MENT, 17. PLATE I.
Fig. 3.
HOLT ON EYE-MOVEMENT.]

To take the first case, Fig. 3:1. The eye fixates the light _L_, then
sweeps 40 deg. toward the right to the point _B'_. The retina is
stimulated throughout the movement, _l-l'_. These conditions yield the
phenomenon of both streaks, appearing as shown on the black rectangle.

In the second case (Fig. 3:2) the wall _W_ is in position and the eye
so adjusted in the eye-rest that the light _L_ is not seen until the
eye has moved about 10 deg. to the right, that is, until the axis of
vision is at _Ex_. Clearly, then, the image of _L_ falls at first a
little to the right of the fovea, and continues in indirect vision to
the end of the movement. The stimulated part of the retina is _l-l'_
(Fig. 3:2). Here, then, we have no stimulation of the eye during the
first part of its movement. The corresponding appearance of the streak
is also shown. Only the correctly localized streak is seen, extending
from the light _L_ toward the right but not quite reaching _B'_. Thus
by cutting out that portion of the stimulation which was given during
the first part of the movement, we have eliminated the whole of the
false image, and the right-hand (foveal) part of the correct image.

Fig. 3:3 shows the reverse case, in which the stimulation is given
only during the first part of the movement. The wall is fixed on the
right of _L_, and the eye so adjusted that _L_ remains in sight until
the axis of vision reaches position _Ex_, that is, until it has moved
about 10 deg.. A short strip of the retina next the fovea is here
stimulated, just the part which in case 2 was not stimulated; and the
part which in case 2 was, is here not stimulated. Now here the false
streak is seen, together with just that portion of the correct streak
which in the previous case was not seen. The latter is relatively dim.

Thus it looks indeed as if the streak given during the first part of
an eye-movement is seen twice and differently localized. But one may
say: The twice-seen portion was in both cases on the fovea; this may
have been the conditioning circumstance, and not the fact of being
given in the early part of the movement.

We must then consider Fig. 3, case 4. Here the eye moves from _B_ to
_B'_, through the same arc of 40 deg.. The wall _W_ is placed so that _L_
cannot be seen until the axis of vision has moved from _EB_ to _EL_,
but _then L_ is seen in direct vision. Its ima