144
THE ART OF PROJECTING.
the sensitiveness of the eyes for various colors. Gen¬
erally, after looking steadily at a given color, and the
disk is made suddenly white, the outline of the colored
part will be seen in a color complementary to the one
looked at first. Thus, if a square red glass should be
projected the residual image would be a square green
one. If a blue one was projected its complementary
image would be orange, and so on. A great variety of
such effects are obtainable with various colored pieces
of glass, or of films of gelatine, by projecting them
singly, in juxtaposition, or superposed.
Let disks of white cardboard a foot or two in diame¬
ter have partial sectors painted black, with india ink,
so that the white and black parts alternate four or five
times in the circumference. This is to be rotated while
a powerful beam of light falls upon it. The persist¬
ence of some of the elements of white light being
greater than of others, the disk will appear of various
colors ; purple, green, and yellow being generally well
developed.
HEAT. — AIR THERMOMETER.
A bulb blown upon one end of a small glass tube,
five or six inches long, answers for this experiment. A
drop of colored water can be made to enter the tube
by first heating the bulb a little by holding it in the
fingers with the open end of the tube a little below the
surface of the water. A bubble or two of air will be
expelled, and the fingers may be removed from the
bulb. As it cools a drop will be driven into the tube,
and with a little painstaking it can be brought to any
required place by cooling or heating the bulb. These
movements can be shown with the porte lumière and a
single lens, as shown in Fig. 17, or it can be put in