THE SENSATION OF SIGHT. 227 



has scarcely any power of appreciating differences of locality. 

 But it is certainly matter for astonishment to anyone who 

 trusts to the direct information of his natural senses, that 

 neither the limits within which the spectrum affects our eyes 

 nor the differences of colour which alone remain as the simpli- 

 fied effect of all the actual differences of light in kind, should 

 have any other demonstrable import than for the sense of sight. 

 Light which is precisely the same to our eyes, may in all other 

 physical and chemical effects be completely different. - Lastly, 

 we find that the unmixed primitive elements of all our sensa- 

 tions of colour (the perception of the simple primary tints) 

 cannot be produced by any kind of external light in the natural 

 unfatigued condition of the eye. These elementary sensations 

 of colour can only be called forth by artificial preparation of the 

 organ, so that, in fact, they only exist as subjective phenomena. 

 We see, therefore, that as to any correspondence in kind of ex- 

 ternal light with the sensations it produces, there is only one 

 bond of connection between them, a bond which at first sight 

 may seem slender enough, but is in fact quite sufficient to lead 

 to an infinite number of most useful applications. This law of 

 correspondence between what is subjective and objective in 

 vision is as follows : 



Similar light produces under like conditions a like sensation 

 of colour. Light which under like conditions excites unlike 

 sensations of colour is dissimilar. 



When two relations correspond to one another in this manner, 

 the one is a sign for the other. Hitherto the notions of a ' sign ' 

 and of an 'image' or representation have not been carefully 

 enough distinguished in the theory of perception; and this 

 seems to me to have been the source of numberless mistakes 

 and false hypotheses. In an ' image ' the representation must 

 be of the same kind as that which is represented. Indeed, it is 

 only so far an image as it is like in kind. A statue is an image 

 of a man, so far as its form reproduces his : even if it is exe- 

 cuted on a smaller scale, every dimension will be represented in 

 proportion. A picture is an image or representation of the 

 original, first because it represents the colours of the latter by 

 Q2 



