daylight and
small tubes containing this gas are in use to-day where accurate
discrimination of color is essential.

The flame-arcs afford a means of obtaining a variety of illuminants
differing in spectral character or color. By impregnating the carbons
with various chemical compounds the color of the flame can be widely
altered. The white flame-arc obtained by the use of rare-earth compounds
in the carbons provides an illuminant closely approximating average
daylight. By using various substances besides carbon for the
electrodes, illuminants differing in spectral character can be
obtained. These are usually rich in ultra-violet rays and therefore have
their best applications in processes demanding this kind of radiant
energy. The arc-lamp is limited in its application by its unsteadiness,
its bulkiness, and the impracticability of subdividing it into
light-sources of a great range of luminous intensities.

The most extensive applications of artificial daylight have been made by
means of the electric incandescent filament lamp, equipped with a
colored glass which alters the light to the same quality as daylight.
The light from the electric filament lamp is richer in yellow, orange,
and red rays than daylight, and by knowing the spectral character of the
two illuminants and the spectral characteristics of colored glasses in
which various chemicals have been incorporated, it is possible to
develop a colored glass which will filter out of the excess of yellow,
orange, and red rays so that the transmitted light is of the same
spectral character as daylight. Thousands of such artificial daylight
units are now in use in the industries, in stores, in laboratories, in
dye-works, in print-shops, and in many other places. Currency and
Liberty Bonds have been made under artificial daylight and such units
are in use in banks for the detection of counterfeit currency. The
diamond expert detects the color of jewels and the microscopist is
certain of the colors of his stains under artificial daylight. The dyer
mixes his dyes for the coloring of tons of valuable silk and the artist
paints under this artificial light. These are only a few of a vast
number of applications of artificial daylight, but they illustrate that
mankind is independent of natural light in another respect.

There are various kinds of daylight, two of which are fairly constant in
spectral character. These are noon sunlight and north skylight. The
former may be s