anaesthesia the
after-image on the retina would have faded). This last interpretation
would be entirely in accordance with the observations of
McDougall,[17] who reports some cases in which after-images are
intermittently present to consciousness, and fade during their
eclipse, so that they reappear always noticeably dimmer than when they
disappeared.

[17] McDougall, _Mind_, N.S., X., 1901, p. 55, Observation II.

Now if the event of such an anaesthesia could be established, we should
know at once that it is not a retinal but a central phenomenon. We
should strongly suspect, moreover, that the anaesthesia is not present
during the very first part of the movement. This must be so if the
interpretation of Schwarz is correct, for certainly no part of the
streak could be made before the eye had begun to move; and yet
approximately the first third was seen at once in its original
intensity, before indeed the 'innervation-feelings' had reached
consciousness. Apparently the anaesthesia commences, it at all, after
the eye has accomplished about the first third of its sweep. And
finally, we shall expect to find that movements of the head no less
than movements of the eyes condition the anaesthesia, since neither by
Schwarz nor by the present writer was any difference observed in the
phenomena of falsely localized after-images, between the cases when
the head, and those when the eyes moved.

III. THE PERIMETER-TEST OF DODGE, AND THE LAW OF THE LOCALIZATION OF
AFTER-IMAGES.

We have seen (above, p. 8) how the evidence which Dodge adduces to
disprove the hypothesis of anaesthesia is not conclusive, since,
although an image imprinted on the retina during its movement was
seen, yet nothing showed that it was seen before the eye had come to
rest.

Having convinced himself that there is after all no anaesthesia, Dodge
devised a very ingenious attachment for a perimeter 'to determine just
what is seen during the eye-movement.'[18] The eye was made to move
through a known arc, and during its movement to pass by a very narrow
slit. Behind this slit was an illuminated field which stimulated the
retina. And since only during its movement was the pupil opposite the
slit, so only during the movement could the stimulation be given. In
the first experiments nothing at all of the illuminated field was
seen, and Dodge admits (_ibid._, p. 461) that this fact 'is certainly
suggestive of a central explanation for the absence of bands