ON THE RELATION OF OPTICS TO PAINTING. 107 



difference between the impression of very powerful 

 and very feeble illumination. 



When painters wish to represent glowing sunshine, 

 they make all objects almost equally bright, and thus 

 produce with their moderately bright colours the im- 

 pression which the sun's glow makes upon the dazzled 

 ^ye of the observer. If, on the contrary, they wish 

 to represent moonshine, they only indicate the very 

 brightest objects, particularly the reflection of moon- 

 light on shining surfaces, and keep everything so dark 

 as to be almost unrecognisable ; that is to say, they 

 make all dark objects more like the deepest dark 

 which they can produce with their colours, than should 

 be the case in accordance with the true ratio of the 

 luminosities. In both cases they express, by their 

 gradation of the lights, the insensitiveness of the eye 

 for differences of too bright or too feeble lights. If 

 they could employ the colour of the dazzling bright- 

 ness of full sunshine, or of the actual dimness of 

 moonlight, they would not need to represent the 

 gradation of light in their picture other than it is in 

 nature; the picture would then make the same im- 

 pression on the eye as is produced by equal degrees of 

 brightness of actual objects. The alteration in the 

 scale of shade which has been described is necessary 

 because the colours of the picture are seen in the 

 mean brightness of a moderately lighted room, for 



