The stereoscope

by David Brewster

9. The Camera Stereoscope.

The object of this instrument is to unite the transient pictures of groups of persons or landscapes, as delineated in two dissimilar pictures, on the ground-glass of a binocular camera. If we attach to the back of the camera a lenticular stereoscope, so that the two pictures on the ground-glass occupy the same place as its usual binocular slides, we shall see the group of figures in relief under every change of attitude, position, and expression. The two pictures may be formed in the air, or, more curiously still, upon a wreath of smoke. As the figures are necessarily inverted in the camera, they will remain inverted by the lenticular and every other instrument but the opera-glass stereoscope, which inverts the object. By applying it therefore to the camera, we obtain an instrument by which the photographic artist can make experiments, and try the effect which will be produced by his pictures before he takes them. He can thus select the best forms of groups of persons and of landscapes, and thus produce works of great interest and value.

10. The Chromatic Stereoscope.

The chromatic stereoscope is a form of the instrument in which relief or apparent solidity is given to a single figure with different colours delineated upon a plane surface.

If we look with both eyes through a lens LL, [Fig. 42], about 2½ inches in diameter or upwards, at any object having colours of different degrees of refrangibility, such as the coloured boundary lines on a map, a red rose among green leaves and on a blue background, or any scarlet object whatever upon a violet ground, or in general any two simple colours not of the same degree of refrangibility, the differently coloured parts of the object will appear at different distances from the observer.

Fig. 42.

Let us suppose the rays to be red and violet, those which differ most in refrangibility. If the red rays radiate from the anterior focus R, or red rays of the lens LL, they will emerge parallel, and enter the eye at m; but the violet rays radiating from the same focus, being more refrangible, will emerge in a state of convergence, as shewn at mv, nv, the red rays being mr, nr. The part of the object, therefore, from which the red rays come, will appear nearer to the observer than the parts from which the violet rays come, and if there are other colours or rays of intermediate refrangibilities, they will appear to come from intermediate distances.

If we place a small red and violet disc, like the smallest wafer, beside one another, so that the line joining their centres is perpendicular to the line joining the eyes, and suppose that rays from both enter the eyes with their optical axes parallel, it is obvious that the distance between the violet images on each retina will be less than the distance between the red images, and consequently the eyes will require to converge their axes to a nearer point in order to unite the red images, than in order to unite the violet images. The red images will therefore appear at this nearer point of convergence, just as, in the lenticular stereoscope, the more distant pair of points in the dissimilar images appear when united nearer to the eye. By the two eyes alone, therefore, we obtain a certain, though a small degree of relief from colours. With the lens LL, however, the effect is greatly increased, and we have the sum of the two effects.

From these observations, it is manifest that the reverse effect must be produced by a concave lens, or by the common stereoscope, when two coloured objects are employed or united. The blue part of the object will be seen nearer the observer, and the red part of it more remote. It is, however, a curious fact, and one which appeared difficult to explain, that in the stereoscope the colour-relief was not brought out as might have been expected. Sometimes the red was nearest the eye, and sometimes the blue, and sometimes the object appeared without any relief. The cause of this is, that the colour-relief given by the common stereoscope was the opposite of that given by the eye, and it was only the difference of these effects that ought to have been observed; and though the influence of the eyes was an inferior one, it often acted alone, and sometimes ceased to act at all, in virtue of that property of vision by which we see only with one eye when we are looking with two.