at just the rate of the pendulum, it moved _more rapidly_ rather than
more slowly. The image is thus horizontally elongated, by an amount
which varies from the least possible up to 9 cm. (the width of the
opening in _T_), or _even more_. And while the last of the movement
(_O_ to _P_, or _N_ to _P'_), in which the stimulation of _H'_ is
supposed to subside, is indeed executed, it may yet be done so
_rapidly_ that after all _H'_ cannot subside, not even although it is
now less intense by being horizontally spread out (that is, less
concentrated than the vanished _h_ of shape 4). This explanation is
rendered more probable by the very rare appearance of shape 5, which
must certainly emerge if ever the eye were to move more slowly than
the pendulum.

The critical fact is, however, that shape 4 _does_ appear to a trained
subject in about one half the trials--a very satisfactory ratio when
one considers the difficulty of timing the beginning of the movement
and its rate exactly to the pendulum.

Lastly, in some cases no image appears at all. This was at first a
source of perplexity, until it was discovered that the image of the
dumb-bell, made specially small so as to be contained within the area
of distinct vision, could also be contained on the blind-spot. With
the pendulum at rest the eye could be so fixed as to see not even the
slight halo which diffuses in the eye and seems to lie about the
dumb-bell. It may well occur, then, that in a movement the image
happens to fall on the blind-spot and not on the fovea. That this
accounts for the cases where no image appears, is proved by the fact
that if both eyes are used, some image is always seen. A binocular
image under normal convergence can of course not fall on both
blind-spots. It may be further said that the shape 4 appears as well