One of the most beautiful as well as the most remarkable pictorial illusions is produced by the combination of two views into one by the recently invented instrument called the Stereoscope. In the Diorama, in the Magic Disc, and in the Dissolving Views, separate paintings combine to produce different effects; but in the Stereoscope the two pictures unite into one to give additional effect to the same view, and to make that which is a flat surface, when seen singly, appear to project like a solid body. The principle of the Stereoscope depends on the different appearance which near objects present when seen by the right or by the left eye. For instance, on looking at a book placed edgewise, with the right eye, the back and one side of the book will be perceived; and on closing the right eye and opening the left, the back and the other side of the book will be seen, and the right-hand side will be invisible. It is the combination of both these views by vision with two eyes that produces the impression of solidity of objects on the mind; and if the different appearances which the book presents to each eye be copied in separate drawings, and they can afterwards be placed in such a position as to form a united image on the retinæ of the eyes, the same effect is produced as if the book itself were looked upon. [Illustration] This diagram represents the outlines of a near object, as seen by each eye separately. The one on the right hand shows it as seen with the right eye, and the other as it looks with the left eye; and if both drawings be combined into one image, it stands out in bold relief. This may be done without any instrument, by squinting at them; but the effect is more readily and far more agreeably produced by the Stereoscope, so named from the Greek words στερος {steros}, solid, and σκοπεω {skopeô}, to see. Professor Wheatstone claims to be the first who contrived an instrument to illustrate this effect of binocular vision, and he also claims to be the first who brought to notice the different appearances of objects seen with each eye separately. Sir David Brewster, however, disputes, on behalf of Mr. Elliot, of Edinburgh, Professor Wheatstone's claim to the invention of the first stereoscopic instrument; and he has shown that the difference of vision with each eye was remarked by Galen, 1,700 years ago; that it was noticed by Leonardo da Vinci in 1500, and formed the subject of a treatise by a Jesuit, named Francis Aquilonius, in 1613; and that it was a well-known phenomenon of vision long before it was mentioned by Professor Wheatstone.[5] Mr. Elliot, though he conceived the idea, in 1834, of constructing an instrument for uniting two dissimilar pictures, did not carry it into effect until 1839, the year after Mr. Wheatstone had exhibited his reflecting Stereoscope to the Royal Society, and at the meeting of the British Association.