About the stereoscopic phenomena and Wheatstone's attack on the theory of the identical points of the retinas: Part 2.

From these general objections [as discussed in the first part of the translation] I proceed to the experiments which Wheatstone adduces as evidence against the basic idea of this theory [that the visual impression of an object is not single and not absolutely instantaneous but rather the aggregate of many impressions that come to the sense one after the other], noting that, until now, he has only drawn negative conclusions from them and I therefore will have completed my task if I confirm that his experiments offer no valid proof against the above theory. If I repeat Wheatstone’s experiment, in which two different letters are presented to the two eyes in two equal circles, which are so positioned toward the eyes that they are imaged on identical points of the retina, I first see the two letters intertwined, but weaker than the surrounding circle, then the alternation described by Wheatstone begins, in which one letter becomes visible with more strength, the other pales by the same measure, first disappears bit by bit, then completely, then it reappears while the other begins to dwindle, etc. This change, as Wheatstone also remarks, is independent of the will, it occurs without it and cannot be modified by it. This experiment arouses a peculiarly unpleasant feeling in me, and the incipient alternation after the previous presence of both images apparently indicates that our visual sense reacts to double images as to a non-homogeneous stimulus and, if I may put it that way, tries to change the double vision into single vision, and actually completely independently of the will. Only to determine this fact, I have performed the above experiment. It cannot be counted among those with which Wheatstone fights against the theory of the identical points of the retinas. The essence of Wheatstone’s stereoscopic experiments, strictly speaking, is that he put an object in perspective twice at the same distance, in the same position relative to the baseline and horizon, but once at 1.25 inches (half the distance of the crossing points of the visual rays in both eyes, which according to him is 2.5 inches) to the left, the other at 1.25 inches to the right of the vertical line through the point of the eye. In other words, he drew the object as it appeared to him perceived only with the right eye at the given distance, the other perceived only with the left eye. He presented these two drawings to the eyes with two mirrors assembled at an angle in such a way that the right eye saw only the image of the drawing corresponding to the left, the left eye only that of the drawing corresponding to the right (so that each eye saw the image of its corresponding view of the object) and that the two images overlapped in the visual field. Under such conditions one no longer sees the reflections of the two drawings, but rather the drawn object in relief. Applying now the above theory to this phenomenon, it is evident that when the variations in visual distance shown to be present are as large as the distance from the horopter for the most distant of the visible points of the object drawn, up to the nearest point of the object, all corresponding points of the images are projected once on identical points of the retinas, and we thereby see single for this reason as if the object were actually standing in front of us. This necessary change in visual distance