The celebrated philosopher Bacon, the founder of the experimental method, claimed that we see better with one eye than with two, because the attention is more concentrated and becomes profounder. "On looking in a mirror," says he, "we may observe that, if we shut one eye, the pupil of the other dilates." To this question: "But why, then, have we two eyes?" he responds: "In order that one may remain if the other gets injured." Despite the reasoning of the learned philosopher, we may be permitted to believe that the reason that we have two eyes is for seeing better and especially for perceiving the effects of perspective and the relief of objects. We have no intention of setting forth here the theory of binocular vision; one simple experiment will permit any one to see that the real place of an object is poorly estimated with one eye. Seated before a desk, pen in hand, suddenly close one eye, and, at the same time, stretch out the arm in order to dip the pen in the inkstand; you will fail nine times out of ten. It is not in one day that the effects of binocular vision have been established, for the ancients made many observations on the subject.

It was in 1593 that the celebrated Italian physicist Porta was the first to give an accurate figure of two images seen by each eye separately, but he desired no apparatus that permitted of reconstituting the relief on looking at them. Those savants who, after him, occupied themselves with the question, treated it no further than from a theoretical point of view. It was not till 1838 that the English physicist Wheatstone constructed the first stereoscopic apparatus permitting of seeing the relief on examining simultaneously with each of the eyes two different images of an object, one having the perspective that the right eye perceives, and the other that the left eye perceives.

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.

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, and the draughtsman has well rendered the effect of the two luminous and differently colored fascicles superposed upon the screen.

In a preceding note upon the same subject, Mr. Hospitalier remarked that upon combining these effects of perspective with those of the praxinoscope, which give the sensation of motion, we would obtain entirely new effects. It would be perhaps complicated as to the installation, and especially as to the making of the images, but, in certain special cases (for giving the effect of a machine in motion, for example), it might render genuine services. - La Nature.