POLARIZING APPARATUS. 



110 



Fig. 67 



seldom obtained perfect of sufficient size to afford a good illu- 

 mination, and. when perfect are not always to be depended on 

 for permanence. The Polarizing Prism is usually fitted into a 

 tube (Fig. 66, 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, where 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 of a tube (Fig. 66, b) that slides 

 into the 'secondary body' beneath the stage (Plate vn.). The 

 Analyzer, which may be either a 'Nicol' prism, a Tourmaline, or 

 a crystal of Herapathite, is usually placed either in the interior of 

 the microscope, 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 just above the Objective, or may be 

 fitted over the Eye-piece in place of its ordinary cap (Fig. 67) : 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 bright- 

 ness of the image, but cuts off a good deal 

 of the margin of the field. In using the 

 Polarizing apparatus with the Binocular 

 Microscope, the Analyzing prism must be 

 placed between the Wenham prism and 

 the Objective ; and its holder should be so 

 constructed as to allow of being rotated. 

 By combining the Polarizing Apparatus with 

 the Achromatic Condenser, it may be used 

 with very high powers and with very oblique 

 or even black -ground illumination. And 

 when low powers are employed with the 

 Webster Condenser or with a Spot-Lens, a very beautiful effect may 

 be produced in the case of many large semi-transparent objects 

 (such as the horny polyparies of Zoophytes, Fig. 277) by illuminat- 

 ing them on a black ground with Polarized rays reflected upwards 

 from the bundle of thin -glass plates which may be substituted for 

 the mirror, and then viewing them through the Analyzing prism 

 in the usual manner.* 



89. For bringing out certain effects of Colour by the use of 

 Polarized Light (Chap, xx.), it is desirable to interpose a plate of 

 Selenite beneath the polarizer and the object; and it is advan- 

 tageous that this should be made to revolve. A very convenient 

 mode of effecting this is to mount the Selenite plate in a revolving 



* A Polarizer of Herapathite or To urmaline may be used for this 

 purpose instead of the glass-plate polarizer, by mounting it in a cap 

 fitted above the Condenser or Spot-Lens, at such a distance as to receive 

 its converging hollow pencil near its termination in the object. 



Fitting of Analyzing 

 Prism upon the Eye- 

 piece. 



