The relief was seen, it appears, but we cannot
very well explain why; the idea, however, had no outcome, because the
image, being quite small, could be observed by but three or four
persons at once. It was Mr. D'Almeida, a French physicist, who toward
the same epoch solved the problem in a most admirable manner, and we
cannot explain why his process (that required no special apparatus)
fell into the desuetude from 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.
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