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doing this by lenses may be explained, to persons not acquainted with optics, in the following manner :—
If we look at A with one eye through the centre of a convex glass, with which we can see it distinctly at the distance of 6 inches, which is called its focal distance, it will be seen in its place at A. If we now move the lens from right to left, the image of A will move towards B; and when it is seen through the right-hand edge of the lens, the image of A will have reached the position c, half-way between A and B. If we repeat this experiment with the portrait B, and move the lens from left to right, the image of B will move towards A ; and when it is seen through the left-hand edge of the lens, the image of B will have reached the position c. Now, it is obviously by the 1-ight-hmd half of the lens that we have transferred the image of A to c, and by the left-hand half that we have transferred the image of B to c. If we cut the lens in two, and place the halves—one in front of each picture at the distance of 2^ inches—in the same position in which they were when A was transferred to c and B to c, they will stand as in Fig. 12, and we shall see the
portraits A and B united into one at c, and standing out in beautiful relief,—a result which will be explained in a subsequent chapter.
The same effect will be produced by quarter lenses, such as those shewn in Fig. 13. These lenses are cut into a round
or square form, and placed in tubes, as represented at r, L in Fig. 14, which is a drawing of the Lenticular Stereoscope. This instrument consists of a pyramidal box, Fig. 14, blackened inside, and having a lid, c D, for the admission of light when required. The top of the box consists of two parts, in one of which is the right-eye tube, r, containing the lens G, Fig. 13, and in the other the left-eye tube, L, containing the lens H. The two parts which hold the lenses, and which form the top of the box, are often made to slide in grooves, so as to suit different persons whose eyes, placed at R, L, are more or less distant. This adjustment may be made by various pieces of mechanism. The simplest of these is a jointed parallelogram, moved by a screw forming its longer diagonal, and working in nuts fixed on the top of the box, so as to separate the semi-lenses, which follow the movements of the obtuse angles of the parallelogram. The tubes r, L move up and down, in order to suit eyes of different focal lengths, but they are prevented from turning round by a brass pin, which runs in a groove cut through the movable tube. Immediately below the eyetubes R, L, there should be a groove, G, for the introduction of convex or concave lenses, when required for very longsighted or short-sighted persons, or for coloured glasses and other purposes.
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 origiginally of card-board, tin-plate, wood, and brass; but wood is certainly the best material when cheapness is not an object.
One of the earliest forms which I adopted was that which is shewn in Fig. 15, as made by M. Duboscq in Paris, and which may be called stereoscopic spectacles. The
two-eye lenses L, R are held by the handle H, So that we can, by moving them to or from the binocular pictures, obtain distinct vision and unite them in one. The effect, however, is not so good as that which is produced when the pictures are placed in a box.
The same objection applies to a form otherwise more convenient, which consists in fixing a cylindrical or square rod of wood or metal to c, the middle point between L and R. The binocular slide having a hole in the middle between the two pictures is moved along this rod to its proper distance from the lenses.