trast, for not only is the simultaneous contrast of hues obtained most strikingly when these are of equal brightness, but we constantly experience brightness contrast 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 grays 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 overlap 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 decrease 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 saturation.
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.[1] 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. When he is compelled to lower the saturation of one color he must be careful to apply those neighboring on it in such a manner as to give the impression that the whole of that portion of the picture is of the same brightness. This he may do, either by making his pigments of similar saturation or by assorting the size of the colored areas, so that they appear by contrast to be of similar saturation.
It is a well-known fact that our judgment of the relative brightness of colors, and to a certain extent of their hues, becomes altered when the conditions of illumination are changed. A picture viewed in broad daylight may create a very different impression from that which it produces in dull illumination. For example, its hues may be dull and muddy under the conditions of illumination that are ordinarily present in a dwelling, or even in a gallery, whereas when viewed in broad day
- ↑ Brightness must be distinguished from color intensity which is purely a physical property and depends upon the amplitude of the wave-lengths.
