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 110[sigma][19]. If
the eye sweeps from _O_ to _N_ in the same time, it will be moving at
an angular velocity of 1° in 11.98[sigma] (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 99.9[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° 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.