254 RECENT PROGRESS OF THE THEORY OF VISION. 



tional activity; that is to say, removing the source of 

 fatigue. 



The process of fatigue as the result of nervous action, 

 takes place in the eye as well as other organs. When the 

 entire retina becomes tired, as when we spend some time 

 in the open air in brilliant sunshine, it becomes insensible 

 to weaker light, so that if we pass immediately into a 

 dimly lighted room we see nothing at first ; we are blinded, 

 as we call it, by the previous brightness. After a time 

 the eye recovers itself, and at last we are able to see, and 

 even to read, by the same dim light which at first ap- 

 peared complete darkness. 



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 por- 

 tion 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 illumina- 

 tion. What was dark appears bright, and what was 

 bright dark, like the first negative of a photographer. By 

 carefully 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 phe- 

 nomenon 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 uniform 

 surface, corresponding in extent to the surface of the 

 retina which before received the bright light. 



