out from the background. Try
looking at the front cover of Popular Mechanics through these
colored gelatine openings and the effect will be produced.

If one looks at the picture first with the right eye alone through
the orange glass, and then with the left eye through the blue
glass, one will understand the principle on which the little
instrument works. Looking through the blue glass with the left
eye, one sees only those portions which are red on the picture.
But they seem black. The reason is that the red rays are absorbed
by the blue filter. Through the orange gelatine all the white
portions of the picture seem orange, because of the rays coming
from them, and which contain all the colors of the spectrum; only
the orange rays may pass through. The red portions of the picture
are not seen, because, although they pass through the screen, they
are not seen against the red ground of the picture. It is just as
though they were not there. The left eye therefore sees a black
picture on a red background.

In the same way the right eye sees through the orange screen only
a black picture on a red background; this black image consisting
only of the blue portions of the picture. Any other part of
complementary colors than blue and orange, as for instance red and
green, would serve the same purpose.

The principle on which the stereograph works may be demonstrated
by a very simple experiment. On white paper one makes a picture or
mark with a red pencil. Looking at this through a green glass it
appears black on a green ground; looking at it through a red glass
of exactly the same color as the picture, it, however, disappears
fully.

Through the glass one will see only a regular surface of the color
of the glass itself, and without any picture. Through a red glass
a green picture will appear black.

So with the stereograph; each eye sees a black picture
representing one of the pictures given by the stereoscope; the
only difference being that in the case of the stereograph the
background for each eye is colored; while both eyes together see a
white background.

In the pictures the red and the green lines and dots must not
coincide; neither can they be very far apart in order to produce
the desired result. In order that the picture shall be "plastic,"
which increases the sense of depth and shows the effect of
distance in the picture, they must be a very trifle apart. The
arrangement of the two pictures can be so that one