THE SENSATION OF SIGHT. 265 



then be impossible to tell that there is any artificial il- 

 lumination at all. The grey looks white. 1 



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 circumstances, the brightness of any proper 

 colour diminishes in proportion as the illumination is 

 diminished, or the fatigue of the retina increased. 



This relation holds even for extreme degrees of illu- 

 mination, so far as the objective intensity of the light is 

 concerned, but not for our sensation. Under illumination 

 so brilliant as to approach 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 unable to appreciate 

 slight differences in the amount of light reflected 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 uni- 

 form daylight, do not really admit of any difference of 

 brightness approaching that between sunshine and moon- 

 light. 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 sen- 

 sation produced by the same actual conditions of light and 

 colour. If the brightness of various colours is equally in- 

 creased, 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, 



1 The demonstration is more striking if the grey disk is placed on a sheet 

 of white paper in diffused light. TB. 



