Page 251 - Wilhelm Wundt zum siebzigsten Geburtstage

Fluctuations of Attention and After-images. 239

spots, as is usually tlie case. The resemblance, of course, becomes
closer when the slit is given a circular or semi-circular form.
The first series of experiments had for its object the study of
the retinal conditions at the moment of disappearance. The method
adopted was as follows: at a signal from the experimenter, the ob-
server fixated the luminous band upon the plaque, allowed it to
vanish and return, and finally, as it disappeared for the second time,
released the screen from the supporting magnet. The second phase
of invisibility was selected because it is much freer than the first
from complications due to the effort at adapting and accommodating
the eye for a portion of the field whieh is but slightly different from
the rest. Any of the subsequent phases in a series might be chosen,
but allowance should be made for the (presumably) progressive change
in the condition of the retina. Whether this change takes place at
the same rate in each and every series, is not yet determined.
Immediately after the fall of the screen, an after-image of the
band appears in the place of the band upon the plaque, the rest of
the field remaining apparently unchanged. The characteristics of the
after-image may be noted here: a) for a luminous band it is dark,
for a shadow-band it is bright, and for a spot or band of colored
light it appears in the complementary color; b) it is strongest when
it first appears and diminishes gradually until it can no longer be
discemed against the white back-ground; c) there is no sign of fluc-
tuation in the after-image itself, that is to say, it does not retum
once it has disappeared.
The essential feature lies in the fact that when the Stimulus,
which has ceased to be visible, is cut off, the after-image appears.
This suggests, on the general theory of the after-image, that the dis-
appearance of the primary Stimulus, whatever be its cause, is ac-
companied by a decided alteration in the retinal condition, and, since
the after-image is negative or complementary, it would indicate that
the retina, at the moment of disappearance, is exliausted or fatigued.
Under these conditions of the visual organ, the Stimulus, though it
persist, fails of its effect.
It must, however, be granted that the appearance of the after-
image at this one point, that is, at the moment when the Stimulus
has just vanished, does not of itself fumish a parallel to the whole