In this experiment with the book, and in all cases where the object is near the eye, we not only see _different pictures_ of the same object, but we see _different things_ with each eye. Those who wear spectacles see only the left-hand spectacle-glass with the left eye, on the left side of the face, while with the right eye they see only the right-hand spectacle-glass on the right side of the face, both glasses of the spectacles being seen united midway between the eyes, or above the nose, when both eyes are open. It is, therefore, a fact well known to every person of common sagacity that _the pictures of bodies seen by both eyes are formed by the union of two dissimilar pictures formed by each_. This palpable truth was known and published by ancient mathematicians. Euclid knew it more than two thousand years ago, as may be seen in the 26th, 27th, and 28th theorems of his Treatise on Optics.[1] In these theorems he shews that the part of a sphere seen by both eyes, and having its diameter equal to, or greater or less than the distance between the eyes, is equal to, and greater or less than a hemisphere; and having previously shewn in the 23d and 24th theorems how to find the part of any sphere that is seen by one eye at different distances, it follows, from constructing his figure, that each eye sees different portions of the sphere, and that it is seen by both eyes by the union of these two dissimilar pictures. [1] Edit. of Pena, pp. 17, 18, Paris, 1577; or _Opera_, by Gregory, pp. 619, 620. Oxon. 1703. More than _fifteen hundred_ years ago, the celebrated physician Galen treated the subject of binocular vision more fully than Euclid. In the _twelfth_ chapter of the tenth book of his work, _On the use of the different parts of the Human Body_, he has described with great minuteness the various phenomena which are seen when we look at bodies with both eyes, and alternately with the right and the left. He shews, by diagrams, that dissimilar pictures of a body are seen in each of these three modes of viewing it; and, after finishing his demonstration, he adds,— “But if any person does not understand these demonstrations by means of lines, he will finally give his assent to them when he has made the following experiment:—Standing near a column, and shutting each of the eyes in succession;—when the _right_ eye is shut, some of those parts of the column which were previously seen by the _right_ eye on the _right_ side of the column, will not now be seen by the _left_ eye; and when the _left_ eye is shut, some of those parts which were formerly seen by the _left_ eye on the _left_ side of the column, will not now be seen by the _right_ eye. But when we, at the same time, open both eyes, both these will be seen, for a greater part is concealed when we look with either of the two eyes, than when we look with both at the same time.”[2]