This apparatus is described in almost all treatises on physics.  We may merely recall the fact that it operated by reflection, that is to say, the two images were seen through the intermedium of two mirrors making an angle of 45 degrees.  The instrument was very cumbersome and not very practical.  Another English physicist, David Brewster, in 1844 devised the stereoscope that we all know; but, what is a curious thing, he could not succeed in having it constructed in England, where it was not at first appreciated.  It was not till 1850 that he brought it to Paris, where it was constructed by Mr. Soleil and his son-in-law Duboscq.  Abbot Moigno and the two celebrated opticians succeeded, not without some difficulty, in having it examined by the official savants; but, at the great exposition of 1851, it was remarked by the Queen of England, and from this moment Messrs. Soleil & Duboscq succeeded with difficulty only in satisfying the numerous orders that came from all parts.  As photography permitted of easily making identical images, but with different perspective, it contributed greatly to the dissemination of the apparatus.

The stereoscope, such as we know it, presents the inconvenience of being incapable of being used by but one person at once.  Several inventors have endeavored to render the stereoscopic images visible to several spectators at the same time.  In 1858, Mr. Claudet conceived the idea of projecting the two stereoscopic images upon ground glass in superposing them.  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.  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.