Case 1. The eye is fixed in the direction EA. The pendulum is allowed to swing through its 47°. The resulting visual image is shown in Fig. 7:1. Its shape is of course like T, Fig. 6, but the part H is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure. Sheets of milk-glass are now dropped into the back groove of BB, until the light is so tempered that part H (Fig. 7:1) is barely but unmistakably visible as luminous. The intensity actually used by the writer, relative to that of EE, is fairly shown in the figure. (See Plate III.)
It is clear, if the eye were now to move with the pendulum, that the same amount of light would reach the retina, but that it would be concentrated on a horizontally narrower area. And if the eye moves exactly with the pendulum, the visual image will be no longer like 1 but like 2 (Fig. 7). We do not as yet know how the intensities of e, e and h will relatively appear. To ascertain this we must put card I into groove x, and let card T swing with the pendulum in groove y. If the eye is again fixed in the direction EA (Fig. 5), the retina receives exactly the same stimulation that it would have received before the cards were shifted if it had moved exactly at the rate of the pendulum. In the experiments described, the handle h of this image (Fig. 7:2) curiously enough appears of the same brightness as the two ends e, e, although, as we know, it is stimulated for a briefer interval. Nor can any difference between e, e and h be detected in the time of disappearance of their after-images. These conditions are therefore generous. The danger is that h of the figure, the only part of the stimulation which could possibly quite elapse during the movement, is still too bright to do so.
Case 2. The cards are replaced in their first positions, T in groove z, I in groove y which swings. The subject is now asked to make voluntary eye-sweeps from P to P' and back, timing his moment of starting so as to bring his axis of vision on to the near side of opening ON at approximately the same time as the pendulum brings I on the same point. This is a delicate matter and requires practice. Even then it would be impossible, if the subject were not allowed to get the rhythm of the pendulum before passing judgment on the after-images. The pendulum used gives a slight click at each end of its swing, and from the rhythm of this the subject is soon able to time the innervation of his eye so that the exposure coincides with the middle of the eye-movement.
Psychological Review. Monograph Supplement, 17. Plate III.
Fig. 7.
HOLT ON EYE-MOVEMENT.
It is true that with every swing the pendulum moves more slowly past ON, and the period of exposure is lengthened. This, however, only tends to make the retinal image brighter, so that its disappearance during an anæsthesia would be so much the less likely. The pendulum may therefore be allowed to 'run down' until its swing is too slow for the eye to move with it, that is, too slow for a distinct, non-elongated image of i to be caught in transit on the retina.
With these eye-movements, the possible appearances are of two classes, according to the localization of the after-image. The image is localized either at A (Fig. 5), or at the final fixation-point (P or P', according to the direction of the movement). Localized at A, the image may be seen in either of two shapes. First, it may be identical with 1, Fig. 7. It is seen somewhat peripherally, judgment of indirect vision, and is correctly localized at A. When the subject's eye is watched, it is found that in this case it moved either too soon or too late, so that when the exposure was made, the eye was resting quietly on one of the fixation-points and so naturally received the same image as in case 1, except that now it lies in indirect vision, the eye being directed not toward A (as in case 1) but towards either P or P'.
Second, the image correctly localized may be like 2 (Fig. 7), and then it is seen to move past the opening ON. The handle h looks as bright as e, e. This appearance once obtained generally recurs with each successive swing of the pendulum, and scrutiny of the subject's eye shows it to be moving, not by separate voluntary innervations from P to P' and then from P' to P, but continuously back and forth with the swing of the pendulum, much as the eye of a child passively follows a moving candle. This movement is purely reflex,[20] governed probably by cerebellar centers. It seems to consist in a rapid succession of small reflex innervations, and is very different from the type of movement in which one definite innervation carries the eye through its 42°, and which yielded the phenomena with the perimeter. A subject under the spell of this reflex must be exercised in innervating his eye to move from P to P' and back in single, rapid leaps. For this, the pendulum is to be motionless and the eye is not to be stimulated during its movement.
These two cases in which the image is localized midway between P and P' interest us no further. Localized on the final fixation-point, the image is always felt to flash out suddenly in situ, just as in the case of the 'correctly localized' after-image streaks in the experiments with the perimeter. The image appears in one of four shapes, Fig. 7: 2 or 3, 4 or 5.