This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Depth Of Printing. Stereoscopic pictures, when made on printing-out paper, should be printed to a good depth. A light print when viewed through a stereoscope will lack in depth and roundness, and there will appear little or no relief. While we do not recommend flat prints, wanting in vigor, they often look the best through the stereoscope. The prints should be full of detail. Avoid contrast. When printing on gaslight or developing papers, extreme care must be taken to see that both prints are given the same exposure. When printing on printing-out paper, be careful to print both sections to the same depth. For instruction on toning and manipulation of all papers see Volume IV.
Trimming. Where much stereo work is being done, the paper should be purchased already cut the exact size, so that no matter whether you use the self-transposing printing-frame or if you transpose the negative before printing, or even if you print direct from the negative and transpose the prints afterward, the accurately cut sheets can be used just the same. In the latter case all that is required is to separate the two prints and transpose them in mounting.
560. Where stereo prints are only made occasionally and untrimmed sheets of paper used, then considerable care must be exercised in the trimming. First, determine the base line. A suitable size for the individual prints is 2 1/2 x 3 1/2. Provide yourself with a glass form this size. If oval prints are preferred, a celluloid form should be used. On one side of the form draw a vertical and a horizontal line, crossing exactly at the center. These lines will enable you to readily place the film in proper position when trimming.
Position Of Cutting Form. Place the prints to be mounted face up on a sheet of plate glass or zinc, and lay the glass form with the etched lines in contact with the print. The vertical line on the glass should be parallel to the vertical lines in the picture, such as the sides of a building, while the horizontal line should be made to intersect corresponding points in both pictures. Unless you do this the two different pictures will be on different planes, and not on a level with each other - an error that will make it difficult to combine the two pictures when they are viewed in the stereoscope - for when the subject is examined it should present the appearance of relief, with all the composition lying beyond the margins of the print. To obtain this effect a little more of the subject should be kept on the left-hand edge of the right picture, and a little more on the right-hand edge of the left picture.
Mounting. Where prints are made direct from the stereo negative, and are not transposed in the printing, then before the double photograph is separated you should mark the pair of prints in some way so as to be able to match them in the mounting. The left end prints should be marked on the back with the letter L, and the right end with the letter R. They may then be numbered in pairs and cut apart and trimmed, and in mounting be guided by the letters and numbers on the back.
563. If prints should be mounted without reversing them, the effect known as pseudoscopic will be produced, and when the pictures are examined through the stereoscope, objects which should occupy a remote position appear in the foreground, while objects which should stand out in the photograph tend to recede. Where the negatives are cut in two and transposed before printing, then one sheet of paper may be used, covering both negatives, and prints from such negatives may be mounted without cutting. The best stereoscopic pictures are made, however, where the prints are made from the original negative and transposed in the mounting, supplying two separate prints.