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.
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.
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° of its arc, the complete swing being therefore 47°. The middle point of the pendulum is then found to move from O to N in 110σ[19]. If the eye sweeps from O to N in the same time, it will be moving at an angular velocity of 1° in 11.98σ (since the 9 cm. are 9° 11' of eye-movement). This rate is much less than that found by Dodge and Cline (op. cit., p. 155), who give the time for an eye-movement of 40° as 99.9σ, which is an average of only 2.49σ to the degree. Voluntary eye-movements, like other voluntary movements, can of course be slow or fast according to conditions. After the pendulum has been swinging for some time, so that its amplitude of movement has fallen below the initial 47° and therewith its speed past the middle point has been diminished, the eye in its movements back and forth between the fixation-points can still catch the after-image of i perfectly distinct and not at all horizontally elongated, as it would have to be if eye and pendulum had not moved just together. It appears from this that certain motives are able to retard the rate of voluntary movements of the eye, even when the distance traversed is constant.
The experiment is now as follows. The room is darkened. Card T is dropped into groove z, while I is put in groove y and swings with the pendulum. One eye alone is used.