m which Mr. Molteni has just rescued it and
obtained much success.

[Illustration: STEREOSCOPIC PROJECTIONS]

This is in what it consists: The impression of the relief appears when
each eye sees that one of the two images which presents the
perspective that it would perceive if it saw the real object. If we
take two transparent stereoscopic images and place each of them in a
projection lantern, in such a way that they can be superposed upon the
screen, we shall obtain thereby a single image. It will always be a
little light and soft, as the superposition cannot be effected
accurately, the perspective not being the same for each of them. It is
a question now to make each eye see the one of the two images proper
to it. To this effect, Mr. D'Almeida conceived the very ingenious idea
of placing green glass in the lantern in front of the image having the
perspective of the right eye, and a red glass in front of the other
image. As green and red are complementary colors, the result was not
changed upon the screen; there was a little less light, that was all.
But if, at this moment, the spectator places a green glass before his
right eye and a red one before his left, he will find himself in the
condition desired for realizing the effect sought.

Each eye will then see only the image responding to the coloration
chosen, and, as it is precisely the one which has the perspective
proper to it, the relief appears immediately. The effect is striking.
We perceive a diffused image upon the screen with the naked eye, but
as soon as we use one special eye-glass the relief appears with as
much distinctness as in the best stereoscope. One must not, for
example, reverse his eye-glass, for if (things being arranged as we
have said) he looks through a red glass before his right eye, and
through a green one before his left, it is the image carrying the
perspective designed for the right eye that will be seen by the left
eye, and reciprocally. There is then produced, especially with certain
images, a very curious effect of reversed perspective, the background
coming to the front.

Now that photography is within every one's reach, and that many
amateurs are making stereopticon views and own projection lanterns,
we are persuaded that the experiment will be much more successful than
it formerly was. An assemblage of persons all provided with colored
eye-glasses is quite curious to contemplate. Our engraving represents
a stereopticon seance,