RESPONSE-CURVES OF THE SENSITIVE SILVER CELL
Showing greater persistence of after-effect when the stimulus is strong.
(_a_) Short exposure of 2" to light of intensity 1; (_b_) short
exposure of 2" to light nine times as strong.]

#Effect of light of short duration.#--If we subject the sensitive cell to
a flash of radiation, the effect is not instantaneous but grows with
time. It attains a maximum some little time after the incidence of
light, and the effect then gradually passes away. Again, as we have seen
previously with regard to mechanical strain, the after-effect persists
for a slightly longer time when the stimulus is stronger. The same is
true of the after-effect of the stimulus of light. Two curves which
exhibit this are given below (fig. 110). With regard to the first
point--that the maximum effect is attained some time after the cessation
of a short exposure--the corresponding experiment on the eye may be made
as follows: at the end of a tube is fixed a glass disc coated with
lampblack, on which, by scratching with a pin, some words are written in
transparent characters. The length of the tube is so adjusted that the
disc is at the distance of most distinct vision from the end of the tube
applied to the eye. The blackened disc is turned towards a source of
strong light, and a short exposure is given by the release of a
photographic shutter interposed between the disc and the eye. On closing
the eye, immediately after a short exposure, it will at first be found
that there is hardly any well-defined visual sensation; after a short
time, however, the writing on the blackened disc begins to appear in
luminous characters, attains a maximum intensity, and then fades away.
In this case the stimulus is of short duration, the light being cut off
before the maximum effect is attained. The after-effect here is
_positive_, there being no reversal or interval of darkness between the
direct image and the after-image, the one being merely the continuation
of the other. But we shall see, if light is cut off after a maximum
effect is attained by long exposure, that the immediate after-image
would be negative (see below). The relative persistence of after-effect
of lights of different intensities may be shown in the following manner:

If a bold design be traced with magnesium powder on a blackened board
and fired in a dark room, the observer not being acquainted with the
design, the instantaneous flash of light, besides