The Stereoscope. The name Stereoscope, from the Greek words stereos, solid, and skopein, to see, has been given to an instrument of recent invention, for exhibiting in true relief and apparent solidity all objects, or groups of objects, by combining into one picture two representations of these objects on a plane, as seen separately by each eye.

The Stereoscope 605

If we hold up a thin book between our two eyes, with its back towards us, and at the distance of about a foot, we shall see the back and the two sides of the book when both eyes are open ; but if we shut the right eye, we shall see with the left eye only the back and left side of the book, and if we shut the left eye, we shall see only the back and the right side of it. Or, to use a more homely illustration, when we shut the Ieft eye. we see only the right side of our nose with the right eye , and when we shut the right eye, we see only the left side of our nose with the left eye. And in general, when we look at any solid object whatever, the right eye sees parts of it towards the right hand not seen by the Left eye, and the left eye sees part of it. towards the left hand not seen by the right eye. Hence we arrive at the first and fundamental truth on which the theory and construction of the Stereoscope depend, viz. : 1, When we look with two eyes upon any solid body or object whose parts are at different distances from us, the picture of it which we see with the right eye, or the image of it which is formed on the retina of the right eye, is different from the picture of it which we see with the left eye, or from the image of it which is formed on the retina of the left eye.

This important fact was known to Euclid more than 2000 years ago, and was illustrated by him in the case of a sphere, the pictures of which as seen by each eye he proved to be dissimilar. Upwards of 1500 i years ago, Galen described the different pictures formed on each eye in the vision of a column. Baptista Porta, in 1593, repeats the proposition of Euclid on the vision of a sphere with one and both eyes; and he quotes the experiments of Galen on the vision of a column with both eyes, and with each eye alternately. Leonardo da Vinci was well acquainted with the same facts; and Aguilonius, in 1613, wrote a whole book on the vision of solids (ta sterea) with one and both eyes, and explained the dissimilarity of the pictures thus seen by the observer.

Optical writers of more recent times, such as Dr. Smith of Cambridge, Mr. Harris, and Dr. Porterfield, were all acquainted with the dissimilarity of the pictures of solids as seen by each eye separately; and hence we see the extreme injustice of the claim made by Mr. Wheatstone to be the discoverer of this truth. In quoting the experiments of Leonardo da Vinci, Mr. Wheatstone maintains that he was not aware "that the object (a sphere) presented a different appearance to each eye;" and he adds, " he Juiled to observe this; and no subsequent writer, to my knowledge, has supplied the omission. The projection of two obviously dissimiler pic-tures on the two retinae, when a tingle object is viewed, while the optic axes converge, mast therefore be regarded as a new fact in the theory of vision." This claim to a discovery made 2000 years ago by Euclid, and explained and illustrated by so many of his distinguished successors, is the more remarkable, as Mr. Wheatstone, though he may have never seen the writings of Euclid or Galen, makes repeated reference to the observations of Porta and Aguilonius, in which the discovery is distinctly described.

The second fundamental truth on which the theory and construction of the Stereoscope depend is: 2, When the two dissimilar pictures of any solid body, as seen by each eye separately, are superimposed, or laid the one above the other by the conver-gency of the axes of the two eyes, the object which these pictures represent is seen in relief, or as a solid body, with its different parts at different distances from the observer.

Altough this truth is not distinctly stated either by Euclid or Galen, we can hardly suppose that they were ignorant of it, as it is a necessary result of their observations. Since we do see an object in true relief by both eyes, and since the picture of the object which we see is formed by the superposition of the one dissimilar picture above the other, the vision in relief is the necessary result of the combination of the pictures. They must have known it simply as a fact, though they did not know its cause.

Baptista Porta and Aguilonins, however, were well acquainted with this second truth. In explaining the experiments of Galen on the dissimilarity of the pictures of an object as seen by each eye and by both, Porta employs the annexed diagram, which is much more distinct than that which is given by the Greek physician. " Let a," he says, "be the pupil of' the right eye, B that of the left, and d c the body to be seen. When we look at the body with both eyes, we see d c, while with the left eye we see e f, and with the right eye G h. But if it is seen with one eye, it will be seen otherwise; for when the left eye B is shut, the body C D, on the left side, will be seen in h G; but when the right eye A is shut, the body C D will be seen in f e; whereas when both eyes are opened at the same time it will be seen in C D" Porta then proceeds to explain these results by quoting the passage from Galen in which he supposes the observer to repeat these experiments when he is looking at a solid column. In the preceding diagram we see not. only the principle but the construction of the Ocular Stereoscope, or the method by which we combine the two pictures by looking at a point between them and the observer, or beyond the pictures. The two dissimilar pictures are represented by he; the picture as seen by one eye by ho; the picture as seen by the other by e f , and the picture of the solid column in full relief by d c. as produced midway between the two dissimilar pictures h g and f e by their union, precisely as in the Stereoscope.