f the arc volatilizes the mercury
so that an arc of considerable length is maintained. The tilting is done
by electromagnets. Starting has also been accomplished by means of a
heating coil and also by an electric spark. The lamps are stabilized by
resistance and inductance coils.

One of the defects of the light emitted by the incandescent vapor of
mercury is its paucity of spectral colors. Its visible spectrum consists
chiefly of violet, blue, green, and yellow rays. It emits virtually no
red rays, and, therefore, red objects appear devoid of red. The human
face appears ghastly under this light and it distorts colors in general.
However, it possesses the advantages of high efficiency, of reasonably
low brightness, of high actinic value, and of revealing detail clearly.
Various attempts have been made to improve the color of the light by
adding red rays. Reflectors of a fluorescent red dye have been used with
some success, but such a method reduces the luminous efficiency of the
lamp considerably. The dye fluoresces red under the illumination of
ultra-violet, violet, and blue rays; that is, it has the property of
converting radiation of these wave-lengths into radiant energy of longer
wave-lengths. By the use of electric incandescent filament lamps in
conjunction with mercury-arcs, a fairly satisfactory light is obtained.
Many experiments have been made by adding other substances to the
mercury, such as zinc, with the hope that the spectrum of the other
substance would compensate the defects in the mercury spectrum. However
no success has been reached in this direction.

By the use of a quartz tube which can withstand a much higher
temperature than glass, the current density can be greatly increased.
Thus a small quartz tube of incandescent mercury vapor will emit as much
light as a long glass tube. The quartz mercury-arc produces a light
which is almost white, but the actual spectrum is very different from
that of white sunlight. Although some red rays are emitted by the quartz
arc, its spectrum is essentially the same as that of the glass-tube arc.
Quartz transmits ultra-violet radiation, which is harmful to the eyes,
and inasmuch as the mercury vapor emits such rays, a glass globe should
be used to enclose the quartz tube when the lamp is used for ordinary
lighting purposes.

It is fortunate that such radically different kinds of light-sources are
available, for in the complex activities of the present time all are