VISUAL ANALOGUES 171 
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. 
Effect of light of short duration.—If we subject the 
sensitive cell to a flash of radiation, the effect is not 
instantaneous but grows with time. It attains a 
maximum some little time after the incidence of light, 
and the effect then gradually passes away. Again, as 
we have seen previously with regard to mechanical 
strain, the after-effect persists for a slightly longer time 
when the stimulus is stronger. The same is true of the 
after-effect of the stimulus of light. Two curves which 
exhibit this are given below (fig. 110). With regard to 
the first point—that the maximum effect is attained 
some time after the cessation of a short exposure—the 
corresponding experiment on the eye may be made as 
follows: at the end of a tube is fixed a glass disc 
coated with lampblack, on which, by scratching with 
a pin, some words are written in transparent characters. 
