Even with the disks it is impossible to imitate purest tints of all the standard colors, because in some of the colors, as peculiarly in red and blue, the rotation of the white disk seems to develop a slightly violet gray, for which effect there has as yet been no scientific explanation. This gray dulls the purity of the tint as compared with that which is found in the color under a bright illumination, but on the whole both tints and shades as well as the hues can be better illustrated with the disks than in any other way, and in addition, the advantage is secured of being able to measure and record the tone by the graduated disk in the same way as the hues are measured and recorded. A further advantage is secured in the use of disks in color instruction because with pigments, the only other method by which colors can be combined, much time must be lost not only in the mixing aucl applying of the colors but in the delay necessary to allow them to dry before the true results can be seen.
The shades of yellow as shown on the wheel will not be generally accepted without criticism, but careful comparison with yellow paper in shadow will prove the substantial truth of the disk results. This experiment may be tried as follows: Join An image should appear at this position in the text.Fig. 8. two cards with a hinge of paper or cloth to form a folding screen like the covers of a book as in Fig. 8. On the surface A, paste a piece of standard yellow paper and on B, a piece of yellow shade No. 1. Hold these two surfaces toward the class in such a position that the strong light will fall on B, which is the yellow shade, and thus bring the face A, which is a standard yellow, in a position to be shaded from the light. By varying the angle of the covers with each other and turning them as a whole from side to side, a position will be secured in which the two faces will seem so nearly alike as to convince the class that this color which they may have
