PHYSIOLOGY OF COLOR VISION 463 



that the pigment particles have a certain degree of transparency so that 

 light partly penetrates them, certain rays being then reflected and cer- 

 tain absorbed according to the hue. A blue pigment, for example, ab- 

 sorbs all constituent rays of white light except the blue and the hues 

 which border on blue in the spectrum, it being impossible to procure 

 pigments which are so pure that they do not let some other hues besides 

 their own characteristic one pass through them. Similarly with yellow, 

 it absorbs all the spectral rays save the yellow, the orange, and the 

 green. Adding these two pigments together, we get every spectral ray 

 absorbed except green, a certain amount of which both pigments have 

 allowed to pass. In a similar way we can explain why blue and red give 

 purple and why a mixture of all the spectral colors as pigments pro- 

 duces a dark gray of uncertain hue. 



The above applies to a matt surface; when there is any trace of 

 glaze there comes into play another factor which we must now consider, 

 namely, surface reflection of some white light which has not penetrated 

 the pigment particles at all and which therefore causes the color to be 

 more or less unsaturated. It is by diminishing surface reflection of 

 white light that the colors of a picture may be raised in saturation by 

 subjecting it to alcohol vapor, which softens the medium and removes 

 surface cracks. Reflection of white light also takes place at the sur- 

 face of the pigment particles themselves and is greatly diminished when 

 these are extremely small, hence the importance in the manufacture of 

 pigments of thorough grinding. It is further minimized by suspending 

 the pigments in oil, because this causes the light before it strikes the 

 surface of the pigment particles to pass through a medium which is of 

 approximately the same density as that of the particles themselves. This 

 reduces the reflection, because the greater the difference of density be- 

 tween two media the greater the reflection of light at the interface 

 between them. 



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

 flected more readily 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 reason 

 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 reflected 

 from the surface of the picture and of the hue due to the particular pig- 

 ments employed. Thus, if we regard a picture in yellow light (gas, 

 carbon filament, etc.) the pale blues may appear white (mixing of com- 

 plementary colors), the deeper blues assume a greenish hue, and the reds 

 turn to orange. 



