THE EYE. 



comparing the perception derived from the sense of touch with those 

 lights and shades, we are enabled by experience and long habit 

 to judge of the figure of the object from these lights and shades 

 and tints of colour. It is true that we are not conscious of this 

 act of the understanding in inferring shape from colour and from 

 light and shade ; but the act is nevertheless performed by the 

 mind. The first experience of inference is the comparison of the 

 impressions of sight with the impressions of touch ; and one of the 

 earliest acts of the mind is the inference of the one from the other. 

 It is the character of all mental acts, that their frequent per- 

 formance produces an unconsciousness of them ; and hence it is 

 that when we look at a cube or a sphere of a uniform colour, 

 although the impression upon the sense of sight is that of a nab 

 plane variously shaded, and having a certain outline, the mind 

 instantly substitutes the thing signified for the sign, the cause 

 for the effect ; and the conclusion of the judgment, that the object 

 before us is a sphere or a cube of uniform colour, and not as it 

 appears, a flat plane variously shaded, is so instantaneous, that 

 the act of the mind passes unobserved. 



The whole art of the painter consists in an intimate practical 

 knowledge of the relation between these two effects of perception 

 of sight and touch. The more accurately he is able to delineate 

 upon a flat surface those varieties of light and shade which 

 visible objects immediately produce upon the sense, the more 

 exact will be his delineation, and the greater the vraisemblance 

 of his picture. 



What is called relief in painting is nothing more than the exact 

 representation on a flat surface of the varieties of light and shade 

 produced by a body of determinate figure upon the eye ; and it is 

 accordingly found that the flat surface variously shaded produced 

 by the art of the painter has upon the eye exactly the same effect 

 as the object itself, which is in reality so different from the coloured 

 canvas which represents it. 



95. The immediate impressions received from the sense of sight 

 are those of light and colour. The impressions of distance, mag- 

 nitude, form, and motion are the mixed results of the sense of 

 sight and the experience of touch. Even the power of distinguish- 

 ing colours is not obtained immediately by vision without some 

 cultivation of this sense. The unpractised eye of the new-born 

 infant obtains a general perception of light ; and it is certain that 

 the power of distinguishing colours is only found after the organ 

 has been more or less exercised by the varied impressions produced 

 by different lights upon it. It would not be easy to obtain a 

 summary demonstration of this proposition from the experience; 

 of infancy, but sufficient evidence to establish it is supplied b}- 

 94 





