ion.

Fig. 5 shows the exact arrangement of the experiment. The figure
represents a horizontal section at the eye-level of the pendulum of
Fig. 4, with accessories. _E_ is the eye which moves between the two
fixation-points _P_ and _P_'. _WONW_ is a wall which conceals the
mechanism of the pendulum from the subject. _ON_ is a rectangular hole
9 cm. wide and 7 cm. high, in this wall. _SS_ is the shield which
swings with the pendulum, and _BB_ is the background (cf. Fig. 4).
When the pendulum is not swinging, a hole in the shield lies behind
_ON_ and exactly corresponds with it. Another in the background does
the same. The eye can thus see straight through to the light _L_.

Each of these three holes has grooves to take an opaque card, _x_,
_y_, or _z_; there are two cards for the three grooves, and they are
pierced with holes to correspond to _i_ and _tt_ of Fig. 4. The
background _BB_ has a second groove to take a piece of milk-glass _M_.
These cards are shown in Fig. 6 (Plate II.) Card _I_ bears a hole 5
cm. high and shaped like a dumb-bell. The diameter of the end-circles
(_e_, _e_) is 1.3 cm., and the width of the handle _h_ is 0.2 cm. Card
_T_ is pierced by two slits _EE_, _EE_, each 9 cm. long and 1.3 cm.
high, which correspond to the two ends of the dumb-bell. These slits
are connected by a perforation _H_, 1.5 cm. wide, which corresponds to
the handle of the dumb-bell. This opening _EEHEE_ is covered by a
piece of ground-glass which serves as a radiating surface for the
light.

[Illustration: Fig. 5.]

The distance _EA_ (Fig. 5) is 56 cm., and _PP_' is 40 cm.; so that the
arc of eye-movement, that is, the angle _PEP_', is very nearly 40 deg.,
of which the 9-cm. opening _ON_ 9 deg. 11'. _SS_ is 2 cm. behind _ON_, and
_BB_ 2 cm. behind _SS_; these distances being left to allow the
pendulum to swing freely.

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 deg. of its arc,
the complete swing being therefore 47 deg.. 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 deg. in 11.98[sigma] (since the 9 cm. are 9 deg. 11'
of eye-movement). This rate is much less than that found by Dodge and
Cline (_op. cit._, p. 155), who give the time