1892.] MICROSCOPICAL JOURNAL. 261 



glass to the line R^ which represents the axis of the microscope, 

 is very important. 



This angle should be 146 degrees, which will cause the reflected 

 beam ^to form an angle of ii3 degrees with the line E^ whicli is 

 the correct angle for a reflector of polished German plate-glass, 

 now to be described. 



If a piece of black glass cannot be obtained, procure a piece 01 

 perfectly polished German plate looking-glass 2g inches long 

 and 1:1^ inches wide. Scrape off the silvered surface and thoroughly 

 clean. Paint the cleaned surface quite heavily with black paint. 

 Plate-glass of a dark green color, when examined edgewise, is 

 best. 



A diaphragm, Z, with opening about the diameter of the field 

 lens of the eye-piece should be placed at the lower end of tube C 



It is hardly necessary to state that this piece of apparatus is 

 used as an analyzing arrangement instead of the Nicols prism 

 analyzer placed above the objective. The following are some of 

 the valuable features of this arrangement : 



It allows the entii'e angular aperture of all objectives to be used 

 which is not the case when using the Nicols analyzer and large- 

 angle low-power objectives. The stage can be kept in a horizon- 

 tal position in chemical experiments and in the examination of 

 fluids, and the line of vision for the worker is the very convenient 

 one shown at F. The image of very delicate objects is free from 

 distortions, which is rarely the case when using a Nicols analyzer. 



The analyzing eye-piece can be revolved in the draw-tube of 

 the microscope by means of the tube C, giving the usual effects of 

 a revolving analyzer. 



It is well to use a hemispherical lens of about % of an inch 

 diameter above the selenite film and polarizing prism, with the 

 convex side of the lens toward the object upon the stage, and the 

 upper part of this convex surface about \ of an inch from the ob- 

 ject. 



A modification of the revolving mica film, which I described 

 in " The Omaha Clinic," to be used with the Nicols analyzer, is 

 of special value used with the above-described e3'e-piece arrange- 

 ment, and it is represented in figs. 2 and 3. 



The apparatus consists of a plate of mica placed between the 

 analyzing plate and the object upon the stage of the microscope, 

 in such a manner that rotation can be given to it, and it can be 

 instantly removed if desired. 



Let fig. 3 represent a side sectional view of the apparatus. C 

 is an adapter carrying the rotating mica plate. E is the objec- 

 tive screwed into the lower part of the adapter. A^is the screw 

 for the body tube of the microscope. The mica film B is to be 

 cemented between two plates of perfectly polished glass, F^ of 

 suflficient thickness to prevent distortion of image. This disc is 

 to be fitted in the ring A^ to one side of which is screwed a small 

 handle, D^ to be used in giving rotation to the plates B^ E. A 



