230 RECENT PROGRESS OF THE THEORY OF VISION. 



represented in the sensations of the human eye. The notion of 

 such properties is a contradiction in itself. They cannot possibly 

 exist, and therefore we cannot expect to find any coincidence of 

 our sensations of colour with qualities of light. 



These considerations have natui-ally long ago suggested 

 themselves to thoughtful minds ; they may he found clearly ex- 

 pressed in the writings of Locke and Herbart, 1 and they are 

 completely in accordance with Kant's philosophy. But in former 

 times, they demanded a more than usual power of abstraction 

 in order that their truth should be understood ; whereas now 

 the facts which we have laid before the reader illustrate them 

 in the clearest manner. 



After this excursion into the world of abstract ideas, we 

 return once more to the subject of colour, and will now ex- 

 amine it as a sensible ' sign ' of certain external qualities, either 

 of light itself or of the objects which reflect it. 



It is essential for a good sign to be constant that is, the 

 same sign must always denote the same object. Now we have 

 already seen that in this particular our sensations of colour are 

 imperfect; they are not quite uniform over the entire field of 

 the retina. But the constant movement of the eye supplies 

 this imperfection, in the same way as it makes up for the un- 

 equal sensitiveness of the different parts of the retina to form. 



We have also seen that when the retina becomes tired, the 

 intensity of the impression produced on it rapidly diminishes, 

 but here again the usual effect of the constant movements of the 

 eye is to equalise the fatigue of the various parts, and hence we 

 rarely see after-images. If they appear at all, it is in the case 

 of brilliant objects like very bright flames, or the sun itself. 

 And, so long as the fatigue of the entire retina is uniform, the re- 

 lative brightness and colour of the different objects in sight re- 

 mains almost unchanged, so that the effect of fatigue is gradually 

 to weaken the apparent illumination of the entire field of vision. 



1 Johann Friedrich Herbart, born .1776, died 1841, professor of philosophy 

 at Konigsberp an<l Gottingen, author of Psychologic ah Wissenschaft, neu- 

 yeyruitdet auf Erfahrung, Metaphysik und Mathematik. TR. 



