the fine arts a vitreous substance or glass, opaque or
transparent, and variously colored, applied as a coating on a surface of
metal or of porcelain.

GRATING, DIFFRACTION.--A series of fine parallel lines on a surface of
glass, or polished metal, ruled very close together, at the rate of
10,000 to 20,000 or even 40,000 to the inch; distinctively called a
diffraction or a diffraction grating, much used in spectroscopic work.

GRAY.--A color having little or no distinctive hue (CHROMA) and only
moderate luminosity.

GREEN.--The color of ordinary foliage; the color seen in the solar
spectrum between wave lengths 0.511 and 0.543 micron.

EMERALD GREEN.--A highly chromatic and extraordinarily luminous green of
the color of the spectrum at wave length 0.524 micron. It recalls the
emerald by its brilliancy, but not by its tint; applied generally to the
aceto-arsenate of copper. Usually known as Paris green.

HIGH COLOR.--A hue which excites intensely chromatic color sensations.

+HUE.--Specifically and technically, distinctive quality of coloring in
an object or on a surface; the respect in which red, yellow, green,
blue, etc., differ one from another; that in which colors of equal
luminosity and CHROMA may differ.+

INDIGO.--The violet-blue color of the spectrum, extending, according to
Helmholtz, from G two-thirds of the way to F in the prismatic spectrum.
The name was introduced by Newton, but has lately been discarded by the
best writers.

LIGHT.--Adjective applied to colors highly luminous and more or less
deficient in CHROMA.

LUMINOSITY.--Specifically, the intensity of light in a color, measured
photometrically; that is to say, a standard light has its intensity, or
_vis viva_, altered, until it produces the impression of being equally
bright with the color whose light is to be determined; and the measure
of the _vis viva_ of the altered light, relatively to its standard
intensity, is then taken as the luminosity of the color in question.

MAXWELL COLOR DISCS.--Discs having each a single color, and slit
radially so that one may be made to lap over another to any desired
extent. By rotating these on a spindle, the effect of combining certain
colors in varying proportions can be studied.

MICRON.--The millionth part of a metre, or 1/23400 of an English inch.
The term has been formally adopted by the International Commission of
Weights and Measures, representing the civilized nations of the world,
and is ado