170 RESPONSE IN THE LIVING AND NON-LIVING 
CHAPTER XIX 
VISUAL ANALOGUES 
Effect of light of short duration—A fter-oscillation—Positive and negative 
after-images—Binocular alternation of vision—Period of alternation 
modified by physical condition—After-images and their reyival—Un- 
conscious 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 morganic 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 corre- 
sponding 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 
