The combination of stereoscopy and color would seem almost ideal, and should present no particular difficulties to the expert worker in stereoscopy. But there are one or two points that it may be as well to explain. Naturally, if the separate system is adopted and two separate plates are used, no advice is needed, as it is a simple matter then to obtain the right-hand picture for the right eye and the left-hand picture for the left eye. But when we use a long plate on the combined system (and this is the only way to get correct results as regards evenness), the matter is not so simple. It would seem that one might merely turn the plate longways with the glass toward the eyes, and the result should be correct. As a matter of fact, one does get relief; but it is not stereo-scopy, but pseudo-stereoscopy, that is to say, the more distant objects stand out in front of the near ones. It is a curious fact that many people cannot see this until it is pointed out. There is also another point that not all people can see, and that is that the color elements appear on different planes, so that instead of having red and green dots side by side, the red appear farther away. This is particularly apparent when the lenses of the stereoscope magnify, and is primarily due to the want of achromatism of the eye. It can be lessened by using lenses of longer focus.
It will be assumed that the worker knows the ordinary rules of mounting stereoscopic pictures, that the separation of objects should be approximately 82 mm, and the total length of the pictures should be 180 mm. Then it will be found as a rule that part of each picture must be cut off. The cutting of an autochrome plate is not an easy matter until you know how: for if the glass is cut in the usual way and then snapped it is ten chances to one that the screen-film will tear irregularly and the plate be completely spoiled.
The proper way to set to work is to place the picture, film up, on a flat support, and then with a straight edge and a sharp penknife cut right through the gelatine and screen-film down to the glass, about one-sixteenth of an inch on either side of the middle dividing line. Then turn the plate over and with the aid of the straight edge cut the glass. But, as it is not easy to see the dividing line, the best way to go to work is to draw a straight line on a sheet of card or paper, set the straight edge to this and with the diamond draw another line, bearing rather heavily. The cutting point of the diamond will mark the card and give a gauge by which to cut, for if the dividing line be made to coincide with the pencil mark, which must be longer than the width of the plate, the diamond line will be the position of the straight edge. It is advisable to rule the diamond line in pencil also so that it may be seen projecting beyond the edges of the plate. If the cut then be properly made so as to fall between the cuts through the film, the glass may be snapped without the film tearing except at the cuts. The distance of the diamond must be found for each tool, as they differ slightly, but the normal working distance is about 2 mm.
The pictures having been cut and transferred, the film sides should face the observer, and the plates may be stuck down to a glass with a little dab of seccotine or other stickfast at each corner, then masked and a cover glass bound up. Possibly it should be pointed out also that the longer the focus of the camera lenses the better, and that 18 to 25 cm is better than the usual 9 to 15 cm, as the illumination at the margins of the pictures is better, and too large a diaphragm should not be used.