EECENT PROGRESS OF THE THEORY OF VISION. 



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 simplified 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 sensations of 

 colour (the perception of the simple primary tints) can- 

 not be produced by any kind of external light in the 

 natural unfatigued condition of the eye. These ele- 

 mentary 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, there- 

 fore, that as to any correspondence in kind of exter- 

 nal 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 



