260 RECEXT PROGRESS OF THE THEORY OF VISION. 



the composition of light that was the first germ of the 

 modern doctrine of the true functions of the senses ; and 

 in the writings of his contemporary, Locke, were correctly 

 laid down the most important principles on which the 

 right interpretation of sensible qualities depends. But, 

 however clearly we may feel that here lies the difficulty 

 for a large number of people, I have never found the 

 opposite conviction of certainty derived from the senses 

 so distinctly expressed that it is possible to lay hold of 

 the point of error ; and the reason seems to me to lie in 

 the fact that beneath the popular notions on the subject 

 lie other and more fundamentally erroneous concep- 

 tions. 



We must not be led astray by confounding the notions 

 of a phenomenon and an appearance. The colours of 

 objects are phenomena caused by certain real differences 

 in their constitution. They are. according to the scientific 

 as well as to the uninstructed view, no mere appearance, 

 even though the way in which they appear depends chiefly 

 upon the constitution of our nervous system. A ' decep- 

 tive appearance ' is the result of the normal phenomena 

 of one object being confounded with those of another. 

 But the sensation of colour is by no means deceptive 

 appearance. There is no other way in which colour 

 can appear ; so that there is nothing which we could 

 describe as the normal phenomenon, in distinction 

 from the impressions of colour received through the 

 eye. 



Here the principal difficulty seems to me to lie in the 

 notion of quality. All difficulty vanishes as soon as we 

 clearly understand that each quality or property of a thing 

 is, in reality, nothing else but its capability of exercising 

 certain effects upon other things. These actions either 

 go on between similar parts of the same body, and so 

 produce the differences of its aggregate condition ; or 



