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Stereoscopic Pictures with Autochromes 567
Brewster3 had described the depth arrangement in 1848 and 1851. The explanation of the phenomenon is that actually each eye looks through a prism, the refractive angle of which is towards the temples of the observer. If any dark spot on a white ground is examined with one eye through a prism, held in the same way, the image will be seen displaced, and surrounded by colored fringes, as the result of the dispersion of the rays from the white ground, and a blue fringe lies on the nasal side and a red fringe on the temporal. If the spots on a dark ground be observed \vith the naked eye, the red spots will appear more distant than the blue ones, and this is due to the non-achromatism of the eye.
R. Luther4 said that it is advisable to use long focus lenses for the stereoscope, so as to avoid as far as possible enlargement of the grain, and that longer focus lenses should be used in the camera, because while the exposure is made through the glass of a screen-plate, a stereoscopic picture is examined from the film side, and, therefore, towards the sides of the picture especially, parallax may come into play, and the portion of the picture which is actually red may appear of another color, due to the angle at which the colored element is looked at. Though it is actually an open question whether parallax comes into play with screen-plates, with which the emulsion is coated on the elements.
Luther gave such explicit directions for obtaining stereo Autochromes, that his remarks are given in extenso : Anyone who has prepared an Autochrome stereoscopic transparency must have asked himself the question, from which side, film or glass, should the stereogram be viewed? Should the picture be reversed as usual in stereoscopic work, or can the undivided plate be employed? These doubts arise from the fact that the Autochrome differs from the ordinary transparency. The plate is exposed through the glass and afterwards converted directly into a positive, a process which must be considered when using the plates for stereoscopy. Every beginner knows that in making an ordinary stereo transparency the picture taken with the left-hand lens must be viewed with the left eye, and vice versa. Nothing seems easier, therefore, than doing this with Autochromes made on a single plate.
Without cutting the plate, that is, without exchanging the right-hand and left-hand print, the pictures are brought into position by turning the plate longways. The right-hand print is then before the right eye, and the left-hand print before the left eye, and the glass side of the transparency is towards the observer. If such an Autochrome is observed in the ordinary stereoscope, solidity is seen, provided, of course, that the separation of the lenses is considerably greater than that of the taking lenses. Yet the effect is not stereoscopic, but pseudoscopic. In reality the more distant objects appear nearest, whilst those really nearest appear more remote. It is a curious fact that many people can not distinguish this pseudostereoscopy from the natural effect until the difference is pointed out to