ISIETHOD OF VARYINO THE COLORS. 
89 
and analyzer, on rotating tlie film a brilliant color is perceived 
at every quadrant of a circle, but in intermediate positions it 
vanishes altogether. We observe, however, that the tint does 
not change, but only varies in intensity. 
If, now, the film of selenite is fixed and the polarizer is 
rotated, we also observe color at every quadrant of revolution 
wdiich disappears in intermediate positions, but the tint changes 
and becomes complementary at every quadrant, — the same tint 
reappearing at every half-revolution. Thus, when the film 
alone is revolved, one color only is seen, but when the polarizer 
is revolved two complementary colors are seen. 
The following is Sir David Brewster’s table of complemen- 
tary colors : 
Red, 
complementary. 
Bluish green. 
Orange, 
Blue, 
Yellow, 
Indigo, 
Green, 
Violet reddish. 
Blue, 
Orange red. 
Indigo, 
Orange yellow, 
Violet, 
a 
Yellow green. 
Black, 
White, 
White, 
Black. 
Films of selenite, varying between .00124 and .01818 of an 
inch in thickness, will give every variety of tint in the solar 
spectrum. If two films of selenite are placed over each other, 
with their crystallographic axes parallel, the color produced 
wdll be that which belongs to the sum of their thicknesses. 
But when the two films are placed with similar axes at right 
angles, the resulting tint is that which belongs to the difference 
of their thicknesses. 
A film of selenite or mica of such thickness as to produce a 
bright purple, or a light blue color, will be found to present the 
most agreeable contrast, and, as a single plate, prove most 
generally useful to the microscopist. Three films of selenite, 
which separately give three different colors, may each be 
mounted in Canada balsam, between slips of thin glass, and 
used singly, or in double, or triple combinations. As many as 
thirteen different tints may thus be obtained. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
