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have been published, the original Atlas in 1915 [105], now chiefly a collector's item, the Munsell Book of Color in 1929 and 1942 [106], and the Munsell Color Standards in High-Gloss Surface in 1957. The book consists of rectangles of mat-finish handpainted paper permanently mounted on charts in a loose-leaf binding. The high-gloss color standards are detachably mounted. The neutrals form a one-dimensional color scale extending from N 1/ to N 9/. Each chromatic sample, of which there are about 1000, takes its place on three color scales: a hue scale, a value scale, and a chroma scale; and the spacing of these scales is intended to be perceptually uniform. The pocket edition, adapted for determining Munsell notation of unknown colors by visual comparison, consists of forty constant-hue charts, so called because all the samples on each chart have the same Munsell hue. These samples are arranged in rows and columns, the rows being chroma scales at constant Munsell value, the columns being value scales at constant Munsell chroma. Comparison of an unknown color with these two families of scales gives by interpolation the Munsell value and Munsell chroma of the unknown. Interpolation between adjacent constant-hue charts gives the Munsell hue. Unknowns not too far outside the range of the Munsell charts may be evaluated with some reliability by extrapolation along the value and chroma scales. Table 9 gives Munsell Book notations of the four printing-ink specimens reproduced on figure 4. The greenish yellow specimen was evaluated by interpolation, the other three by extrapolation of varying degrees of uncertainty, the red-purple specimen being furthest outside the range of the Munsell charts and hence the least certainly evaluated by visual estimate.

Table 9. Munsell renotations, luminous reflectances, chromatieity coordinates, Munsell Book Notations, and ISCC-NB8 color designations of four printing-ink specimens

Hue destination of specimen	Luminous reflectance ($Y/Y_{o}$)	Chromaticity coordinates		Munsell renotation (hue value/chroma)		Munsell book notation (hue value/chroma)		ISCC-NBS color designation
		$x$	$y$
Red Purple	0.221	0.430	0.239	5.5RP	5.25/16.1	6.0RP	4.8/16	Vivid purplish red
Greenish Yellow	.704	.426	.476	8.3Y	8.60/10.6	7.5Y	9.0/9.5	Brilliant greenish yellow
Greenish Blue	.242	.194	.248	5.6B	5.46/8.5	3.0B	5.4/9	Strong greenish blue
Blue	.246	.190	.213	0.8PB	5.50/9.6	10.OB	5.4/11	Strong blue

To facilitate the comparison of the unknown color with those of the permanently mounted paper rectangles on the Munsell charts, either the unknown color must be brought into juxtaposition with the rectangle and held in nearly the same plane, or, if the form of the unknown prevents such juxtaposition, two masks of thin cardboard having a rectangular opening to fit the paper rectangles should be used. One mask should be placed over the unknown color; the other over one or another of the Munsell colors in succession to obtain the interpolated Munsell Book notation.

Kelly [69, 76] made effective use of a form of mask with three rectangular openings particularly adapted to comparisons involving powdered chemicals and drugs viewed through a cover glass. It is advantageous to have the color of the mask fairly close to that of the unknown, particularly in Munsell value; that is, if the unknown color is dark, the mask should be of a dark color also. Use of a light mask for a dark color prevents the observer from making as precise a visual estimate as he can make with a mask more nearly a color match for the unknown.

Because of the visual uniformity of the scales, the estimates of Munsell notation for unknown colors within the color range of the charts have a reliability corresponding to the use of a much larger collection of unequally spaced color standards. On this account the pocket edition of the Munsell Book of Color is widely used as a practical color standard for general purposes.

The standard or library edition shows the same colors as the pocket edition, but it shows them on constant-value charts in a polar-coordinate system, and on constant-chroma charts in a rectangular coordinate system, as well as on contant-hue charts. This edition has full explanatory matter in the text and is adapted particularly for teaching color. It is too bulky for convenient practical use in determining the Munsell notation of an unknown color.

The samples of the 1929 Munsell Book of Color have been measured by means of the spectrophotometer twice independently with generally concordant results [39, 78]. In both these studies luminous reflectance, $Y/Y_{0}$, relative to magnesium oxide and chromaticity coordinates, $x$,$y$, of the roughly 400 samples were computed for source C. Glenn and Killian [39] have also published the dominant wavelength and purity for each of the colors; Kelly, Gibson, and Nickerson [78] have published specifications ($Y,x,y$) for three additional sources (source A, Macbeth daylight, and limit blue sky. Nickerson and Wilson [118] have extended them to nine sources. Furthermore, they have published [78] a series of ($x,y$) chromaticity diagrams showing the position of the Munsell colors for each of the Munsell values from 2/ through 8/. From these diagrams it is possible to find the chromaticity coordinates, $x,y$, correspond-