POLARIZING APPARATUS. 141 



the artificial tourmalines composed of the disulphate of iodine 

 and quinine, now known by the designation of "Herapathite," 

 after the name of their discoverer. Of these methods, the 

 "Nicol" prism is the one generally preferred ; the objection to 

 the reflecting polarizer being, that it cannot be made to rotate ; 

 the tourmaline being undesirable, on account of the color 

 which it imparts when sufficiently thick to produce an effective 

 polarization ; and the crystals of Herapathite being seldom 

 obtained perfect, of sufficient size to afford a good illumination. 

 The polarizing prism is usually fitted into a tube (Fig. 41, A, a) 

 with a large milled head (c) at the bottom, by which it is made 

 to rotate in a collar (b) that is attached to the microscope ; this 

 collar may be fitted to the under side of the stage-plate, or, 

 w T here a secondary stage is provided, it may be attached to this ; 

 in the microscope of Messrs. Smith and Beck, it screws into the 

 lower part (b) of a tube (Fig. 41, B) that slides into the " cylin- 

 drical fitting" beneath the stage (Fig. 29). The analyzer, which 



FIG. 41. FIG. 42. 



Fitting of Polarizing Prism in Smith and Beck's Fitting of Analyzing Prism 



Microscope. upon the Eye-piece. 



may be either a " Nicol" prism, a Tourmaline, or a crystal of 

 Herapathite, is usually placed either in the interior of the micro- 

 scope, or between the eye-piece and the eye. If it be a prism, 

 it is mounted in a tube, which may either be screwed into the 

 lower end of the body in the situation of the erector (Fig. 32), 

 or may be fitted over the eye-piece in place of its ordinary cap 

 (Fig. 42) ; in the former situation it has the advantage of not 

 limiting the field, but it stops a considerable proportion of the 

 light ; in the latter, it detracts much less from the brightness of 

 the image, but cuts off a good deal of the margin *of the field. 

 A plate of Tourmaline or Herapathite, if obtainable of sufficient 

 size and freedom from color, has a decided advantage above the 

 Nicol prism, as an analyzer, in being free from both these 

 inconveniences ; and it may be set in a cap which fits over the 

 ordinary cap of the eye-piece. For bringing out certain effects 

 of color by the use of Polarized light (Chap. XX), it is desir- 

 able to interpose a plate of Selenite beneath the polarizer and 



