e most
efficient lamp in use at the present time, but before the firefly is
pronounced ideal, the efficiency of its light-producing process must be
known.

There are many ways of exciting phosphorescence and fluorescence, the
latter being merely an unenduring phosphorescence, which ceases when the
exciting energy is cut off. Ultra-violet, violet, and blue rays are
generally the most effective radiant energy for excitation purposes.
X-rays and the high-frequency discharge are also powerful excitants. As
already stated, virtually nothing is known of the efficiency of this
mode of light-production or of the mechanism within the substance, but
on the whole it is a remarkable phenomenon.

Radium is also a possibility in light-production and in fact has been
practically employed for this purpose for several years. It or one of
its compounds is mixed with a phosphorescent substance such as zinc
sulphide and the latter glows continuously. Inasmuch as the life of some
of the radium products is very long, such a method of illuminating
watch-dials, scales of instruments, etc., is very practicable where they
are to be read by eyes adapted to darkness and consequently highly
sensitive to light. Whether or not radium will be manufactured by the
ton in the future can only conjectured.

Owing to the limitations imposed by physical laws of radiation and by
the physiological processes of vision the highest luminous efficiency
obtainable by heating solid materials is only about 15 per cent. of the
luminous efficiency of the most luminous radiant energy. At present
there are no materials available which may be operated at the
temperature necessary to reach even this efficiency. Great progress in
the future of light-production as indicated by present knowledge appears
to lie in the production of light which is unaccompanied by invisible
radiant energy. At present such phenomena as fluorescence,
phosphorescence, the light of the firefly, chemi-luminescence, etc., are
examples of this kind of light-production. Of course, if science ever
obtains control over the constitution of matter, many difficulties will
disappear; for then man will not be dependent upon the elements and
compounds now available but will be able to modify them to suit his
needs.

XII

LIGHTING THE STREETS

In this age of brilliantly lighted boulevards and "great white ways"
flooded with light from shop-windows, electric signs, and street-lamps,
it is diffic