This instrument has now attained a popularity quite equal to, if it does not surpass, that formerly enjoyed by the kaleidoscope, and without entering upon the much-vexed question of priority of discovery, it is sufficient again to mention with the highest respect the names of Sir David Brewster and Professor Wheatstone as identified with the discovery and use of this most pleasing optical instrument. The principle of the stereoscope (meaning, _solid I see_) is copied from nature: _i.e._, when both eyes are employed in the examination of an object, two separate pictures, embracing dissimilar forms, are impressed upon the retinæ, and produce the effect of solidity; if the pictures formed at the back of the eyes could be examined by another person with a stereoscope, they would come together, and also produce the effect of solidity. Stereoscopic pictures are obtained by exposing sensitized paper in the camera to the picture of an object taken in two positions, or two cameras are employed to obtain the same result. If the latter mode is adopted, the stereoscopic pictures must not be taken from positions too widely separated from each other; or else, when the two pictures are placed in the stereoscope, they will stand out with a relief that is quite unnatural, and the object will appear like a very reduced solid model, instead of having the natural appearance presented by pictures which have been taken at positions too distant from each other. Sir David Brewster says, "In order to obtain photographic pictures mathematically exact, we must construct a binocular camera which will [Page 321] take the pictures simultaneously, and of the same size; that is, by a camera with two lenses of the same aperture and focal length, placed at the same distance as the two eyes. As it is impossible to grind and polish two lenses, whether single or achromatic, of exactly the same focal lengths, even if we had the very same glass for each, I propose to bisect the lenses, and construct the instrument with semi-lenses, which will give us pictures of precisely the same size and definition. These lenses should be placed with their diameters of bisection parallel to one another, and at a distance of 2½ inches, _which is the average distance of the eyes in man_; and when fixed in a box of sufficient size, will form a binocular camera, which will give us at the same instant, with the same lights and shadows, and of the same size, such dissimilar pictures of statues, buildings, landscapes, and living objects, as will reproduce them in relief in the stereoscope." Thus with a single camera provided with semi-lenses, or two lenses of the same focal length, stereoscopic pictures can be obtained. To bring the images of the two pictures together, and produce the effect of solidity; either of two instruments may be employed. The reflecting stereoscope is the invention of Professor Wheatstone. The refracting or lenticular stereoscope that of Sir David Brewster.