_a_ Red _e_ Yellow _i_ Blue 1 Red 1 Yellow 1 Blue 2 Red 2 Yellow 2 Blue 3 Red 3 Yellow 3 Blue 4 Red 4 Yellow 4 Blue 5 Red 5 Yellow 5 Blue 139. _b_ Red-orange _f_ Yellow-green _k_ Blue-violet 1 Red-orange 1 Yellow-green 1 Blue-violet 2 Red-orange 2 Yellow-green 2 Blue-violet 3 Red-orange 3 Yellow-green 3 Blue-violet 4 Red-orange 4 Yellow-green 4 Blue-violet 5 Red-orange 5 Yellow-green 5 Blue-violet 140. _c_ Orange _g_ Green _l_ Violet 1 Orange 1 Green 1 Violet 2 Orange 2 Green 2 Violet 3 Orange 3 Green 3 Violet 4 Orange 4 Green 4 Violet 5 Orange 5 Green 5 Violet 141. _d_ Orange-yellow _h_ Green-blue _m_ Violet-red 1 Orange-yellow 1 Green-blue 1 Violet-red 2 Orange-yellow 2 Green-blue 2 Violet-red 3 Orange-yellow 3 Green-blue 3 Violet-red 4 Orange-yellow 4 Green-blue 4 Violet-red 5 Orange-yellow 5 Green-blue 5 Violet-red I attach no importance to this nomenclature; I employ it only as the simplest to distinguish the seventy-two scales just described. The number may be increased indefinitely, by inserting as many as we choose between the above. 142. Let us now represent the gradations of each colour in the scales of the circle by the addition to it of black, progressively increasing till it becomes pure black. Imagine a quadrant whose radius is equal to that of the circle, and arranged so as to turn upon an axis perpendicular to the plane of the circle. Divide this quadrant, 1st, by concentric arcs _y yʹ_, which coincide with the circles denoted by the same letters; 2nd, by ten radii, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. Divide each of these radii into twenty parts, representing twenty tones, corresponding to the tones of the scales represented on the circle. 143. I suppose that the tenth radius comprises the gradations of normal black, covering the half-circle described by the movement of the quadrant upon its axis; this black mixed in decreasing quantities, with increasing quantities of white, gives the twenty tones of normal grey, and ends by being lost in the white situated above the tone 1. I suppose, further, that the normal tone of each of the scales taken upon each of the radii of the quadrant 1, 2, 3, 4, 5, 6, 7, 8, 9, is formed of the mixture of black with the colour of any of the scales that the circle contains, and in such a proportion that the normal tone 15 of that scale is represented by the unit of surface covered with 1, or ¹⁰/₁₀ of red. 144. The tone 15 of the scale of the