The conclusion above reported that the visual inhibition during the more rapid phase of the nystagmus in no wise depends on an inadequate stimulation of the retina, due to the greater speed of the rapid movements, and that the inhibitory process is purely central, is further supported by the following phenomenon. If before the rotation has commenced, the eyes are so strongly stimulated that a lasting after-image is obtained, this after-image will, during the rotation, always be seen to swim in the direction opposite to the rotation, that is, with the slow eye-movements; but when the rate of rotation begins to decrease, and as Mach, Breuer, and Delage have shown, the slow eye-movements reverse their direction, the after-image also reverses its direction, and now swims in the direction of rotation, that is, still with the slow eye-movements. If the after-image persists long enough, it may still be observed, after the rotation has ceased, swimming in the same direction as the surviving slow eye-movements. If, for instance, the slow movements are from left to right, the after-image (best seen with the eyes closed) swims from the left to the right hand side of the field and disappears, reappears at the left and swims again toward the right, and continues to do this until the nystagmus entirely ceases.

This experiment was repeated several times, with four subjects, and with both clockwise and anti-clockwise rotations, and the results were uniformly as described above. In order to see whether this motion of the after-image really depended on the slower nystagmiform movements, the following variation was tried. It will be recalled that if the head is rotated not about a vertical (longitudinal) axis, but about a transverse axis, as, say, one passing through the ears, a nystagmus is produced in which during the rotation the slower eye-movements are opposite to the direction of rotation, while when the rotation is checked or stopped, the nystagmus, as before, reverses. The same is true if the rotation is about a sagittal axis. These conditions were approximately realized by having the subject sit as before on the rotary chair, but during the rotation hold his head horizontally to the right or left, forward or back. With any of these positions of the head, however, the rotation produced, on all of the subjects tried, extreme dizziness and a feeling of nausea that lasted in some cases for several hours. This fact made it impossible to ask for a set of the four possible positions of the head from any of the subjects. The following are the records that were obtained:

Subject Fl.Head horizontally to left; rot. anti-clockwise.
During rot.; after-im. moved clockwise, i. e., from subject's brow to chin.
Eye-mov. not observable during rot.
After rot.; after-im. moved anti-clockwise, chin to brow.
Slow eye-mov. anti-clockwise, chin to brow.
Vis. field clockwise, brow to chin.
Subject H.Head horizontally to right; rot. anti-clockwise.
During rot.; after-im. clockwise, chin to brow.
Eye-mov. not observable.
After rot.; after-im. anti-clockwise, brow to chin.
Slow eye-mov. anti-clockwise, brow to chin.
Vis. field clockwise, chin to brow.
Subject H.Same repeated, with same results.
Subject H.Same as case of Fl., with identical results.
Subject K.Head horizontally to left; rot. anti-clockwise.
During rot.; after-im. not observed.
After rot.; after-im. anti-clockwise, chin to brow.
Slow eye-mov. anti-clockwise, chin to brow.
Vis. field not observed.

So far as these records go, they entirely confirm the results of other investigators as to the direction and the reversal of the nystagmus. In each of the cases the after-image moved with the slow eye-movements, reversing its direction with these slow movements, while the visual field whenever it was observed (the eyes were kept closed during the rotation) moved in the opposite direction to that of the after-image and the slow eye-movement. It is well known that after-images move with every involuntary eye-movement, and although they disappear during voluntary eye-jumps,[21] they reappear at the end of the jump in a position that is related to the new fixation-point exactly as the old position was to the former fixation-point. These after-images, then, are seen during the slow eye-movements whose direction they follow; but are not seen during the quick movements, when they must naturally move in the direction of these quick movements. And aside from this it is possible to observe introspectively that the after-image disappears at that side of the visual field toward which the slow eye-movements tend, and is for a moment invisible before it reappears on the other side of the field. As was shown above, the visual field always moves opposite to the direction of the slow eye-movements, as must of course be the case if there is no inhibition of vision during these movements. The simultaneous appearance of the after-image moving with, and the rest of the visual field moving contrary to, the direction of the slow eye-movements, with a uniform absence of the converse phenomena, seems to prove that vision is unimpaired during these slow movements, while it is completely inhibited during the rapid phases of the nystagmus.

Purkinje himself[22] called the slower phases "involuntary and unconscious," meaning by "unconscious" not that the visual field was not seen (for it just then is seen), but that the movement of the eyeball during the slow phases was not felt. I have observed, with the confirmation of several subjects, that this movement can also not voluntarily be inhibited; whereas the swift movement is so far voluntary that it can be inhibited at pleasure. It is possible, that is, to fix the eyes on that side of the field toward which the slow movements are directed, but not on any point at the other side of the field. The slow movements, then, during which vision is possible, are purely reflex. These slow movements, purely reflex and yielding clear vision, with the rapid movements, partly under voluntary control and attended by an inhibition of vision, present a parallelism, that may be not without significance, to the "pursuit" eye-movements (Dodge's "second type"), that are likewise relatively slow, are reflex, and yield remarkably clear vision, and the ordinary voluntary eye-jumps (Dodge's "first type"), that are relatively rapid, and are, like the rapid nystagmiform movements, attended by a central inhibition of vision.

VISUAL IRRADIATION

BY FOSTER PARTRIDGE BOSWELL

There are various kinds of visual irradiation, of which perhaps the best-known variety is that which appears as the enlargement of a brightly illuminated surface at the expense of a contiguous one of less intensity. This has been until recently the only form recognized, and until very lately the greater part of the literature has dealt with it alone.