of light
it is still slowly diminished to a point where it remains tolerably
constant, and on the removal of light there is a sudden increase of the
E.M. power nearly up to its original position.'[18]

(3) I have sometimes obtained the following curious result. On the
incidence of light there is a response, say, upward. On the continuation
of light the response declines to zero and remains at the zero position,
there being no further action during the continuation of stimulus. But
on the cessation or 'break' of light stimulus, there is a response
downwards, followed by the usual recovery. This reminds us of a somewhat
similar responsive action produced by constant electric current on the
muscle. At the moment of 'make' there is a responsive twitch, but
afterwards the muscle remains quiescent during the passage of the
current, but on breaking the current there is seen a second responsive
twitch.

#Resume.#--So we see that the response of the sensitive inorganic cell, to
the stimulus of light, is in every way similar to that of the retina. In
both we have, under normal conditions, a positive variation; in both the
intensity of response up to a certain limit increases with the duration
of illumination; it is affected, in both alike, by temperature; in both
there is comparatively little fatigue; the increase of response with
intensity of stimulus is similar in both; and finally, even in
abnormalities--such as reversal of response, preliminary negative twitch
on commencement, and terminal positive twitch on cessation of
illumination, and decline and reversal under continued action of
light--parallel effects are noticed.

[Illustration: FIG. 109.--CERTAIN AFTER-EFFECTS OF LIGHT]

We may notice here certain curious relations even in these abnormal
responses (fig. 109). If the equilibrium position remain always
constant, then it is easy to understand how, when the rising curve has
attained its maximum, on the cessation of light, recovery should proceed
_downwards_, towards the equilibrium position (fig. 109, _a_). One can
also understand how, after reversal by the continued action of light,
there should be a recovery _upwards_ towards the old equilibrium
position (fig. 109, _b_). What is curious is that in certain cases we
get, on the stoppage of light, a preliminary twitch away from the zero
or equilibrium position, upwards as in (_c_) (compare also fig. 107) and
downwards as in (_d_) (compare also fig. 108 _b_).

In mak