THE MICROSCOPE. 
72. If either the reflecting surface or the crystal, placed under 
the necessary conditions, be carried round a polarised ray, a' b' c'd', 
so as to he successively presented to all sides of it, the ray will 
he completely reflected or transmitted when it is presented to a\ 
the middle of the side A' b'. As it is moved from a ' towards b \ 
the quantity of light reflected or transmitted will he less and less, 
until it comes to b\ when none will he reflected or transmitted, the 
ray being wholly extinguished. As it is moved from V to c', 
the light reflected or transmitted, small in quantity at first, will 
he continually greater and greater until it comes to & the middle 
of c' D', when the ray will he wholly reflected or transmitted. 
As it is moved from c' towards d', the quantity of light reflected or 
transmitted is less and less, until arriving at d ' the ray is alto¬ 
gether extinguished. After passing from d towards a the light 
reflected, at first small, is more and more in quantity until it 
comes in fine to a', when the ray is, as at first, wholly reflected or 
transmitted. 
73. An instrument adapted to show the effects of polarised light 
upon bodies oh which it is incident or through which it is trans¬ 
mitted, is called a polariscope, fig. 35, p. 65, and a polarising 
microscope or micro-polariscope, is a microscope by which the 
observer is enabled to project polarised light upon the objects, and 
to observe its effects when transmitted or reflected by them. 
Micro-polariscopes have been constructed in various forms, 
some depending on polarisation by reflection, and some on polari¬ 
sation by transmission. 
One of the most simple and most generally useful, consists of 
two prisms of Iceland-spar, one of which, p, is placed under the 
stage, so that the light by which the object is illuminated must 
previously pass through it, and the other p' is placed in the body 
of the instrument between the object-glass and the eye-glass, so 
that before producing the image, the rays must pass through it. 
The light proceeding from p, and projected upon the object, 
being polarised, and received, after passing through the object- 
glass, by p', will be wholly or partially transmitted, or altogether 
extinguished, according to the sides or poles of the ray to which 
certain faces of the prism are presented. If, therefore, the instru¬ 
ment be so mounted that the prism p' can be turned round its 
axis, its faces can be presented successively to all sides of the 
rays, so that the light will be in a certain position wholly trans¬ 
mitted, and the image will be seen strongly illuminated. When 
the prism is gradually turned round, the light transmitted will 
be less and less, until the prism has been turned through a 
quarter of a revolution, when the light will be wholly 
extinguished, and the image will disappear. Continuing to turn 
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