128 ON THE EELATION OF OPTICS TO PAINTING. 



any tremulousness, any rising or falling in it is at 

 once more distinctly recognised by the hearer than 

 could be the case with a less regular sound ; and it 

 seems also that the powerful excitation which it pro- 

 duces in the ear of the listener, arouses trains of ideas 

 and passions more strongly than does a feebler excita- 

 tion of the same kind. A pure, fundamental colour 

 bears to small admixtures the same relation as a dark 

 ground on which the slightest shade of light is visible. 

 Any of the ladies present will have known how sensi- 

 tive clothes of uniform saturated shades are to dirt, 

 in comparison with grey or greyish-brown materials. 

 This also corresponds to the conclusions from Young's 

 theory of colours. According to this theory, the per- 

 ception of each of the three fundamental colours 

 arises from the excitation of only one kind of sensitive 

 fibres, while the two others are at rest ; or at any rate 

 are but feebly excited. A brilliant, pure colour pro- 

 duces a powerful stimulus, and yet, at the same time, 

 a great degree of sensitiveness to the admixture of 

 other colours, in those systems of nerve-fibres which 

 are at rest. The modelling of a coloured surface 

 mainly depends upon the reflection of light of other 

 colours which falls upon them from without. It is 

 more particularly when the material glistens that the 

 reflections of the bright places are preferably of the 

 colour of the incident light. In the depth of the 



