ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
247 
I have had many letters from microscopists since that time asking about 
the appliance, and I have been strongly urged lately to present the 
matter through the * American Monthly Microscopical Journal.’ 
This apparatus consists of a box A, of the form shown in the side 
view, fig. 25, 1, made of either metal or wood, and containing a plate of 
Fig 25. 
polished black glass B. At the lower part of this box is a short tube C, 
which fits into the draw-tube of the Microscope, and at the opposite 
angle of the box is another short tube D, which receives the eye-piece. 
The glass plate is used for the purpose of reflecting the beams of 
polarized light at the best analysing augle. It will be necessary, of 
course, to use some form of polarizer below the object upon the stage 
of the Microscope, and the best is the Nicol’s prism. 
The line E represents a ray of light which has been reflected by the 
concave mirror through the Nicol’s prism and objective, and is reflected 
by the polished surface of the glass B through the axis of the eye-piece, 
as shown by the line F. C represents the eye-piece. 
The exact angle of inclination of the polished surface of the glass to 
the line E, which represents the axis of the Microscope, is very important. 
This angle should be 146°, which will cause the reflected beam F to form 
an angle of 112° with the line E, which is the correct angle for a reflector 
of polished German plate glass, now to be described. 
If a piece of black glass cannot bo obtained, procure a piece of per- 
fectly polished German plate looking-glass 2J in. long and 1^ in. wide. 
Scrape off the silver surface and thoroughly clean. Paint the cleaned 
surface quite heavily with black paint. Plate glass of a dark-green 
colour when examined edgeways is best. 
A diaphragm 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 analysing 
arrangement instead of the Nicol’s prism analyser placed above the 
