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.
We have already referred to the electrical theory of the visual impulse. We have seen how a flash of light causes a transitory electric impulse not only in the retina, but also in its inorganic substitute. Light thus produces not only a visual but also an electrical impulse, and it is not improbable that the two may be identical. Again, varying intensities of light give rise to corresponding intensities of current, and the curves which represent the relation between the increasing stimulus and the increasing response have a general agreement with the corresponding curve of visual sensation. In the present chapter we shall see how this electrical theory not only explains in a simple manner ordinary visual phenomena, but is also deeply suggestive with regard to others which are very obscure.