olors with which to
contrast this color, the world would be colorless. This should be
obvious when it is considered that an object which is red under an
illuminant containing all colors such as sunlight would be black or dark
gray under monochromatic yellow-green light. The red under present
conditions is kept alive by contrast with other colors, because the
latter live by virtue of the fact that most of our present illuminants
contain their hues. It is assumed that the reader knows that a red
object, for example, appears red because it reflects (or transmits) red
rays and absorbs the other rays in the illuminant. In other words, color
is due to selective absorption reflection, or transmission.

Perhaps the ideal illuminant, which is most generally satisfactory for
general activities, is a white light corresponding to noon sunlight. If
this is chosen as the scientific ideal, the illuminants of the present
time are much more "efficient" than if the most efficient light is the
ideal.

The luminous efficiency of the radiant energy most efficient in
producing the sensation of light (yellow-green) is about 625 lumens per
watt. That is, if energy of this wave-length alone were radiated by a
hypothetical light-source, each watt would produce 625 lumens. The
luminous efficiency of the most efficient white light is about 265
lumens per watt; in other words, if a hypothetical light-source radiated
energy of only the visible wave-lengths and in proportions to produce
the sensation of white, each watt would produce 265 lumens. If such a
white light were obtained by pure temperature radiation--that is, by a
normal radiator at a temperature of 10,000 deg.F., which is impracticable at
present--the luminous efficiency would be about 100 lumens per watt. The
normal radiator which emits energy by virtue of its temperature without
selectively radiating more or less energy in any part of the spectrum
than indicated by the theoretical radiation laws is called a
"black-body" or normal radiator. Modern illuminants have luminous
efficiencies ranging from 5 to 30 lumens per watt, so it is seen that
much is to be done before the limiting efficiencies are reached.

The amount of light obtained from various gas-burners for each cubic
foot of gas consumed per hour varies for open gas-flames from 5 to 30
lumens; for Argand burners from 35 to 40 lumens; for regenerative lamps
from 50 to 75 lumens; and for gas-mantles from 200 to 250 lumens.

In t