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.

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