_CD_ is a square hole which is closed by the screen _S_ fitted into
the front pair of the grooves _GG_. In the center of _S_ and on a
level with the eye _E_ is a hole _A_, 2 cm. in diameter, which
contains a 'jewel' of red glass. The other two pairs of grooves are
made to hold pieces of milk-or ground-glass, as _M_, which may be
needed to temper the illumination down to the proper intensity. _L_ is
an electric lamp. _B_ and _B'_ are two white beads fixed to the
perimeter at the same level as _E_ and _A_, and used as
fixation-points. Although the room is darkened, these beads catch
enough light to be just visible against the black perimeter, and the
eye is able to move from one to the other, or from _A_ to either one,
with considerable accuracy. They leave a slight after-image streak,
which is, however, incomparably fainter than that left by _A_ (the
streak to be studied), and which is furthermore white while that of
_A_ is bright red. _B_ and _B'_ are adjustable along a scale of
degrees, which is not shown in the figure, so that the arc of
eye-movement is variable at will. _W_ is a thin, opaque, perpendicular
wall extending from _E_ to _C_, that is, standing on a radius of the
perimeter. At _E_ this wall comes to within about 4 mm. of the cornea,
and when the eye is directed toward _B_ the wall conceals the red spot
_A_ from the pupil. _W_ can at will be transferred to the position
_ED_. _A_ is then hidden if the eye looks toward _B'_.

The four conditions of eye-movement to be studied are indicated in
Fig. 3 (Plate 1.). The location of the retinal stimulation is also
shown for each case, as well as the corresponding appearance of the
streaks, their approximate length, and above all their localization.
For the sake of simplicity the refractive effect of the lens and
humors of the eye is not shown, the path of the light-rays being in
each case drawn straight. In all four cases the eye moved without