From ten to forty feet there seems to be but little difference in the color perception, but for best tests fifteen to twenty feet is the most desirable position.
For private practice with the color wheel a small mirror may be placed five or six feet in front of the wheel in such position as to furnish an image of the disks to the person operating the machine. Owing to a slight loss of light by reflection the closest criticism may not be possible when working with a mirror in this way, but if a plate mirror is used the results are very good and a bevel plate mirror about 7×9 inches without frame, can usually be procured at small cost; this method is much more satisfactory for personal experimenting than an assistant to turn the wheel.
These disks have heretofore been used as a curious piece of philosophical apparatus rather than because they have been supposed to have any practical value in color training, but in establishing a color nomenclature based on six spectrum colors the disks at once assume a great value and are indispensable in a system of color instruction founded on the science of color and on the psychological. perception of colors.
Let us suppose that the two disks shown in Fig. 7 are yellow and green, 80 parts yellow and 20 parts green; then by rotation we shall have a green yellow indicated by the symbol Y. 80, G. 20. No argument is necessary to prove that when an exact expression of color effect is required this is better than the simple statement that it is a greenish yellow.
For practice it is profitable to commence with the red and orange disks combined on the spindle, with a smaller red disk in front of them, the smallest being preferable. Begin by introducing say five per cent of orange and notice that a change from the standard red at the center is visible. Gradually increase the orange until it seems difficult to say whether the resulting color is more like red or orange, and then exchange the small red disk for an orange disk of the same size, and con-
