Case 1. The eye is fixed in the direction _EA_. The pendulum is
allowed to swing through its 47°. 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.