[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH SUPPLEMENT, 17. PLATE PLATE II. Fig. 4. Fig. 6. HOLT ON EYE-MOVEMENT.] Thus much of preliminary discussion to show how, by means of a pendulum, identical stimulations can be given to the moving and to the resting eye. We return to the problem. It is to find out whether a stimulation given during an eye-movement can be perceived if its after-image is so brief as wholly to elapse before the end of the movement. If a period of anæsthesia is to be demonstrated, two observations must be made. First, that the stimulation is bright enough to be _unmistakably visible_ when given to the eye at rest; second, that it is not visible when given to the moving eye. Hence, we shall have three cases. Case 1. A control, in which the stimulation is proved intense enough to be seen by the eye at rest. Case 2. In which the same stimulation is given to the eye during movement. Case 3. Another control, to make sure that no change in the adaptation or fatigue of the eye has intervened during the experiments to render the eye insensible to the stimulation. Fig. 5 shows the exact arrangement of the experiment. The figure represents a horizontal section at the eye-level of the pendulum of Fig. 4, with accessories. _E_ is the eye which moves between the two fixation-points _P_ and _P_'. _WONW_ is a wall which conceals the mechanism of the pendulum from the subject. _ON_ is a rectangular hole 9 cm. wide and 7 cm. high, in this wall. _SS_ is the shield which swings with the pendulum, and _BB_ is the background (cf. Fig. 4). When the pendulum is not swinging, a hole in the shield lies behind _ON_ and exactly corresponds with it. Another in the background does the same. The eye can thus see straight through to the light _L_. Each of these three holes has grooves to take an opaque card, _x_, _y_, or _z_; there are two cards for the three grooves, and they are pierced with holes to correspond to _i_ and _tt_ of Fig. 4. The background _BB_ has a second groove to take a piece of milk-glass _M_. These cards are shown in Fig. 6 (Plate II.) Card _I_ bears a hole 5 cm. high and shaped like a dumb-bell. The diameter of the end-circles (_e_, _e_) is 1.3 cm., and the width of the handle _h_ is 0.2 cm. Card _T_ is pierced by two slits _EE_, _EE_, each 9 cm. long and 1.3 cm. high, which correspond to the two ends of the dumb-bell. These slits are connected by a perforation _H_, 1.5 cm. wide, which corresponds to the handle of the dumb-bell. This opening _EEHEE_ is covered by a piece of ground-glass which serves as a radiating surface for the light. [Illustration: Fig. 5.] The distance _EA_ (Fig. 5) is 56 cm., and _PP_' is 40 cm.; so that the arc of eye-movement, that is, the angle _PEP_', is very nearly 40°, of which the 9-cm. opening _ON_ 9° 11'. _SS_ is 2 cm. behind _ON_, and _BB_ 2 cm. behind _SS_; these distances being left to allow the pendulum to swing freely.