MICRO-POL AKISCOPE. 
rented to the reflecting surface. If a polarised ray, however, fail 
at a certain inclination upon the same surface, it will he reflected 
or absorbed according to the side of it which is turned towards 
the reflecting surface. Thus, if the side A' B' or c' D' be pre¬ 
sented towards the reflecting surface, the ray will be reflected as 
if it were common light, but if the side b' o' or a'd' be turned 
towards the reflecting surface, it will not be reflected at all, but 
will be, as it were, smothered or extinguished. 
The sides a' b' and c' d', which are opposite to each other, 
have, therefore, a property contrary to that of the sides b' c' and 
A' b', so that they are respectively called the poles of the ray, 
just as the ends of a voltaic circuit having contrary electric pro¬ 
perties are called the positive and negative poles of the voltaic 
battery, and the ends of a magnet are called its boreal and austral, 
•or south and north poles. 
The effects which polarised light produces when it falls upon, 
•or is transmitted through, various substances, more especially 
such as are in the state of crystallisation, are of the highest 
physical importance, being in most cases the indication of mole¬ 
cular and other properties, by which optics has been placed in 
relation with, and has become the handmaid of, almost every 
other branch of physical science. 
71. There are various expedients by which a ray of common 
light can be polarised. It will be polarised if it be reflected at 
a certain inclination, called from that circumstance the angle of 
polarisation, from certain surfaces. Each substance has its own 
angle of polarisation. That of glass, for example, is 35j°. It 
is also polarised if it pass through certain transparent crystals. 
‘Some of these, while they polarise the ray, split it into two, both 
Leing polarised, but in planes at right angles to each other; that 
ns, for example, the sides A' b' and c' D' being white in one, and 
black in the other. 
The well-known mineral called Iceland spar is an example of 
this class of crystals. 
Such crystals are called double-refracting crystals, because the 
two rays into which the ray of common light is split are refracted 
by the crystal in different directions, and according to different 
laws. 
When a polarised ray is transmitted through such a crystal, 
according to certain conditions, it will either pass through it, as 
it would through any ordinary transparent medium, or will be 
extinguished by it, according to the side of the ray to which 
certain faces of the crystal are presented. Such crystal is related 
to the poles of the ray, therefore, in the same manner as the 
reflecting surface already described. 
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