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HERMANN VON HELMHOLTZ 



objects represented. In addition to the form of the objects, 

 degrees of brightness have to be considered. Since it is im- 

 possible for the painter to depict the light and shade in a 

 picture as they are presented in nature, he can only strive by 

 his colours to produce the same impression upon the eye of the 

 spectator. He does this unconsciously in virtue of Fechner's 

 psycho-physical law, that within very wide limits of brightness 

 differences of light-intensity, if they form an equal fraction of 

 the total quantity of light compared, are equally distinct and 

 therefore appear equal in sensation. The ratio of brightness is 

 our only sensory sign of the lighter or darker coloration of 

 bodies, and the painter therefore need but select in his colours 

 the same ratio of brightness as is exhibited by the bodies them- 

 selves. But when the mean limits of Fechner's law are trans- 

 gressed, then with lessened illumination the darker objects 

 become more like the darkest, and with greater illumination the 

 brighter objects become more like the brightest, and so in 

 representing glowing sunshine the painter is obliged to make 

 all objects almost equally bright, while in moonlight only the 

 very brightest objects can be bright, and the others must be 

 unrecognizably dark. 



But the question of degrees of brightness is complicated by 

 colour differences, since the scale of intensity of sensation is 

 different for different colours. The phenomena of dazzle are 

 weaker with increased brightness for red than for blue, and 

 Helmholtz observed that even with a small proportional increase 

 of intensity this was especially striking in the red and violet 

 colours of the spectrum, so that with mixed colours very bright 

 white appeared yellowish, dull white bluish in colour. The 

 painter accordingly, to reproduce the impression of sunlit white 

 with faint colours, must by an admixture of yellow in his white 

 make this colour preponderate just as it would in actually 

 brighter white. 



Lastly, the phenomena of contrast also come under considera- 

 tion. These cannot be represented in paintings as they are in 

 the real objects, since the colours of pictures are not as bright 

 and intensely luminous as they are in reality. The painter 

 accordingly must represent an evenly illuminated surface as 

 brighter where it is contiguous with a darker part, and darker 

 where it impinges on what is bright. The artist again has to 



