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. [Illustration: 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.