[12] In fixing the exact position or wave-length of the spectrum colors considerable latitude is allowable, the element of "personal equation"—that is, difference in the conception of different persons as to just where the reddest red, greenest green, etc., are located, accounting for the considerable disagreement among chromatologists as to the wave-lengths. The following table, showing the average, mean, and extreme wave-length of each of the spectrum colors as given by nine or more authorities together with those of the present work (as determined by Dr. P. G. Nutting, Associate Physicist of the U. S. Bureau of Standards) is of interest in this connection: Average Extremes Mean This work. of 9-12 of 9-12 of 9-12 authorities. authorities. authorities. Red 644 6770 6440-7028 6734 (10) Orange 598 ± 2 6074 5892-6300 6096 (9) Yellow 577 ± 1 5786 5640-5850 5745 (10) Green 520 ± 10 5235 5050-5335 5193 (11) Blue 473 ± 3 4738 4520-4861 4680 (12) Violet 410 4176 4050-4330 4190 (10) From this table it will be seen that the red of this work is appreciably more orange than that of others, the orange slightly more yellowish, and the violet a little less bluish than the average; but the author is assured by Dr. Nutting that these standards are exceptionally accurate. [13] The percentages are given in tables on pages 23 and 25. [14] That is to say, theoretically. Unfortunately it seems to be beyond the colorists' skill to reproduce true shades of the pure colors, all showing a more or less decided admixture of gray, resulting in a series of broken or dull shades. (See pages 23 and 24.) [15] Although only 1115 different colors are actually shown on the plates the system is really equivalent to the presentation of considerably more than 4000 distinguishable and designatable colors. [16] The Theory of Color (American edition, 1876), p. 99. [17] In the present work the possibility of variation between different copies is wholly eliminated by a very different process of reproduction. Each color, for the entire edition, is painted uniformly on large sheets of paper from a single mixture of pigments, these sheets being then cut into the small squares which represent the colors on the plates. [18] See Rood: Modern Chromatics, pages 50-52. [19] Some criticisms of Popular Color Definitions and Suggestions for a better Color Nomenclature. Milton Bradley Co., Springfield, Mass. (Small pamphlet of 15 pages). [20] Milton Bradley: Elementary Color, p. 25.