THE SENSATION OF SIGHT. 259 



executed on a smaller scalej 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 similar colours, secondly be- 

 cause it represents a part of its relations in space— those, 

 namely, which belong to perspective — by corresponding 

 relations in space. 



Functional cerebral activity and the mental conceptions 

 which go with it may be ' images ' of actual occurrences 

 in the outer world, so far as the former represent the 

 sequence in time of the latter, so far as they represent 

 likeness of objects by likeness of signs — that is, a regular 

 arrangement by a regular arrangement. 



This is obviously sufficient to enable the understanding 

 to deduce what is constant from the varied chanoes of 

 the external world, and to formulate it as a notion or a 

 law. That it is also sufficient for all practical purposes 

 we shall see in the next chapter. But not only un- 

 educated persons, who are accustomed to trust blindly to 

 their senses, even the educated, who know tliat their 

 senses may be deceived, are inclined to demur to so com- 

 plete a want of any closer correspondence in kind between 

 actual objects and the sensations they produce than the 

 law I have just expounded. For instance, natural philo- 

 sophers long hesitated to admit the identity of the rays 

 of light and of heat, and exhausted all possible means of 

 escaping a conclusion which seemed to contradict the 

 evidence of their senses. 



Another example is that of Groethe, as I have en- 

 deavoured to show elsewhere. He was led to contradict 

 Newton's theory of colours, because he could not persuade 

 himself that white, which appears to our sensation as the 

 purest manifestation of the brightest light, could be com- 

 posed of darker colours. It was Newton's discovery of 



