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 deg., of which the 9-cm. opening ON 9 deg. 11’. SS is 2 cm. behind ON, and BB 2 cm. behind SS; these distances being left to allow the pendulum to swing freely.

It is found under these conditions that the natural speed made by the eye in passing the 9-cm. opening ON is very well approximated by the pendulum if the latter is allowed to fall through 23.5 deg. of its arc, the complete swing being therefore 47 deg..  The middle point of the pendulum is then found to move from O to N in 110[sigma][19].  If the eye sweeps from O to N in the same time, it will be moving at an angular velocity of 1 deg. in 11.98[sigma] (since the 9 cm. are 9 deg.