224 RECENT PROGRESS OF THE THEORY OF VISION. 



It is thus that fatigue of the entire retina shows itself. 

 But it is possible for separate parts of that membrane to become 

 exhausted, if they alone have received a strong light. If we 

 look steadily for some time at any bright object, surrounded by 

 a dark background it is necessary to look steadily in order that 

 the image may remain quiet upon the retina, and thus fatigue 

 a sharply defined portion of its surface and afterwards turn 

 our eyes upon a uniform dark-grey surface, we see projected 

 upon it an after-image of the bright object we were looking at 

 just before, with the same outline but with reversed illumin- 

 ation. What was dark appears bright, and what was bright 

 dark, like the first negative of a photographer. By care- 

 fully fixing the attention, it is possible to produce very elaborate 

 after-images, so much so that occasionally even printing can 

 be distinguished in them. This phenomenon is the result of 

 a local fatigue of the retina. Those parts of the membrane 

 upon which the bright light fell before, are now less sensitive to 

 the light of the dark-grey background than the neighbouring 

 regions, and there now appears a dark spot upon the really uni- 

 form surface, corresponding in extent to the surface of the retina 

 which before received the bright light. 



( 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 direct or positive 

 after-image, which at first unites with the negative or indirect 

 one produced 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 di- 

 minish 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 



