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. 



