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Stereoscopic Pictures with Autochromes 571
to make use of the least degree of enlargement, that is to say, to choose lenses for the stereoscope of long focus, and in order to retain the geometrical truth of the stereoscopic effect, it is likewise advisable to choose a pair of lenses for the camera of 18 to 25 cm., in place of 9 to 15 cm., which are usually employed.
This choice of longer focal length for all the lenses has a further advantage, which is connected with the character of the screen-plate, and does not apply to ordinary slides. This lies in the fact that while the exposure is made through the glass, the transparency is viewed from the film side. The direction, therefore, in which oblique rays fall upon the elements and emulsion during exposure, does not correspond with that in which the rays pass through the film when observed in the stereoscope. In Fig. 155 an exaggerated representation is given of the marginal portion of a screen-plate transparency, in which oblique rays are falling on the plate. It is seen that the portion which appears actually red, may appear green to the eye. This difficulty is gotten over by looking at the positive through the glass but the picture is then reversed.
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Fig. 155 (Page 575).
There is one point in connection with the cutting of screen-plates, and that is the very great possibility of the film splitting from the glass, if the latter be cut in the usual manner, that is to say with the diamond on the glass. The method of avoiding this was suggested by Garel.5 Place the plate, film up on the table and with a sharp knife cut right through the film down to the glass, making two such incisions about one-eighth of an inch apart and equidistant from the medial line of the plate. To ensure the diamond cutting exactly between the two incisions, a cardboard gauge should be used, which can be easily made by drawing a line on a sheet of smooth card, equal in length to the base line of the plate. Near the center, at right angles to the base line of the plate, should be drawn two lines, the distance between them being equal to the cut of the diamond. This, as a rule, varies slightly, but it is extremely easy to find, by placing a foot rule, or straight edge, on the card drawing a pencil line as closely as possible