nicol’s single image prism. 
77 
and every other line will appear double, when viewed re- 
spectively in a direction perpendicular to a face of the prism. 
101. Polarization produced by double refraction. If 
one face of a prism of Iceland spar is covered by an opaque 
substance, (as paper or tin foil,) in which a small hole has been 
pierced, and this hole is viewed from the opposite face of the 
prism, held before a beam of light, two illuminated discs will 
be seen. As the spar is revolved before the eye, one image 
{fhe ordinary image) remains stationary, while the other {or 
extraordinary image) appears to revolve around the first. 
Examined by an analyzing plate of tourmaline, both of these 
images are found to be perfectly polarized. Tliis is at once 
evident when the analyzer is rotated in front of the stationary 
prism, the two images alternately disappear and appear again 
at every 90° of the revolution of the tourmaline plate, one 
arriving at its maximum brightness when the other disap- 
pears, and vice versa, the maximum brightness of both images 
being equal. 
If now, in place of the tourmaline, a second prism of Iceland 
spar is used as an analyzer, and is held with its principal sec- 
tion parallel to that of the first prism, both images will still be 
seen, but separated twice as far as when one prism only is used. 
If, now, however, the second prism be revolved 90®, 180°, or 
270®, only one image remains. But in all other positions of 
the second prism, each of the images produced by the first 
prism is doubled, so that four images will be seen at the same 
time. 
The ordinary and extraordinary rays, on issuing from a doubly 
refracting prism, are parallel to each other, and it is clear from 
the preceding observations, that they are polarized in planes at 
right angles to each other. A substance having such proper- 
ties as we find in Iceland spar, must be of great value as 
a means of analysis and polarization of light. 
102. Nicol’s Single linage Prism. This beautiful con- 
trivance, devised by Mr. Nicol of Edinburg, is of the greatest 
value to the microscopist of all known means of polarization 
and analysis, since it furnishes a perfectly colorless pencil of 
polarized light, and when of sufficient size allows of brilliant 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
