Stereoscopic effect or the appearance of relief depends upon the combining in one in the stereoscope of two representations of the same scene taken from slightly different points of view. Stereoscopic photographs, therefore, are best obtained with a camera having a pair of lenses fitted side by side. These lenses should be accurately matched as regards focus, ratio, aperture, colour, etc., and should be 2 3/4 in. apart, which is about the distance between the eyes. With this camera two pictures will be taken at the same time. Paired lenses are sold for the purpose. A method of taking stereoscopic photographs with one lens only (a half-plate camera being used) is to employ a couple of mirrors set at such an angle as to have two points of sight. These mirrors are placed in front of the lens and reflect the image through the lens on to the plate. The instrument is known as a stereoscopic transmitter. Still objects," and ordinary landscapes in which there are no moving figures, can be taken with only one lens if the camera is fitted with a sliding front. Such a camera must have square bellows. The above sketch explains the construction of a sliding front.

The first exposure is made, and A is then pushed along until the mark B points to the mark C. The opening in the front board of the camera is shown by dotted lines. The distance between the two points may be varied according to the distance of the principal object. The farther the principal object is from the camera the greater must be the separation between the two points. Sometimes it is possible to obtain stereoscopic photographs by moving the object, as, for example, a vase of flowers. In this case the camera and lens are stationary and an ordinary quarter-plate camera can be used. Such a camera may also be used if it is fitted with a board as wide as the base from back to front and about double the length of the original base. Two parallel slots are made in this extra baseboard, and thumbscrews pass through these into the original baseboard. The camera may thus be slid easily from one position to the other and clamped. A great deal depends upon correct mounting of the prints; this is a process that is described on another page, but suffice it to say that the picture that was on the left hand of the camera becomes the right-hand print when mounted.

Sliding Front of Camera for Stereoscopic Photography.

For most subjects, except instantaneous stereoscopic work, an ordinary quarter-plate camera, with one lens only, may be used if provision is made for shifting the camera or the lens from side to side for a distance of from 2 1/4in. to 2 3/4in.; or if the object itself can be moved the same relative distance the camera may remain stationary; or achromatic lenses, paired for stereoscopic work, could be fitted to a half-plate camera. As to plates, in a half-plate camera double quarter-plates (6 1/2 in. by 4 1/4 in.) are often preferred. There should be a partition between the lenses, and this may easily be made in a square-bellows camera by pleating some flexible black material over two slips of elastic and fastening it to hooks in the camera front and in the back frame.