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 anaesthesia 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