THE SENSATION OF SIGHT. 263 



Moreover, the colour of the ilhimination 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 illu- 

 mination 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 wave 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 importance for the painter, for many of 

 his finest efifects 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 pur- 

 pose, 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 

 alone is the expression of a constant property of the 

 object in question. 



