If we now put the slide AB, Fig. 11, into the horizontal opening at S, turning up the sneck above S to prevent it from falling out, and place ourselves behind R, L, we shall see, by looking through R with the right eye and L with the left eye, the two images A, B united in one, and in the same relief as the living person whom they represent. No portrait ever painted, and no statue ever carved, approximate in the slightest degree to the living reality now before us. If we shut the right eye R we see with the left eye L merely the portrait A, but it has now sunk into a flat picture, with only _monocular relief_. By closing the left eye we shall see merely the portrait B, having, like the other, only monocular relief, but a relief greater than the best-painted pictures can possibly have, when seen even with one eye. When we open both eyes, the two portraits instantly start into all the roundness and solidity of life. Many persons experience a difficulty in seeing the portraits single when they first look into a stereoscope, in consequence of their eyes having less power than common over their optic axes, or from their being more or less distant than two and a half inches, the average distance. The two images thus produced frequently disappear in a few minutes, though sometimes it requires a little patience and some practice to see the single image. We have known persons who have lost the power of uniting the images, in consequence of having discontinued the use of the instrument for some months; but they have always acquired it again after a little practice. If the portraits or other pictures are upon opaque paper or silver-plate, the stereoscope, which is usually held in the left hand, must be inclined so as to allow the light of the sky, or any other light, to illuminate every part of the pictures. If the pictures are on transparent paper or glass, we must shut the lid CD, and hold up the stereoscope against the sky or the artificial light, for which purpose the bottom of the instrument is made of glass finely ground on the outside, or has two openings, the size of each of the binocular pictures, covered with fine paper. In using the stereoscope the observer should always be seated, and it is very convenient to have the instrument mounted like a telescope, upon a stand, with a weight and pulley for regulating the motion of the lid CD. The lenticular stereoscope may be constructed of various materials and in different forms. I had them made originally of card-board, tin-plate, wood, and brass; but wood is certainly the best material when cheapness is not an object. [Illustration: FIG. 15.]