(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 making a general retrospect, finally, of the effects produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material. FOOTNOTES: [18] _Proc. Roy. Soc. Edin._, 1873 p. 153. CHAPTER XIX VISUAL ANALOGUES Effect of light of short duration--After-oscillation--Positive and negative after-images--Binocular alternation of vision--Period of alternation modified by physical condition--After-images and their revival--Unconscious visual impression.