LAUSR

In an essay entitled “Contributions to the Physiology of Vision” in which he announces his invention of the stereoscope, Charles Wheatstone denies the theory of the identical points of the retinas to be true and seeks to demonstrate its invalidity by means of a proof that proceeds as follows: We can put an object that is not a sphere into such a position that unequal images of it fall on the two retinas and, nevertheless, we see the object single when we fixate it with both eyes. So we abstract from two dissimilar images, which therefore lie only partially on identical points of the retina, a single visual phenomenon that does not correspond to either of the visual phenomena generated by the retinal images produced by each individual eye. Consequently, it is untrue that only what is imaged on identical points of the retinas can be seen single. There are such good reasons for the theory of the identical points of the retinas, and from it arise such important conclusions for the theory of vision, that it is of the greatest interest to investigate how it can be defended against Wheatstone’s attack. First of all, it is to be observed that we can never perceive an object with the eyes, but only a point of it, for the direction of vision of each eye is determined by a straight line and that can, consequently, only intersect that of the other eye at a point. If I now fixate a point of an object, I see this point single and also all points that lie on the horopter (which expression I use not in the wrong sense of Aguilonius but in the correct sense of Müller) or are so close to it that the difference between the relative positions of their retinal images is imperceptible to the eye; all other points I see double. Apparently, Wheatstone sees things differently and there must be a reason for that. But this reason, as will become clearer later, is none other than that he has not acquired the ability to fixate with sufficient consistency. In order to convince one that this really requires a skill acquired only through persevering practice, I must draw attention to the difficulties involved. As everyone knows, it is in itself painful for the eyes to fixate one point for a long time. Moreover, if we fixate one point in order to see a second double, we are continually seduced by this second, since it is evidently also an object of our attention, to look at it and thereby change the convergence of our visual axes. If one considers that two visual distances (translator: distance of the plane of fixation) relate to each other like the cotangents of the half angles of convergence of the visual axes and, thus, even a small change of the angle of convergence suffices to change the visual distance significantly, it will no longer seem unnatural that our visual distance can change without us paying attention to it. How we become aware of it or, rather, if you take the paradox of the expression to my advantage, how we become aware of the unconscious awareness of the degree of convergence of our visual axes, will be shown later. By this I think I have explained how it is possible for me to see double under the same external conditions under which Wheatstone saw only single. Another factor, the most important one, which makes involuntary double vision more difficult, will become clear later. Next, we should examine how we perceive an object with our visual sense as a single one, respecting the theory of the identical points of the retinas. Here, I may be allowed to explain in a few words what I consider to be the correct theory on