pted by all metrologists.

ORANGE.--A reddish yellow color, of which the orange is the type.

VISION, PERSISTENCE OF.--The continuance of a visual impression upon the
retina of the eye after the exciting cause is removed. The length of
time varies with the intensity of the light and the excitability of the
retina, and ordinarily is brief, though the duration may be for hours,
or even days. The after image may be either positive or negative, the
latter when the bright part appears dark and the colored parts in their
corresponding contrast colors. It is because of this persistence that,
for example, a firebrand moved very rapidly appears as a band or circle
of light.

PHOTOMETER.--An instrument used to measure the intensity of light.
Specifically, to compare the relative intensities of the light emitted
from various sources.

PIGMENT.--Any substance that is or can be used by painters to impart
color to bodies.

PINK.--A red color of low chroma, but high luminosity, inclining toward
purple.

PRIMARY COLORS.--See Colors, primary.

PURE COLOR.--A color produced by homogeneous light. Any very brilliant
or decided color.

PURPLE.--A color formed by the mixture of blue and red, including the
violet of the spectrum above wave length 0.417, which is nearly a violet
blue, and extending to, but not including, crimson.

RAINBOW.--A bow or an arc of a circle, consisting of the prismatic
colors, formed by the refraction and the reflection of rays of light
from drops of rain or vapor, appearing in the part of the heavens
opposite to the sun.

RED.--A color more or less resembling that of blood, or the lower end of
the spectrum. Red is one of the most general color names, and embraces
colors ranging in hue from aniline to scarlet iodide of mercury and red
lead. A red yellower than vermilion is called scarlet. One much more
crimson is called crimson red. A very dark red, if pure or crimson, is
called maroon; if brownish, chestnut or chocolate. A pale red--that is,
one of low CHROMA and high LUMINOSITY--is called a pink, ranging from
rose pink or pale crimson to salmon pink or pale scarlet.

VENETIAN RED.--An important pigment used by artists, somewhat darker
than brick red in color, and very permanent.

RETINA.--The innermost and chiefly nervous coat of the posterior part of
the eyeball.

SATURATION, OF COLORS.--In optics the degree of admixture with white,
the saturation diminishing as the amount of white is increased.