THE SENSATION" OF SIGHT. 255 



(I may here remark that illuminated sheets of white 

 paper are sufficiently bright to produce this after-image. 

 If we look at much brighter objects — at flames, or at the 

 sun itself — the effect becomes complicated. The strong 

 excitement of the retina does not pass away immediately, 

 but produces a dii'ect or positive after-image, which at 

 first unites with the negative or indirect one pioduced by 

 the fatigue of the retina. Besides this, the effects of the 

 different colours of white light differ both in duration and 

 intensity, so that the after-images become coloured, and 

 the whole phenomenon much more complicated.) 



By means of these after-images it is easy to convince 

 oneself that the impression produced by a bright surface 

 begins to diminish after the first second, and that by the 

 end of a single minute it has lost from a quarter to half 

 of its intensity. The simplest form of experiment for 

 this object is as follows. Cover half of a white sheet of 

 paper with a black one, fix the eye upon some point of 

 the white sheet near the margin of the black, and after 

 30 to 60 seconds draw the black sheet quickly away, 

 without losing sight of the point. The half of the white 

 sheet which is then exposed appears suddenly of the most 

 brilliant brightness ; and thus it becomes apparent how 

 very much the first impression produced by the upper half 

 of the sheet had become blunted and weakened, even in 

 the short time taken by the experiment. And yet, what 

 is also important to remark, the observer does not at all 

 notice this fact, until the contrast brings it before him. 



Lastly, it is possible to produce a partial fatigue of the 

 retina in another way. We may tire it for certain colours 

 only, by exposing either the entire retina, or a portion of 

 it, for a certain time (from half a minute to five minutes) 

 to one and the same colour. According to Young's theory, 

 only one or two kinds of the optic nerve fibres will then 

 be fatigued, those namely which are sensitive to impres- 



