DELICATE STRUCTURES, VIEWED BY COLORED POLARIZED LIGHT. 91 
v/hich supports the selenite, another cylinder, carrying the 
Nicol’s prism P, is separately revolved by turning the milled 
b. The Nicol’s prism revolves 
more easily than the selenite 
carrier, so that the latter may 
remain stationary when the 
former is revolved. By turn- 
ing the milled head c, the 
polarizer and selenite are re- 
volved together. This has the 
same effect upon the light as if 
the analyzer were revolved. 
By means of this apparatus, 
the axes of the polarizing prism, 
selenite plate, and analyzer^ 
may be placed in any relative 
position desired, affording great facility for using polarized 
light colored of any tint required in microscopic investigations. 
We are happy to acknowledge our obligations to Dr. White 
of 'New Haven, for suggesting this improved arrangement for 
revolving the plate of selenite. 
119. Delicate structures, viewed by colored polarized 
light, produce much more sensible changes upon the light than 
upon plain polarized lights — microscopic crystals are especially 
beautiful when viewed in this manner. The crystallization of 
various salts, viewed by polarized light, is a subject of great 
importance to the practical chemist and mineralogist. So 
minute a quantity as one thirteen millionth of a grain of 
potassa, when tested with bichloride of platinum, gives a dis- 
tinct and characteristic tint, sufficient to distinguish it from 
every other alkali, when viewed in the microscope by this 
kind of light. Many substances, known in organic chemistry, 
are more readily distinguished by polarized light than by any 
other means. 
The examination of microscopic structures, by polarized 
light, affords to the enterprising student a rich field of investi- 
gation, as yet but partially explored. As a method of investi- 
gating delicate structures it is of the highest value. The chem- 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
