ing 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.

We have seen in our silver cell that if the molecular conditions of the
anterior and posterior surfaces were exactly similar, there would be no
current. In practice, however, this is seldom the case. There is,
generally speaking, a slight difference, and a feeble current in the
circuit. It is thus seen that there may be an existing feeble current,
to which the effect of light is added algebraically. The stimulus of
light may thus increase the existing current of darkness (positive
variation). On the cessation of light again, the current of response
disappears and there remains only the feeble original current.

In the case of the retina, also, it is curious to note that on closing
the eye the sensation is not one of absolute darkness, but there is a
general feeble sensation of light, known as 'the intrinsic light of the
retina.' The effect produced by external light is superposed on this
intrinsic light, and certain curious results of this algebraical
summation will be noticed later.

[Illustration: FIG. 110--