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 (6) that is attached 'to the microscope ; this 

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

 where a seeondary stage is provided, it may be attached to this ; 

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

 lower part (6) 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. 



Fining of Polarizing Prism in Smitli and Beclt's 

 Microscope. 



Fitting of Analyzing Prism 

 upon the Eye-piece. 



may be either a " Nicol" prism, a Tourmahne, 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 



