Very considerable care is required in cutting and mounting stereoscopic pictures, or stereo grams, as they are termed. The first thing is to determine a base line. The cutting edge is laid down on the double print, and the knife drawn along, so that it cuts a given object in the foreground of each half at exactly the same point.* The top of the print is then trimmed parallel to this base line. The two halves are then separated and transposed. The sides of the pictures are then trimmed on the print that is to be mounted on the right hand side, more subject on the right than we leave on the right of the left hand picture ; and on the left hand picture we must leave more of the left side than we leave on the left of the right hand picture. The pic tures are then mounted so that their centers shall be not less than two and three-quarter inches nor more than three inches apart.
Stereoscopic pictures should be printed to full depth, and should be full of detail. Flat prints, wanting in vigor, often look best in the stereoscope.
It is an indisputable fact that stereoscopic transparencies are a long way superior to prints, therefore this portion would be very incomplete without some remarks concerning their manu facture. The simplest method is by contact printing. The negative may be cut and transposed, or it may be printed from in an ordinary printing frame, and the transparency cut and transposed, and bound up between two cover glasses.
The best plan, however, is by means of a special printing frame, shown in the accompany ing sketches.f The principle of the frame is to print each picture separately, and at the same time to effect the necessary transposition by print ing the left-hand side of the negative on the right hand side of the transparency plate, and then to transpose the negative and plate, and print the right end of the negative on the left of the transparency plate. The working of this frame is thus given.* The front of the frame (fig. 441) is of solid wood, with an aperture in the center, through which the printing is done. The frame should be ioi in. long inside, the aperture two inches and seven-eighths wide, and exactly in the middle. In fig. 442 the negative shown by diagonal lines has been placed in the frame film side up, and adjusted with its left side over the aperture. The sensitive transparency plate is shown by horizontal lines, adjusted with its right end over the negative, and the aperture in the frame. The back (fig. 443) is now placed in posi tion, held by the spring and two holders shown in figs. 442 and 444. After exposure the relative
position of negative and plate must be changed, as shown in fig. 444, where the right end of the negative and the left end of the transparency plate are now over the aperture and ready for the second printing When adjusting the negative for the first printing, as in fig 442, observe some particular object—say the edge of a stone, or something of the kind in the foreground that is cut by the left side of the aperture—and by a piece of cardboard to serve for a guide the plate is applied in such a position that the center of it is cut by the left edge of the aperture. The back of the frame is now applied and printing performed. We now remove the sensitive plate, refresh our memory with "the stone in the foreground." and slide the negative to the position, as fig. 444, adjusting the particular " stone in the fore ground " as for the previous printing. Apply the plate by use of the guide, which is now turned to the other end, and if made correctly will produce the two pictures side by side, with a clean cut line between them without any space or overlapping.
In printing from large negatives the printing frame must, of course, be made correspond ingly larger, and provided with suitable guides, enabling various portions of the negative to be printed from.
The plates used for the transparencies should be those specially prepared for that purpose; several excellent kinds are now in the market. When finished the transparency is filled with a paper mask, with openings at 2t centers, and a cover glass. The whole are then bound up to gether with gummed edging paper. To view correctly, the film side must, of course, he nearest to the observer, the cover glass acts as a protector.
Some place a ground glass behind the transparency, but this should be very thin. (See also Transpar ency.
A copying camera for making stereoscopic transparencies from stereoscopic negatives without cutting the negative is shown in fig. 445.
Midway between the ends of the camera is a frame which divides the body of the camera in two parts. A door at the side ( H) gives access to the inside. On a hooded front (B), with a flap shutter, is screwed a pair of 5 or 6 inch lenses. The front carrying the lenses is so arranged that the lenses may, by means of screws (D), be brought nearer to each other or farther If it is intended that the transparency be a little smaller or larger in scale than the negative, the bellows in front and rear of this frame permits the same to be accomplished.