THEORY OF POLARIZED LIGHT. 
79 
ray diverges very widely from the axis of the prism. The 
extraordinary ray has a refractive power so low that it is not 
reflected by the balsam, unless it is very nearly parallel with 
its surface, but passes freely through it into the lower part of 
the prism and emerges in the direction g A, parallel to the 
incident ray.* 
Nicol’s prisms are capable of transmitting pencils of light, 
perfectly polarized, varying from 20° to 27'^. The prices of 
these prisms vary with the size and purity of the crystals. 
103 . Common and Polarized liiglit Contrasted. 
A Ray of Common Lights 
1. Is capable of reflection at oblique 
angles of incidence, in every position of 
the reflector. 
2. Penetrates a plate of tourmaline 
(cut parallel to the axis of the erystal) 
in every position of the plate. 
3. Penetrates a bundle of parallel glass 
plates, in every position of the bundle. 
4. Suffers double refraction by Iceland 
spar, in every direction except that 
parallel to the major axis of the crystal. 
A Ray of Polarized Lights 
1. Is capable of reflection at oblique 
angles of incidence, in certain positions 
only of the reflector. 
2. Penetrates a plate of tourmaline 
(cut parallel to the axis of the crystal) 
in certain positions of the plate, but in 
others it is wholly intercepted. 
3. Penetrates a bundle of parallel 
glass plates in certain positions of the 
bundle, but not in others. 
4. Does not suffer double refraction 
by Iceland spar in every direction, not 
parallel to the major axis of the crystal. 
In some other positions it suffers single 
refraction only. 
THEORY OF POLARIZED LIGHT. 
lOT. CiidHlatory Theory. In attempting to account for 
the phenomena of polarized light, the most satisfactory expla- 
nations are furnished by the undulatory theory of light, first 
proposed by Huyghens and more fully investigated by Dr. 
Thomas Young. 
According to this theory, the particles of luminous bodies 
* The refractive index of Iceland spar, for the extraordinary ray, varies by a 
somewhat complicated law, ranging from 1.483 to 1.654, but for such pencils of 
light as can be transmitted by Nicol’s prism, it varies only between 1.5 and 1.56. 
The extraordinary ray in this prism never suffers total reflection by the balsam, 
unless it approaches within about 10° of its surface. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
