Page:Atlas of the Munsell color system.djvu/7



1. THREE COLOR SCALES UNITE IN A SPHERE.

Imagine a colored sphere with white as its north pole, black as its south pole, and its equator ringed about by a circuit of red, yellow, green, blue and purple hues— each of which melts imperceptibly into its neighbors, Fig, 1, Thus the equator traces the horizontal scale of hues: H.

Imagine each equatorial hue as graded upward to white and downward to black in regular measured steps. Each hue then presents a scale of values over the surface, while the axis traces the vertical scale of gray values: V.

Imagine surface colors weakened by additions of neutral gray as they pass inward to disappear in the vertical axis. The sphere is thus filled with gradations of color,—lighter degrees above the equator, darker degrees below; stronger degrees outward, and weaker degrees inward to the axis, where all color is balanced in neutrality. The degree of color strength at any point is known as chroma and is traced by radii at right angles to the axis. It represents the gradual emergence of each hue from grayness. Each radius serves as a scale of chromas: C.

Every color sensation may be measured and defined by these three scales of hue, value, and chroma. Neglect of either scale—that is, failure to state either the hue, the value, or the chroma of a color—creates doubt and confusion.

2. A COLOR TREE SURROUNDS THE COLOR SPHERE.

Were all pigment colors of equal chroma then a sphere would present an ideal of their relations. But pigments are very unequal in strength, Vermilion red, for example, being twice as strong as its opposite complement, blue-green Viridian. This is shown in chart 40. The unequal scales of pigment chroma may be treated as branches of a Color Tree whose trunk is the neutral axis, while its branches of various lengths and at various levels blossom out with the strongest colors. This tree is imagined as compact of colored leaves—darker leaves below, lighter leaves above; most chromatic leaves on the surface and grayer leaves inward to the trunk, which is colorless. The tree also encloses the Color Sphere, which would appear were the longer branches lopped off to equal the length of the shortest branch. Fig. 2.

3. NOTATION OF COLORS BY SYMBOLS.

The place of each leaf of the Color Tree is determined by the measured scales of hue, value and chroma. These scales also furnish an expressive notation, made by the five color initials with their combinations and ten arabic numbers.

The scale of hue is a sequence of red (R), yellow-red (YR), yellow (Y), green-yellow (GY), green (G), blue-green (BG), blue (B), purple-blue (PB), purple (P), and red-purple (RP). The five principal hues melt perceptibly into intermediates by ten steps, of which the middle or fifth step is typical of that hue. The scale of values is also decimal from 0 (black) to 10 (white), and the scale of chromas likewise from 0 (neutral gray) to 10 (the strongest permanent pigment so far obtained).

A symbol completely describing the character of any color sensation is composed of its degrees of hue, value, and chroma. The symbol for what is commonly known as Vermilion is $5R 4⁄10$ ("five red, four over ten"):—the numeral before R showing that it is the fifth or typical step of red in the hue scale, without tendency either to yellow-red or purple-red; the upper numeral showing that its luminosity equals the fourth step in the value scale, and the chroma numeral ten showing that it is of maximum strength. Chart H.

Should the Vermilion be changed by fading or admixture with another pigment, this would appear in the symbol: — thus a tinge of yellow in the red is written 6R while 4R indicates a tinge of purple; a slight addition of gray reduces the chroma to $R undefined⁄9$, while the addition of white changes the value to $R⁄5$. Grouping all these changes in the symbol, $6R 5⁄9$, shows that the original Vermilion $5R 4⁄10$ is no longer pure, but tinged with yellow, lightened with white, and weakened with gray.

4. CHARTS OF THE COLOR SYSTEM.

The measured scales of hue, value, and chroma are presented in two sets of charts, one made by vertical sections of the Color Tree, and the other by horizontal sections. Figs. 3 and 4.

There are eight vertical charts. 'Chart H is the hue scale arranged as an index for recording colors singly or in groups. Vermilion appears in the column R at the level four and with the chroma symbol ten. Chart V is the value scale upon a hinged and perforated card, behind which to test the value of a color sample. Thus Vermilion seen through the perforations is darker than value five and lighter than value three. It matches value step four. Chart C bears the chroma scales of red, yellow, green, blue and purple as tree branches whose levels and lengths describe the relation of these maxima to the extremes of white and black. Vermilion appears as the strongest red chroma, and the color is written $5R 4⁄10$. The five remaining vertical charts are planes passed through the axis, on opposite sides of which appear the complementary fields of color. Chart R shows the red field with its complementary field of blue-green. By noting the symbol $5 R⁄4$ Vermilion may be balanced with any degree of its opposite blue-green. Chart Y shows yellow with its opposite purple-blue. Charts G, B, and P show green, blue, and purple with their appropriate complements, red-purple, yellow-red (orange), and green-yellow. There are seven horizontal charts. The axis appears on each as the neutral gray centre of a star or radial pattern, the lengths of whose radii indicate the chroma of their hues. These sections present colors at a single uniform level of value:—thus. Chart 50 at the middle of the Color Tree bears only colors which reflect 50 per cent, of the luminosity of white, while Charts 40, 30, and 20 show darker levels, and Charts 60, 70, and 80 show the lighter levels of color.

5. BALANCE OF COLOR BY A SPHERE.

The sphere typifies balance of color. White and black balance at the centre on middle gray, $N⁄5$. Balanced colors appear at the ends of any diameter passing through the centre of the sphere. Also, a lighter color balances a darker, but when unequal values or chromas are employed the color of weaker chroma must be given the larger area. The symbols on each step of these color charts indicate the proportions needed to produce balance, as suggested in the text to be found on each chart.