THE SENSATION OF SIGHT. 231 



This brings us to consider the differences in the pictures 

 presented by the eye, which depend on different degrees of illu- 

 mination. Here again we meet with instructive 'facts. We 

 look at external objects under light of very different intensity, 

 varying from the most dazzling sunshine to the pale beams of 

 the moon ; and the light of the full moon is 150,000 times less 

 than that of the sun. 



Moreover, the colour of the illumination may vary greatly. 

 Thus, we sometimes employ artificial light, and this is always 

 more or less orange in colour ; or the natural daylight is altered, 

 as we see it in the green shade of an arbour, or in a room with 

 coloured carpets and curtains. As the brightness and the colour 

 of the illumination changes, so of course will the brightness and 

 colour of the light which the illuminated objects reflect to our 

 eyes, since all differences in local colour depend upon different 

 bodies reflecting and absorbing various proportions of the 

 several rays of the sun. Cinnabar reflects the rays of great 

 length without any obvious loss, while it absorbs almost the 

 whole of the other rays. Accordingly, this substance appears 

 of the same red colour as the beams which it throws back into 

 the eye. If it is illuminated with light of some other colour, 

 without any mixture of red, it appears almost black. 



These observations teach what we find confirmed by daily 

 experience in a hundred ways, that the apparent colour and 

 brightness of illuminated objects varies with the colour and 

 brightness of the illumination. This is a fact of the first im- 

 portance for the painter, for many of his finest effects depend 

 on it. 



But what is most important practically is for us to be able 

 to recognise surrounding objects when we see them : it is only 

 seldom that, for some artistic or scientific purpose, we turn our 

 attention to the way in which they are illuminated. Now what 

 is constant in the colour of an object is not the brightness and 

 colour of the light which it reflects, but the relation between 

 the intensity of the different coloured constituents of this light, 

 on the one hand, and that of the corresponding constituents of 

 the light which illuminates it on the other. This proportion 



