ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



459 



Fig. 70. 



Fig. 71. 



section carried througli it is in a plane perpendicular to the principal 

 axis of the crystal. The cementing medium is linseed oil, the index 

 of refraction of which is 1*485. The field of view afforded by this 

 construction depends upon 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 the utmost extent of the field 

 is not required, the prism may be shortened by 

 lessening the angle of the section at the expense 

 however of interfering with the symmetrical disposi- 

 tion of the field. 



6. The Glan Prism * (fig. 71).— This is a modi- 

 fication of the Foucault, and in similar manner 

 includes a film 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 broad. It is subject 

 to the same defect as that mentioned 

 in the case of the Foucault, although perhaps not 

 quite to the same extent, j 

 II. — Feussner's Prism (figs. 72-3). — This prism differs very con- 

 siderably from the preceding forms, and consists of a thin plate of a 

 doubly refracting crystal cemented between two wedge-shaped pieces 

 of glass, the terminal 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 the cementing medium should correspond with the 

 greater index of refraction of the crystal, and the direction of greatest 

 and least elasticity in the latter must stand in a plane perpendicular 

 to the direction of the section. One of the advantages 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 crystalline substances may be 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 ex- 

 traordinary rays in the particular 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 (natron- 

 salpeter), in which the above-mentioned values are w = 1 • 587 and 



* Carl's ' Kepertorinm,' xvi. p. .570 and xvii. p. 195. 



t Amongst others, the modifications of the Nicol prism which have recently 

 been devised by Prof. S. P. Thompson (see this Journal, iii. (1883) p. 575), and 

 by Mr. K. T. Glazebrook (Phil. Mag., 1883, p. 352), do not appear to have been 

 known to Dr. Feussner. 



