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°,
of which the 9-cm. opening _ON_ 9° 11'. _SS_ is 2 cm. behind _ON_, and
_BB_ 2 cm. behind _SS_; these distances being left to allow the
pendulum to swing freely.