Stereoscopic Work 
SEPARATION OF THE LeENSES.—This should be equal to the 
interpupillary distance of the eyes, which is approximately 
65mm (2 9/1l6in.), but 76mm (3in.) is usually adopted 
for normal work. When taking near objects, the centers of 
the lenses must be brought nearer to one another, and the 
rule for finding how much is: if the additional extension of 
the camera beyond the infinity mark or focus is equal to the 
focal length multiplied by 1 —~v,, then the separation must be 
reduced by an amount equal to the maximum separation 
multiplied by 1 (r+ 1), in which ¢ is the ratio of image 
to object. Example, the image is to be made one-quarter 
the natural size, then 1 ~ (r + 1) = 1/5, so that the separa- 
tion must be 3 — 3/5 inches. 
MountTiInG Prints.—To secure correct conditions of con- 
vergency, each print must be seen under the same angle of 
view as that at which it was produced, and the two prints 
must be mounted in accord with the following rule: 
Let P = separation of any pair of corresponding points on 
prints 
N = separation of same points on negatives 
F = separation of eyes (average is 64 mm) 
L = separation of camera lenses. 
A non-prismatic stereoscope being used: 
1. If image points represent infinitely distant objects, 
Male eas 
2. If only near objects are shown, and an ordinary single 
plate double lens stereo camera has been used, 
Make P= E+ L—WN 
3. If a single camera is used for two separate exposures, or 
328 
