736 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



the case, and we shall now endeavor to show how certain of these 

 difficulties can be explained. First of all, with regard to the mixing 

 of pigments as compared with the mixing of colored lights, of course 

 the two processes yield very different results: for example, mixing 

 yellow and blue lights, as we have seen, produces almost pure white, 

 whereas mixing these colors as pigments, as every artist knows, 

 produces green. The entire want of similarity in the results which 

 follow tlie mixing of colors by the two methods has had the effect 

 of making some artists conclude tluit the laws of chromatics are use- 

 less as guides in the practical use of pigments. But this is wrong, 

 the apparent difference beins:,, really due to a very simple cause, 

 namely, to the fact that by mixing pigment we substract the color rays 

 from entering the eye, whereas we add such rays when we mix colored 

 lights. To make this clear let us return to our example of blue and 

 yellow. When we use these as pigments, we must remember that 

 the pigment particles have a certain degi'ee of transparency so that 

 light partly penetrates them, certain rays being then reflected and 

 certain absorbed according to the hue. A blue pigment, for example, 

 absorbs 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. Simi- 

 larly, yellow 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 ex- 

 plain why blue and red give purple and why a mixture of all the 

 spectral colors as pigments produces 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 con- 

 sider, 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 sur- 

 face reflection of white light that the colors of a picture may be 

 raised in saturation b}'' subjecting it to alcohol vapor, which softens 

 the medium and removes surface cracks. Reflection of white light 

 also takes place at the surface 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 par- 

 ticles to pass through a medium which is of approximately the same 

 density as that of the particles themselves. This reduces the reflec- 

 tion, because the greater the difference of density between two media 

 the greater the reflection of light at the interface between them. 



