The appearance of high relief given by the stereoscope, is obtained by placing side by side two prints representing the same object, but photographed from slightly different positions, whilst the glass prisms of the stereoscope so direct the visual rays as to superpose the views, and but one picture is seen, although it is in reality a combination of both; thus the same object is seen from two different points of view at the same time, as is always the case when both eyes are looking at one thing, as they, with the object seen, of course form a kind of triangle. Views for the stereoscope are frequently taken simultaneously by two cameras, placed at certain angles and distances from each other, varying with the size and distance of the object to be photographed; but for portraits less trouble is involved by the use of a properly constructed twin camera. For views or groups situated some distance from the operator, two distinct cameras must be used (or one camera moved from place to place); the distance they must be placed apart, and the relative angle in which they should stand to each other, require careful consideration. For portraits or other objects, to which the cameras can be brought rather close, the angle should not be too great, otherwise the effect of relief will be distorted.
In such cases an angle of about 2° must be used. For landscapes, as large an angle as 4° may generally be safely employed. To reckon the angles, suppose the nearest point of the view to be taken to represent the apex of a triangle, from each camera produce an imaginary straight line to the apex, these lines must represent the desired angle. As lines diverging from a centre may be indefinitely produced without altering their relative angle to each other, so the distance between the cameras wilt not affect the angles they should stand in, except that, for pictorial effect, distant objects may be a little distorted, with good results, as will be the case when a large angle is used; whereas for subjects close to the camera, such distortion does not give a pleasing picture. Supposing an angle of 2° to be used, the distance required between the cameras will be about 1 1/4 in. for 1 yard, 2} in. for 2 yards, 3} in. for 3 yards, 5 in. for 4 yards, 6 in. for 5 yards, 7 1/2 in. for 6 yards, 9 in. for 7 yards, 10 in. for 8 yards, 11 1/3 in. for 9 yards, 12 1/2 in. for 10 yards, 19 in. for 15 yards, 25 in. for 20 yards.
These remarks apply equally whether two cameras are used simultaneously, or whether only one camera is used, being moved from one position to the other as required.
This is a camera having two double achromatic combinations of the same focal length, in other respects like an ordinary camera, except that it has two folding shutters at the back. Before focusing the object to be photographed, it is necessary to ascertain that the two lenses are in focus with each other. For this purpose, focus a statuette, or other convenient article, and when a perfectly sharp image is obtained with each of the lenses, upon the ground-glass slide, do not again alter the rack and pinion which adjust the lenses; any further adjustment necessary for portraits to be taken subsequently must be obtained by drawing in or out the expanding body of the camera, as when once the glasses are of the exact focal length, their relative positions to each other will not require any alteration, although the body of the camera will. As the two lenses are necessarily rather close to each other, the twin camera will only answer for photographs taken at a very short distance, otherwise the effect of relief will not be obtained. The glasses used are longer than they are wide, as two negatives are taken at the same time.
All the operations are the same as before described for negatives; when the positive prints are obtained, their position must be reversed in mounting, the left-hand half of the print being pasted on the right-hand side of the card, and the right side of the print on the left side of the card.
The camera must be placed on a board, having a movable slip of wood at each side which can be adjusted to the desired angle, against which the camera must be placed, first on one side to take one view, and then on the other side to take the other view. Mark cross lines on the ground-glass plate, to intersect a central point of the view from whichever side of the board the camera is standing; this is to ensure correct centres for the proofs. Two separate negatives are then taken; when mounting the prints, transpose their position from right to left. Dark slides are made for this process, to hold a glass sufficiently long to contain both views, and fitted with two shutters, by which each half of the glass can be exposed alternately. Having exposed the right-hand half, close its shutter, move the camera the required distance to the left, and expose the left-hand half 'of the glass.
For instantaneous views of any landscape containing animate figures, it is necessary to use two quarter-plate cameras, with lenses of exactly equal focal length; they must be placed on a board provided with movable stops to regulate angle and distance. Great care is necessary in manipulation; the two shutters must be opened and closed at the same time, otherwise the two proofs will develop unequally. The plates should be collodionised and sensitised in the same baths, and to the same extent. For the developing bath, employ a vessel into which the two plates can be placed side by side, so that the same pyrogallic acid may affect both simultaneously. When mounting the positives, transpose the two views, left to right, as before described.