Case 2. _T_ is fixed in the background, _I_ on the pendulum, and the
phenomena are observed with the eye moving.

Case 3. A repetition of case 1, to make sure that no different
adaptation or fatigue condition of the eye has come in to modify the
appearance of the two successive phases as at first seen.

The possible appearances to the moving eye are closely analogous to
those in the dumb-bell experiment. If the eye moves too soon or too
late, so that it is at rest during the exposure, the image is like _T_
itself (Fig. 8) but somewhat fainter and localized midway between the
points _P_ and _P'_. If the eye moves reflexly at the rate of the
pendulum, the image is of the shape _i_ and shows the two phases (3
followed by 5). It is localized in the middle and appears to move
across the nine-centimeter opening.

A difficulty is met here which was not found in the case of the
dumb-bell. The eye is very liable to come to a full stop on one of the
colored surfaces, and then to move quickly on again to the final
fixation-point. And this happens contrary to the intention of the
subject, and indeed usually without his knowledge. This stopping is
undoubtedly a reflex process, in which the cerebellar mechanism which
tends to hold the fixation on any bright object, asserts itself over
the voluntary movement and arrests the eye on the not moving red or
green surface as the exposure takes place. A comparable phenomenon was
found sometimes in the experiment with the dumb-bell, where an
eye-movement commenced as voluntary would end as a reflex following of
the pendulum. In the present experiment, until the subject is well
trained, the stopping of the eye must be watched by a second person