PHYSIOLOGY OF COLOR VISION KELLER AND MACLEOD. 737 



The quickly vibrating (blue) rays of the spectrum tend to be re- 

 flected more readil}^ than the slowly vibrating (red) rays, hence we 

 often find that a substance is bluish by reflected light, whereas it is 

 reddish when the light passes through it. It is, indeed, for this rea- 

 son that during the day the sky looks blue, the light being reflected 

 from the fine particles of dust and moisture which are constantly 

 suspended in it, whereas after the sun has set it is red because the 

 slanting rays come to be transmitted through these particles. 



Artificial illumination alters the hues of pictures mainly because of 

 mixture of colored lights; that is to say, of the hue of the light re- 

 flected from the surface of the picture and of the hue due to the par- 

 ticular pigments employed. Thus, i. we regard a picture in yellow 

 light (gas, carbon filament, etc.), the pale blues may appear white 

 (mixing of complementary colors), the deeper blues assume a green- 

 ish hue, and the reds turn to orange. 



In the colors which we see in nature influences of a similar kind 

 are constantly at play, for every object, besides being illuminated by 

 the prevailing light, has thrown on to it colors which are reflected 

 from near-by objects. In analyzing these influences there are, as 

 Rood has pointed out, at least three factors that must be borne in 

 mind. These are (1) the natural or "local color" of the object, 

 the cause for which we have already explained; (2) the colored light 

 which is reflected unaltered from its surface, just as we have seen 

 white light to be; (3) the portion of this colored light which is not 

 entirely reflected but which penetrates the surface and is then re- 

 flected. Let us suppose that we are regarding a red wall of glazed 

 brick at the edge of a grass lawn: the local brick red of the wall 

 will be materially altered by surface reflection not only of the white 

 light but also of blue-green which, being approximately its comple- 

 mentary, tends to lower its saturation and pull it toward neutrality ; 

 at the same time, the green rays which have penetrated will on re- 

 flection assume a yellowish orange hue. The total efl'ect is therefore 

 that the red is somewhat removed toward neutrality and at the same 

 time made to assume an orange hue. But it is by no means always 

 possible to analyze these color effects, so that we must depend rather 

 on the accuracy of the impression which we receive, at the same time 

 bearing in mind that even objects with which we usually associate 

 the most positive of hues may under certain conditions become en- 

 tirely altered in this regard. In their use of colors, the post-impres- 

 sionists are most careful to allow for these influences, although they 

 may employ hues to produce them which at first sight appear to be 

 entirely out of place. 



Finally, we must say a few words about the relative, refracta- 

 biljty of different colors; that is to say, the ease with which the 



44Sia3°^SM 1913 i7 



