The stereoscope

by David Brewster

3. If a portrait is wanted to assist a sculptor in modelling a statue, a great angle might be adopted, in order to shew more of the head. But in this case the best way would be to take the correct social likeness, and then take photographs of the head in different azimuths.

If we wish to have a greater degree of relief than we have with our two eyes, either in viewing colossal statues, or buildings, or landscapes, where the deviation from nature does not, as in the human face, affect the expression, or injure the effect, we must increase the distance of the lenses in the binocular camera, or the angle of direction of the common camera. Let us take the case of a colossal statue 10 feet wide, and suppose that dissimilar drawings of it about three inches wide are required for the stereoscope. These drawings are forty times narrower than the statue, and must be taken at such a distance, that with the binocular camera the relief would be almost evanescent. We must therefore suppose the statue to be reduced n times, and place the semi-lenses at the distance n × 2½ inches. If n = 10, the statue 10 feet wide will be reduced to ¹⁰/₁₀, or to 1 foot, and n × 2½, or the distance of the semi-lenses will be 25 inches. With the lenses at this distance, the dissimilar pictures of the statue will reproduce, when combined, a statue one foot wide, which will have exactly the same appearance and relief as if we had viewed the colossal statue with eyes 25 inches distant. But the reproduced statue will have also the same appearance and relief as a statue a foot wide reduced from the colossal one with mathematical precision, and it will therefore be a better or more relieved representation of the work of art than if we had viewed the colossal original with our own eyes, either under a greater, an equal, or a less angle of apparent magnitude.

We have supposed that a statue a foot broad will be seen in proper relief by binocular vision; but it remains to be decided whether or not it would be more advantageously seen if reduced with mathematical precision to a breadth of 2½ inches, the width of the eyes, which gives the vision of a hemisphere 2½ inches in diameter with the most perfect relief.[55] If we adopt this principle, and call B the breadth of the statue of which we require dissimilar pictures, we must make n = B/2½, and n × 2½ = B, that is, the distance of the semi-lenses in the binocular camera, or of the lenses in two cameras, must be made equal to the breadth of the statue.

In concluding this chapter, it may be proper to remark, that unless we require an increased relief for some special purpose, landscapes and buildings should be taken with the normal binocular camera, that is, with its lenses 2½ inches distant. Scenery of every kind, whether of the picturesque, or of the sublime, cannot be made more beautiful or grand than it is when seen by the traveller himself. To add an artificial relief is but a trick which may startle the vulgar, but cannot gratify the lover of what is true in nature and in art.

The Single Lens Binocular Camera.

As every photographer possesses a camera with a lens between 2½ and 3 inches in diameter, it may be useful to him to know how he may convert it into a binocular instrument.

In a cover for the lens take two points equidistant from each other, and make two apertures, c, d, [Fig. 43], ²/₁₀ths of an inch in diameter, or of any larger size that may be thought proper, though ²/₁₀ is the proper size. Place the cover on the end of the tube, and bring the line joining the apertures into a horizontal position. Closing one aperture, take the picture of the sitter, or of the statue, through the other, and when the picture is shifted aside by the usual contrivances for this purpose, take the picture through the other aperture. These will be good binocular portraits, fitted for any stereoscope, but particularly for the Achromatic Reading Glass Stereoscope. If greater relief is wanted, it may be obtained in larger lenses by placing the two apertures at the greatest distance which the diameter of the lens will permit.

The Binocular Camera made the Stereoscope.

If the lenses of the binocular camera, when they are whole lenses, be made to separate a little, so that the distance between the centres of their inner halves may be equal to 2½ inches, they become a lenticular stereoscope, in which we may view the pictures which they themselves create. The binocular pictures are placed in the camera in the very place where their negatives were formed, and the observer, looking through the halves of his camera lenses, will see the pictures united and in relief. If the binocular camera is made of semi-lenses, we have only to place them with their thin edges facing each other to obtain the same result. It will appear, from the discussions in the following chapter, that such a stereoscope, independently of its being achromatic, if the camera is achromatic, will be the most perfect of stereoscopic instruments.

The preceding methods are equally applicable to landscapes, machines, and instruments, and to solid constructions of every kind, whether they be the production of nature or of art.[56]