PARTIAL POLARIZATION. 
83 
such a position that its vibrations are oblique to the polarizing 
planes or axes of the medium, they will be resolved into vibra- 
tions in hoth those axes or polarizing planes, and two new 
polarized rays will result, each of which might be again sub- 
divided in the same manner by another polarizing prism placed 
in a position oblique to the new axes. 
108. Familiar illustrations. A ray of common light is 
sometimes compared to a cylindrical rod, whereas the polarized 
rays are like two flat parallel rulers, one of which is laid hori- 
zontally on its broad suface, and the other horizontally on its 
edge. The alternate transmission and obstruction of one of the 
flattened beams, by the tourmaline, is similar to the facility 
with which a card, or flat ruler, may be passed between the 
wires of a cage if presented edgewise, and the impossibility of 
its passing in a transverse position. 
We may also suppose a refracting substance, with a reflect- 
ing surface, made up of parallel fibres. Such a surface would 
allow the passage of all the rays in common light which vibrate 
in a plane parallel to the direction of its fibres and would 
reflect the rest ; while polarized light, if vibrating in a plane 
parallel to the fibres, would be wholly transmitted, but if vibra- 
ting in a plane at right angles to the fibres, it would be wholly 
reflected. 
109. Partial Polarization. Having already explained in 
the previous section how light is polarized, 1st, by reflection ; 
2nd, by refraction ; 3rd, by transmission through bundles of 
thin plates ; and 4th, by double refraction ; it now remains to 
state that a great variety of substances, and in different condi- 
tions, ^xodiWQQ partial polarization of light reflected from their 
surfaces or transmitted through them. 
It is well known that no substance either transmits^ or reflects^ 
all the light that falls upon it ; even the most transparent sub- 
stances reflect a small portion of light, the proportion reflected 
and transmitted varying with every angle of incidence. While 
plate glass polarizes very nearly all the light that falls upon it 
at an angle of 57°, the intensity of the reflected ray is equal to 
only one-half the intensity of the incident ray, the other half 
of the light is transmitted through the glass, and is, at the 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
