s, will continue to glow for
a long time. Despite their brightness they will be cold to the touch.
This phenomenon would be termed "photo-luminescence," although it is
better known as "phosphorescence." It should be noted that the latter
term was carelessly originated, for phosphorus has nothing to do with
it. The glow of the Geissler tube or electrically excited gas at low
pressure would be an example of "electro-luminescence." The luminosity
of various salts in the Bunsen-flame is due to so-called luminescence
and there are many other examples of light-production which are included
in the same general class. Inasmuch as light is emitted from
comparatively cold bodies in these cases, it is popularly known as
"cold" light.

There are many instances of light being emitted without being
accompanied by appreciable amounts of invisible radiant energy and it is
natural to hope for practical possibilities in this direction. As yet
little is known regarding the efficiency of light-production by
phosphorescence. The luminous efficiency of the radiant energy emitted
by phosphorescent substances has been studied, but it seems strange that
among the vast works on phosphorescent phenomena, scarcely any mention
is made of the efficiency of producing light in this manner. For
example, assume that phosphorescent zinc sulphide is excited by the
light from a mercury-arc. All the energy falling upon it is not capable
of exciting phosphorescence, as may be readily shown. Assuming that a
known amount of radiant energy of a certain wave-length has been
permitted to fall upon the phosphorescent material, then in the dark the
latter may be seen to glow for a long time. An interesting point to
investigate is the relation of the output to input; that is, the ratio
of the total emitted light to the total exciting energy. This is a
neglected aspect in the study of light-production by this means.

The firefly has been praised far and wide as the ideal light-source. It
is an efficient radiator of light, for its light is "cold"; that is, it
does not appear to be accompanied by invisible radiant energy. But
little is said about its efficiency as a light-producer. Who knows how
much fuel its lighting-plant consumes? The chemistry of light-production
by living organisms is being unraveled and this part of the phenomenon
will likely be laid bare before long. For an equal amount of energy
radiated, the firefly emits a great many times more light than th