i;o RESPONSE IN THE LIVING AND NON-LIVING 



CHAPTEK 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 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 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 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 



