being too quick for
detailed observation, is obscured by the accompanying smoke. But if the
eyes be closed immediately after the flash, the feebler obscuring
sensation of smoke will first disappear, and will leave clear the more
persistent after-sensation of the design, which can then be read
distinctly. In this manner I have often been able to see distinctly, on
closing the eyes, extremely brief phenomena of light which could not
otherwise have been observed, owing either to their excessive rapidity
or to their dazzling character.[19]

#After-oscillation.#--In the case of the sensitive silver cell, we have
seen (fig. 105), when it has been subjected for some time to strong
light, that the current of response attains a maximum, and that on the
stoppage of the stimulus there is an immediate rebound towards recovery.
In this rebound there may be an over-shooting of the equilibrium
position, and an after-oscillation is thus produced.

If there has been a feeble initial current, this oscillatory
after-current, by algebraical summation, will cause the current in the
circuit to be alternately weaker and stronger than the initial current.

#Visual recurrence.#--Translated into the visual circuit, this would mean
an alternating series of after-images. On the cessation of light of
strong intensity and long duration, the immediate effect would be a
negative rebound, unlike the positive after-effect which followed on a
short exposure.

The next rebound is positive, giving rise to a sensation of brightness.
This will go on in a recurrent series.

If we look for some time at a very bright object, preferably with one
eye, on closing the eye there is an immediate dark sensation followed by
a sensation of light. These go on alternating and give rise to the
phenomena of recurrent vision. With the eyes closed, the positive or
luminous phases are the more prominent.

This phenomenon may be observed in a somewhat different manner. After
staring at a bright light we may look towards a well-lighted wall. The
dark phases will now become the more noticeable.

If, however, we look towards a dimly lighted wall, both the dark and
bright phases will be noticed alternately.

The negative effect is usually explained as due to fatigue. That
position of the retina affected by light is supposed to be 'tired,' and
a negative image to be formed in consequence of exhaustion. By this
exhaustion is meant either the presence of fatigue-stuffs, or t