While the absence (in large part) of pure chromatic shades is much to be regretted, the defect is not so serious, _from the standpoint of utility_, as might appear at first sight; for while saturated or darkened pure colors are not uncommon in the animal, vegetable, and mineral kingdoms, more or less broken dark colors are infinitely more so; and since the latter are greatly increased in number by the defect mentioned the actual result is rather an advantage than otherwise. It will doubtless be noticed that there is a conspicuous difference in relative darkness between shades of yellow and contiguous hues on the one hand and corresponding ones of violet and adjacent hues on the other, as if the percentage of black in each were very different. This, however, is entirely the result of difference of luminosity of the two sets of colors, that of yellow being between 7000 and 8000 while that of violet is only about 13;[28] for the percentage of black in corresponding tones of the vertical scales is precisely the same for each color throughout the chromatic scale of this work. TABLE SHOWING PERCENTAGES OF NEUTRAL GRAY IN THE BROKEN COLOR SCALES. Every Plate in each series of broken colors (′ to ′′′′′) contains exactly the same percentage of neutral gray in each color, the relative amount increasing progressively in the several series, as shown in the following table. The percentages of white in the tints and of black in the shades of the tone scales are in all cases exactly the same as in the tone scales of pure colors. SERIES. PERCENTAGES. Color. Neutral Gray. Pure Colors 100 (′) 68 32 (′′) 42 58 (′′′) 23 77 (′′′′) 10 90 (′′′′′) 4.5 95.5 Neutral Gray 100 TABLE OF PERCENTAGE OF BLACK AND WHITE IN THE DIFFERENT TONES OF CARBON GRAY. TONE NUMBER. PERCENTAGES. Black. White. 1 100 2 98 2 3 94.5 5.5 4 89.5 10.5 5 83 17 6 75 25 7 67.5 32.5 8 58.5 41.5 9 47 53 10 30 70 _Note._—The percentages given in the preceding tables may not in all cases be precisely those actually contained in the colors on the Plates, since absolute precision in reproduction is hardly possible. All that can be claimed is a reasonably close approximation to the ideal. DYES AND PIGMENTS USED IN THE PREPARATION OF THE MAXWELL DISKS, REPRESENTING THE THIRTY-SIX COLORS OF THE PURE SPECTRUM SCALE, FORMING THE BASIS OF THE COLOR-SCHEME OF THIS WORK.[29] =Red.=—Devoe's _geranium lake_ (dry), its orange hue neutralized by a wash of _rhodamin b._ (_Crocein scarlet b._ washed with _rhodamin b._ produces practically the same fine red.)