mitting only invisible
infra-red energy and has an efficiency of zero as a producer of light.
As it becomes hotter it begins to appear red, but as its temperature is
raised it appears orange, until if it could be heated to the temperature
of the sun, about 10,000 deg.F., it would appear white. All this time its
luminous efficiency is increasing, because it is radiating not only an
increasing percentage of visible radiant energy but an increasing amount
of the most effective luminous energy. But even when it appears white, a
large amount of the energy which it radiates is invisible infra-red and
ultra-violet, which are ineffective in producing light, so at best the
substance at this high temperature is inefficient as a light-producer.

In this branch of the science of light-production substances are sought
not only for their high melting-point, but for their ability to radiate
selectively as much visible energy as possible and of the most luminous
character. However, at best the present method of utilizing the
temperature radiation of hot bodies has limitations.

The luminous efficiencies of light-sources to-day are still very low,
but great advances have been made in the past half-century. There must
be some radical departures if the efficiency of light-production is to
reach a much higher figure. A good deal has been said of the firefly and
of phosphorescence. These light-sources appear to emit only visible
energy and, therefore, are efficient as radiators of luminous radiant
energy. But much remains to be unearthed concerning them before they
will be generally applicable to lighting. If ultra-violet radiation is
allowed to impinge upon a phosphorescent material, it will glow with a
considerable brightness but will be cool to the touch. A substance of
the same brightness by virtue of its temperature would be hot; hence
phosphorescence is said to be "cold" light.

An acquaintance with certain terms is necessary if the reader is to
understand certain parts of the text. The early candle gradually became
a standard, and uniform candles are still satisfactory standards where
high accuracy is not required. Their luminous intensity and
illuminating value became units just as the foot was arbitrarily adopted
as a unit of length. The intensity of other light-sources was
represented in terms of the number of candles or fraction of a candle
which gave the same amount of light. But the luminous intensity of the
candle was taken