a discouraging byway.

It is natural to inquire whether or not any mode of light-production now
in use has a limiting luminous efficiency approaching the ultimate limit
which is imposed by the visibility of radiation. The eye is able to
convert radiant energy of different wave-lengths into certain fixed
proportions of light. For example, radiant energy of such a wave-length
as to excite the sensation of yellow-green is the most efficient and one
watt of this energy is capable of being converted by the visual
apparatus into about 625 lumens of light. Is this efficiency of
conversion of the visual apparatus everlastingly fixed? For the answer
it is necessary to turn to the physiologist, and doubtless he would
suggest the curbing of the imagination. But is it unthinkable that the
visual processes will always be beyond the control of man? However, to
turn again to the physics of light-production, there are still several
processes of producing light which are appealing.

Many years ago Geissler, Crookes, and other scientists studied the
spectra of gases excited to incandescence by the electric discharge in
so-called vacuum tubes. The gases are placed in transparent glass or
quartz tubes at rather low pressures and a high voltage is impressed
upon the ends of these tubes. When the pressure is sufficiently low, the
gases will glow and emit light. Twenty-eight years ago, D. McFarlan
Moore developed the nitrogen tube, which was actually installed in
various places. But there is such a loss of energy near the cathode that
short "vacuum" tubes of this character are very inefficient producers of
light. Efficiency is greatly increased by lengthening the tubes, so
Moore used tubes of great length and a rather high voltage. As a tube of
this sort is used, the gas gradually disappears and it must be
replenished. In order to replenish the gas, Moore devised a valve for
feeding gas automatically. An advantage of this mode of light-production
is that the color or quality of the light may be varied by varying the
gas used. Nitrogen yields a pinkish light; neon an orange light; and
carbon dioxide a white light. Moore's carbon-dioxide tube is an
excellent substitute for daylight and has been used for the
discrimination of colors where this is an important factor. However, for
this purpose devices utilizing color-screens which alter the light from
the tungsten lamp to a daylight quality are being used extensively.

The vacuum-tube method o