ZOOLOGY, BOTANY AND MICROSCOPY, ETC. 373 



waste by one-half in the case of large prisms. This mechanical method, 

 however, is not adapted for the smaller prisms. Now a study of his 

 tables shows that the prisms, as used, differ considerably as regard their 

 field of view (opening) and their polarisation-field. Whilst prisms with 

 larger opening usually have a narrowly limited polarisation-field, those 

 with smaller opening, in consequence of their large polarising angle, 

 attain a proportionally large field. It is therefore necessary in selecting 

 a prism to keep one's requirements carefully in view. For an analyser 

 (eye-Nicol) a prism with large polarising angle, and consequently large 

 opening, is desirable ; whereas for a polariser a large beam with few 

 converging rays would be usually recommended. The author describes 

 and figures an apparatus by which the polarisation angle may be 

 measured. The prism A to be investigated is securely mounted on a 

 circular table B, whose centre is C. The analyser is placed on a segment 

 D, which is concentric with C, and rotates round it. The plane of B 

 extends slightly beyond the segment, and its circumference is graduated. 

 The first prism is so placed that a narrow face is at C, and both prisms 

 are so arranged that their extraordinary rays are in the same plane and 

 at the zero of the scale. Illumination (lamp or daylight) reaches the 

 remoter end of the polariser. The analyser and polariser are now inter- 

 changed and the segment rotated leftwise, until a point is reached at 

 which no light passes through to the eye. This is the limit of the 

 polarisation field on the one side, and, in the case of Nicols with 

 inclined end-planes, is marked by a bluish tint. The segment is now 

 brought back to the zero point, and the analyser rotated 90° about its 

 long axis. The field of view is now quite black ; but the segment is 

 rotated rightwise until the blackness disappears, thus marking the 

 other limit. The angle subtended at C by these limits is the value of 

 the required angle of the polarisation field. If the limits are equally 

 distant from the zero the polarisation field is symmetrical. The author 

 gives the following values of the polarisation angle, the field being 

 symmetrical unless otherwise stated : — 



(The reference letters a-l relate to details of construction.) 



The last thr ee are thus only applicable for parallel light, the others 

 being also use ful for more or less convergent light. The forms h, e, d, b 

 are especially suitable for analysers on account of their small cross- 

 section and la rge polarisation angle ; the others serve better as polarisers. 

 The two Ahre ns' prisms seem capable of great reduction in size. The 

 different form s of the Glan-Thompson show that a large opening com- 

 bined with red uced polarisation field is most economically attained by 

 altering the an gle of the prism ; such a change is, however, possible only 

 in prisms with balsam or linseed-oil cement. 



