( ^17 ) 



Fig. 71. 



X. — Note on Polarizing! Apparatus for the Microseope. 

 By Professor Silvanus P. Thompson, D.Sc. 



(Read I2th June, 1889.) 



A FEW montlis ago the writer had adapted to his Microscope (a Beck's 

 " Pathological ") one of Ahrens's triple polarizing prisms * which he 

 had for some eighteen months been using for other purposes. This 

 prism has an angular aperture of some 28°, far exceeding any ordinary 

 Nicol's prism. The prism is so short, in comparison with its area of 

 cross-section, that it appeared particularly convenient for the purpose 

 of a substage polarizer, provided the line of junction across the end- 

 face of the prism did not interfere with the optical performance. 

 The prism when removed from its mountings is a rectangular 

 parallelepiped, having square end-faces. The side of the square end- 

 face is 17 • 5 millimetres, and the length of the prism is but 27 milli- 

 metres. Mounted below a wide-angled Abbe-Beck condenser, as in 

 fig. 71, with an iris diaphragm between, it gives most satisfactory 

 results. The line of junction across the 

 upper end-face is barely to be detected, and 

 gives no trouble in use. 



As analyser several prisms have been 

 tried, the most satisfactory being a small 

 prism lately cut for me by Mr. Ahrens, for 

 the express purpose of use over an eye-piece. 

 The following considerations determined its 

 construction. It is obvious that it is of no 

 use to make the eye-end of the analysing 

 prism of greater diametral aperture than 

 that of the pupil of the eye. Any larger 

 aperture than this is not merely wasted, for 

 the prism that has a larger end-face than 

 necessary means a prism that is longer than 

 necessary, and obliges the observer's eye to 

 be further removed from the eye-piece. In 

 Nicol prisms, as ordinarily made, the two 

 end-faces are of equal size, and the lateral 

 faces are parallel. For analysing purposes, 

 however, the prism must have at the end 



farthest from the eye an aperture greater than that of the pupil, 

 otherwise the eye cannot receive the proper cone of rays. Further, 

 the oblique end-faces habitual in the Nicol prisms of ordinary con- 

 struction are objectionable, as they waste light by reflection, and take 

 up valuable space. The prism which Mr. Ahrens has cut for me is 

 represented in fig. 72. It is about 11 millimetres long, narrower at 

 the end nearest the eye, and has end-faces square to its axis. These 



1889. 



* Phil. Mag., 1886, p. 476. 



2 X 



