If tlie utmost extent of field is not required, 

 the prism may be shortened l)y lessening the angle of the section, 

 at the expense however of interfering with the symmetrical 

 disposition of the field. 



6. The Glan Prism (Carl's " Repertorium," xvi. 570, and 

 xvii. 19s). — This is a modification of tlie Foucault, and in a 

 similar manner includes a fihn of air between the sectional 

 surfaces. The end surfaces and also the cut carried through 

 the prism are parallel to the principal axis of the calc spar. 

 The ends are normal to the length, and the field includes 

 about 8°. This prism is very short, and may indeed be even 

 shorter than it is liroad. It is subject to the -ame defect as that 

 mentioned in the case of the Foucault, although perhaps not 

 quite to the same extent. 



II. — The New Polarising Prism 

 This prism differs very considerably from the preceding forms, 

 and consists of a thin plate of a doubly refracting crystal 

 cemented between two wedge-shaped pieces of glass, the ter- 

 minal faces of which are normal to the length. The external 

 form of the prism may thus be similar to the Hartnack, the 

 calc-spar being replaced by glass. The indices of refraction of 

 the glass and of tlie cementing medium .should correspond with 

 the greater index of refraction of the crystal, and the directions 

 of greatest and least elasticity in the latter must stand in a plane 

 perpendicular to the direction of the section. One of the advan- 

 tages claimed for the new prism is that it dispenses with the 

 large and valuable pieces of spar hitherto found necessary ; a 

 further advantage being that other crystalMne .substances may 

 lie used in this prism instead of calc-spar. The latter advantage, 

 however, occurs only when the difference between the indices of 

 refraction for the ordinary and extraordinary rays in the par- 

 ticular crystal made use of is greater than in calc-spar. When 

 this is the case, the field becomes enlarged, and the length of the 

 prism is reduced. 



The substance which Dr. Feussner has employed as being 

 most suitable for the separating crystal plate is nitrate of soda 

 (natronsalpeler), in which the above-mentioned values are w = 

 I "587 and € = I '336. It crystallises in similar form to ealcite, 

 and in both cases thin plates obtained by cleavage may be used. 

 As the cementing substance for the nitrate of soda, a mixture 

 of gum dammar with monobromonaphthalene was used, which 

 afforded an index of refraction of I '58. In the case of thin plates 

 of calcite, a solid cementing substance of sufficiently high refrac- 

 tive power was not available, and a fluid medium was therefore 

 employed. For this purpose the whole pri^m was inclosed in a 

 short glass tube with air-tight ends, which was filled with mono- 

 bromonaphthalene. In an experimental prism a mixture of 

 balsam of tola was made use of, giving a cement with an index 



Fig.7. 



Fig.O. 



of refraction of i'62, but the low refractive power resulted in a 

 very considerable reduction of the field. The extent and dis- 

 position of the field may be varied by altering the 'nclination at 

 which the crystal lamina is inserted (Fig. 7), and thereby re- 

 ducing the length of the prism, as in the case of the Hartnack. 



In order to obviate the effects of reflection from the internal 

 side surfaces of the prism, the wedge-shaped blocks of glass of 

 which it is built up may be made much broader than would 



otherwise be necessary; the edges of this extra width are cut 

 obliquely, and suitably blackened. 



The accompanying diagram (Fig. 8) represents a prism of 

 cylindrical external form constructed in this manner, the lower 

 surface being that of the incident light. In this the field amounts 

 to 30°, and the breadth is aliout double the length. 



Dr. Feu-sner remarks that a prism similar in some respects to 

 his new arrangement was devised in l86g by M. Jamin {Comptes 

 Kcndus, Ixviii. 221), who nsed a thin plate of calc-spar inclosed 

 in a cell filled with bisulphide of carbon ; and also by Dr. 

 Zenker, v/ho replaced the liquid in M. Jamin's construction by 

 wedges of flint glas^. 



Amongst other.s, the carefully considered modifications of the 

 Nicol prism which have recently lieen devised by Prof. S. P. 

 Thompson {Phil. Mag., November, 1881, 349 ; and jfourn. M. 

 Micros. Soc, August, 1883, 575), and by Mr. R. T. Glazebrook 

 (Phil. Mag., May, 1883, 352), do not appear to have been known 

 to Dr. Feu'Sner. 



The following tabular view of different forms of polarising 

 prisms is taken from the conclusion of Dr. Feussner's paper: — 



As an analysing prism of about 6 mm. clear width, and 

 I3'5 mm. long, the new prism is stated by its inventor to be of 

 the most essential service, and it would certainly appear that the 

 arrangement is rather better adapted for small prisms than for 

 those of considerable size. Any means by which a beam of 

 polarised light of large diameter — say 3 to 3^ inches — could lie 

 obtained with all the convenience of a Nicol would be a real 

 advance, for spar of sufficient size and purity for such a purpose 

 has become so scarce and therefore so valuable that large prisms 

 are difficult to procure at all. So far as an analyser is con- 

 cerned, the experience of the writer of this notice would lead to 

 the opinion that improvements are to be looked for rather in the 

 way of the discovery of an artificial ci-ystal which absorbs one of 

 the polarised rays than by further modifications depending upon 

 total reflection. The researches of Dr. Herapath on iodo- 

 sulphate of quinine {Phil. Mag., March, 1S52, 161, and No- 

 vember, 1853, 346) are in this direction ; but crystals of the 

 co-called heripathite require great manipulative skill for their 

 production. If these could be readily obtained of sufficient 

 size, they would be invaluable as analysers. 



Thi^ opinion is supported by the existence of an inconvenience 

 which attends every form of analysing prism. It is frequently, 

 and especially in projecting apparatus, required to be placed at 



