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 deg. as 99.9[sigma], which is an average of only 2.49[sigma] 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 deg. 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.

[19] The speed of the pendulum is measured by attaching a tuning-fork of known vibration-rate to the pendulum, and letting it write on smoked paper as the pendulum swings past the 9-cm. opening.

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.

Case 1.  The eye is fixed in the direction EA.  The pendulum is allowed to swing through its 47 deg..  The resulting visual image is shown in Fig. 7:1.  Its shape is of course like T, Fig. 6, but the part H is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure.  Sheets of milk-glass are now dropped into the back groove of BB, until the light is so tempered that part H (Fig. 7:1) is barely but unmistakably visible as luminous.  The intensity actually used by the writer, relative to that of EE, is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the same amount of light would reach the retina, but that it would be concentrated on a horizontally narrower area.  And if the eye moves exactly with the pendulum, the visual image will be no longer like 1 but like 2 (Fig. 7).  We do not as yet know how the intensities of e, e and h will relatively appear.  To ascertain this we must put card I into groove x, and let card T swing with the pendulum in groove y.  If the eye is again fixed in the direction EA (Fig. 5), the retina receives exactly the same stimulation that it would have received before the cards were shifted if it had moved exactly at the rate of the pendulum.  In the experiments described, the handle h of this image (Fig. 7:2) curiously enough appears of the same brightness as the two ends e, e, although, as we know, it is stimulated for a briefer interval.  Nor can any difference between e, e and h be detected in the time of disappearance of their after-images.  These conditions are therefore generous.  The danger is that h of the figure, the only part of the stimulation which could possibly quite elapse during the movement, is still too bright to do so.