The history of three-color photography (1925)

568 History of Three-Color Photography them. This fact, however, teaches us that the uncut Autochrome should not be used even though the proper image appears before each eye. The separate images must be reversed in position, yet not in such a way that a left-hand print is brought before the right eye and vice versa. There is, therefore, nothing to do than cut the plate down the middle and to arrange the two plates with their film sides facing the observer, when the correct effect is obtained. The rule may be very simply explained thus : — reverse the positions of the two pictures and view them from the film side. If the two are exchanged, but viewed from the glass side, the effect is stereoscopically correct, but in this case the original is seen reversed as regards right and left, just as one would see the picture in a mirror. In order to understand the principles upon which this rule is based it may be well to consider the case of a very simple object of a diagrammatic kind. Let us assume that we are looking with both eyes at two points, one higher h and the other lower n, Let the higher one be further removed and more to the right than the lower, that is to say, the letter h is higher and further back. If a glass be held some distance from the eyes, the state of things is that shown in Fig. 148. If the left and right eye be alternately shut, it will be seen that for the left eye the lower point appears directly to the right under the higher point on the glass, whilst with the right eye the lower point is shifted to the left from the higher point, Fig. 150. The two images which which we should thus see on a glass plate have the arrangement shown in Fig. 149. The points in a finished stereoscopic print would be arranged in exactly this way in order to give the stereoscopic effect on observation with both eyes. Thus it will be seen that the image seen by the left eye can be distinguished