lem the matter would be easily expressed by the disk nomenclature, For instance, if we are to consider a certain red object which may be represented by the standard red disk, we place a medium sized disk of that color on the spindle, and in front of it, smaller disks of white and black united. By rotation the white and black disks become a neutral gray at the center of the red disk. If this gray is made nearly white all observers will agree that the gray is lighter than the red, and if it is nearly black the opinion will be equally unanimous that it is darker than the red. Consequently there evidently must be a gray somewhere, between these two extremes which a large majority of experts may agree to be equal in depth or tone to the red, i. e., neither lighter nor darker. But the artist-engraver will insist that to him the term "value" expresses much more than this and that he must use different lines in the sky or distance from those which he uses in the foreground; and some engravers will also insist that two different colors in the foreground must receive different treatment with the graver in order to express their true values. We know that true values of colors are not expressed in a photograph, as the warm colors are too dark and the blue far too light. If the term "value of a color" is to be used as expressing something more than a neutral gray of such a tone as to seem equal to it, then possibly this latter quality must be expressed by the word tone, and yet this use of that word will seem to enlarge its scope beyond its present limits as it now is used to express the relations between the different localities in one scale of color, while this new use will extend to the comparison of tones in various color scales, including neutral grays.
Luminosity.—The luminosity of a color is determined by comparing it with a neutral gray. When a color seems to be of the same brightness as a given neutral gray, i. e., not lighter nor darker, then that gray is its measure of luminosity.
A noted authority says: "No colored object can have the luminosity of a white object reflecting practically the whole of
