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The other prism must fit under the stage. If a concave reflect- 
ing mirror, and no condenser is employed, the prism should be 
as close under the slide as possible. If an achromatic condenser 
and plane mirror are used, the lower prism should be between 
the condenser aod the mirror. This prism must be capable of 
rotation round its axis. 
We may, as polarizer, instead of a Nicols’ prism, employ a 
glass plate, blackened behind and fixed at the back of the 
silvered mirror ; or a bundle of glass plates. If this arrange- 
ment is adopted, a mark should be made, so that the reflector 
can at once be fixed at the polarizing angle. The plane of the 
reflecting plate should make an angle of 33;^° with the optic 
axis of microscope, and then the inclination of the whole in- 
strument, or the position of the lamp, can be altered until the 
object on the slide is properly illuminated. 
If the last plan is adopted, the analyzing prism must be in 
the eyepiece, so that it can be conveniently turned round on 
its axis. Each of these plans possesses certain advantages. Com- 
pletely to examine a small crystal, it is necessary to have the 
means of rotating both the analyzing and polarizing apparatus. 
To do this two Nicols’ prisms may be used, one fitted over the 
eyepiece, the other under the stage. The objection to a prism 
over the eyepiece is, that it limits the field of view, but at the 
same time it does not cut off much light. Another plan is to 
have a plate of tourmaline mounted in a piece of tube that 
will fit the mounting of the eyepiece, and replace its ordinary 
cap. Tourmaline, however, has the disadvantage of not being 
colourless as is Icelapd spar. Herapathite, so called from its 
discoverer. Dr. Bird Herapath, is a better polarizing medium 
than tourmaline. Much thinner laminse may be employed, and 
these give no colour to the object. It is difficult to obtain large 
crystals of herapathite. 
In examining those substances by polarized light under the 
microscope, which do not of themselves produce colour, it is 
necessary to place a plate of some doubly refracting crystal over 
or under the object. Silenite is the best. Plates of silenite 
may be mounted in a brass ring, fitting a circular aperture in a 
plate of brass made like an ordinary glass slide. The ring 
should be capable of removal, and of revolving in the brass 
plate by means of a tangent screw. It is easy to select a plate 
of selenite which will give good colours. Place an ordinary 
microscope slide on a piece of black paper on a table in front of 
a lamp, the distance of the slide from the lamp being half as 
great again as the height of the lamp. Then split off a lamina 
with a thin-bladed knife from the crystal of selenite, and hold 
it between the prism and the glass, the prism being held in such 
a position that the glass slide appears black. Turn the selenite 
