THE SENSATION OF SIGHT. 233 



We may assume, and the assumption is justified by certain 

 phenomena of contrast, that illumination of the brightest white 

 we can produce, gives a true criterion for judging of the darker 

 objects in the neighbourhood, since, under ordinary circum- 

 suances, the brightness of any proper colour diminishes in pro- 

 portion as the illumination is diminished, or the fatigue of the 

 retina increased. 



This relation holds even for extreme degrees of illumination, 

 so far as the objective intensity of the light is concerned, but 

 not for our sensation. Under illumination so brilliant as to ap- 

 proach what would be blinding, degrees of brightness of light- 

 coloured objects become less and less distinguishable ; and, in 

 the same way, when the illumination is very feeble, we are un- 

 able to appreciate slight differences in the amount of light re- 

 flected by dark objects. The result is that in sunshine local 

 colours of moderate brightness approach the brightest, whereas 

 in moonlight they approach the darkest. The painter utilises 

 this difference in order to represent noonday or midnight scenes, 

 although pictures, which are usually seen in uniform daylight, do 

 not really admit of any difference of brightness approaching that 

 between sunshine and moonlight. To represent the former, he 

 paints the objects of moderate brightness almost as bright as the 

 brightest ; for the latter, he makes them almost as dark as the 

 darkest. 



The effect is assisted by another difference in the sensation 

 produced by the same actual conditions of light and colour. If 

 the brightness of various colours is equally increased, that of 

 red and yellow becomes apparently stronger than that of blue. 

 Thus, if we select a red and a blue paper which appear of the 

 same brightness in ordinary daylight, the red seems much 

 brighter in full sunlight, the blue in moonlight or starlight. 

 This peculiarity in our pei'ception is also made use of by 

 painters ; they make yellow tints predominate when represent- 

 ing landscapes in full sunshine, while every object of a moon- 

 light scene is given a shade of blue. But it is not only local 

 colour which is thus affected ; the same is true of the colours of 

 the spectrum. 



