Note 453 — Chapter XXVI · [430]

An illusion in principle exactly analogous to that of the patient under discussion can be produced experimentally in anyone in a way which Hering has described in his Lehre von Binocularen Sehen, pp. 13-14. I will quote Helmholtz's account of it, which is especially valuable as coming from a believer in the Innervationsgefühl: "Let the two eyes first look parallel, then let the right eye be closed whilst the left still looks at the infinitely distant object a. The directions of both eyes will thus remain unaltered, and a will be seen in its right place. Now accommodate the left eye for a point f [a needle in Hering's experiment] lying on the optical axis between it and a, only very near. The position of the left eye and its optical axis, as well as the place of the retinal image upon it... are wholly unaltered by this movement. But the consequence is that an apparent movement of the object occurs—a movement towards the left. As soon as we accommodate again for distance the object returns to its old place. Now what alters itself in this experiment is only the position of the closed right eye: its optical axis, when the effort is made to accommodate for the point f, also converges towards this point.... Conversely it is possible for me to make my optical axes diverge, even with closed eyes, so that in the above experiment the right eye should turn far to the right of a. This divergence is but slowly reached, and gives me therefore no illusory movement. But when I suddenly relax my effort to make it, and the right optical axis springs back to the parallel position, I immediately see the object which the left eye fixates shift its position towards the left. Thus not only the position of the seeing eye a, but also that of the closed eye b, influences our judgment of the direction in which the seen object lies. The open eye remaining fixed, and the closed eye moving towards the right or left, the object seen by the open eye appears also to move towards the right or left" (Physiol. Optik, pp. 607-8.)