74 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
plate of tourmaline, presents the same phenomena as a ray 
originally polarized by passing through a tourmaline plate. 
From its great convenience a plate of tourmaline is commonly 
used as a test of polarized light, and when so used it is called 
an analyzer. 
On account of the difficulty of obtaining plates of tourma- 
line of sufficient size and freedom from color, other apparatus 
have been devised for the same purpose. 
98. Polarization by refraction tliroiigli numerous 
plates of transparent media. If light is transmitted ob- 
liquely through a bundle of thin transparent plates, it is polar- 
ized. Plates of the very thin glass used for covering micro- 
scropic objects, form the best polarizer of this kind. Sixteen 
or more of these plates, quite clean, are to be placed close 
together, and the bundle or package then fixed at an angle of 
56° 45', to the ray to be polarized. Common window glass, or 
plates of mica, may be used in the same manner, but the 
polarization is not as complete as that produced by the action 
of the thin glass above referred to. 
Light, which has been transmitted through such a system of 
thin plates, is almost entirely polarized.'^* It may be reflected 
from polished surfaces in certain positions, but not in others ; it 
may also be transmitted through an analyzer in one position, 
but it is wholly intercepted when the analyzer is rotated 90°. 
99. Double Refraetiou. It is well known that when light 
passes obliquely through w^ater and other fluids, glass of uni- 
form density, common salt and all cubical crystals, or crystals 
w’hose form is derived from the cube, (called monometric crys- 
tals,) the light is bent out of its course, but the object from 
which the light proceeds appears single, and in its true propor- 
tions if the transparent medium is bounded by parallel surfa- 
ces. Tliis is called single refraction. Long before tlie discov- 
ery of polarized light, it was known that certain bodies, of 
which the most noted is Iceland spar, give two images of all 
objects seen through them in certain directions. It is now 
* The light which remains unpolarized by this method, can only be appreciated 
by the most delicate analysis. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
