57 VB-V 22 78 58 12 88 59 Violet 100 410 60 3 97 61 VR-V 7 93 62 11 89 63 R-V 18 82 64 24 76 65 RR-V 33 67 66 41 59 67 V-R 52 48 68 64 36 69 RV-R 74 26 70 83 17 71 V-RR 90 10 72 95.5 4.5 TABLE SHOWING PERCENTAGE OF WHITE AND BLACK, RESPECTIVELY, IN EACH TONE OF THE TONE OR LUMINOSITY SCALES. All of the vertical scales in the original Plates of this work (the scale of carbon grays alone excepted) contain the following percentages by color-wheel measurement: TONE. PERCENTAGES. White. Color. Black. (White) 100 (g) 70 30 f 45 55 (e) 32 68 d 22.5 77.5 (c) 15 85 b 9.5 90.5 (a) 5 95 (Full Color) 100 (h) 64 26 i 55 45 (j) 41 59 k 29.5 70.5 (l) 20 80 m 12.5 87.5 (n) 6 94 (Black) 100 One of the most serious difficulties encountered in the preparation of the Plates of this work was the apparent impracticability of reproducing satisfactory shades of pure colors. This originated in the fact that there seems to be no substance (pigment, dye, or fabric) which represents a true black, all reflecting more or less of white light, and consequently producing shades which are dull or broken. The difficulty is increased by the additional fact that any black pigment mixed with almost any color falls short of even the color-wheel mixture in purity of hue in the resulting shades, owing to the very considerable amount of gray in all black pigments. Chromolithography can be made to produce clearer and better shades of the pure colors, but is distinctly objectionable for the purpose of a work of this kind owing to eventual oxidation of the oil or varnish with which the pigments are combined in lithographic inks, causing a change of hue; reds becoming more orange, blues more greenish, etc., in course of time.