734 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



gradually move a small piece of colored or of gray paper, mounted on 

 a suitable handle, from the peripherj^ toward the center of the sur- 

 face. At a certain position the colored paper will be seen as gi'ay, 

 because the rays of light from it are striking the color-blind areas of 

 the retina. Various grays are used until one is found which matches 

 exactly with that created b}^ the colored paper. A still sijnpler 

 method consists in rotating the color on a Maxwell disk along with a 

 synthetic gray. In this case judgment of equality may. however, be 

 somewhat confused on account of the gray assuming the comple- 

 mentary hue. 



Brightness plays a most important part in the phenomenon of con- 

 trast, for not only is the simultaneous contrast of hues obtained most 

 strildngly when these are of equal brightness, but we constantly ex- 

 perience brightness contrasting itself. Thus pieces of the same gray 

 paper placed on gray backgrounds of varying degrees of brightness 

 do not look at all alike. It is particularly at the border between the 

 two gi'ays that contrast brightness is most evident. This subserves 

 the function of creating a sharp border between the grays, and it can 

 be demonstrated by causing strips of different gray papers to over- 

 lap one another like the tiles of a roof or, still more strikingly, by 

 rotating a disc on which when spun appear three circles of different 

 grays, each synthesized from black and white. In both experiments 

 the grays, though really perfectly uniform, will appear as if shaded 

 from their edges. 



Since we measure brightness in terms of grayness, and since it is 

 most marked at the yellow portion of the spectrum, it follows that if 

 we desire, for successful contrast effects in picture painting, to appose 

 yellows with blues or deep reds, we must employ some artificial 

 means either to increase the brightness of the blues or reds or to de- 

 crease that of the yellows. This can be done by mixing the pigments 

 with white (or black), that is to say, we may alter what the artist 

 speaks of as the value of the color but which in so far as white is 

 used for producing the alteration is more correctly called the satura- 

 tion. 



It may indeed be said that the object sought in mixing pigments 

 with white (i. e., changing their saturation) is to give the impression 

 that their properties of brightness have been altered.^ When it is 

 desired to raise the brightness of a given color, we can succeed only 

 to a limited degree by using more pigment ; to obtain it further, we 

 must, as already explained, employ the property of simultaneous 

 contrast. These methods used by the artist to alter the brightness of 

 his colors are, however, liable to have a dulling effect on the whole 

 composition unless they are used with great care and judgment. 



1 Brightness must be distinguished from color Intensity, which Is purely a physical 

 property and depends upon the amplitude of the wave lengths. 



