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The vagaries of color estimate thus disclosed, lead some to seek shelter in “feeling and inspiration”; but feeling and inspiration are temperamental, and have nothing to do with the simple facts of vision. A measured and unchanging scale is as necessary and valuable in the training of the eye as the musical scale in the discipline of the ear.

It will soon be necessary to talk of the values in each color. We may distinguish the values on the neutral axis from color values by writing them N1, N2, N3, N4, N5, N6, N7, N8, N9, N10. Such a scale makes it easy to foresee the result of mixing light values with dark ones. Any two gray values in varying proportions unite to form a gray midway between them. Thus N4 and N6 being equally above and below the centre, unite to form