and the light
was invisible to an observer unless he was equipped with a similar red
screen to eliminate the daylight. It is said that signals were
distinguished at a distance of six miles. By night a screen was used
which transmitted only the ultraviolet rays, and the observer's
telescope was provided with a fluorescent screen in its focal plane. The
ultraviolet rays falling upon this screen were transformed into visible
rays by the phenomenon of fluorescence. The range of this device was
about six miles. For naval convoys lamps are required to radiate toward
all points of the compass. For this purpose a quartz mercury-arc which
is rich in ultraviolet rays was surrounded with a chimney which
transmitted the ultraviolet rays efficiently and absorbed all visible
rays excepting violet light. The lamp appeared a deep violet color at
close range, but the faintly visible light which it transmitted was not
seen at a distance. A distant observer picks up the invisible
ultraviolet "light" by means of a special optical device having a
fluorescent screen of barium-platino-cyanide. This device had a range of
about four miles.

Light-signals are essential for the operation of railways at night and
they have been in use for many years. In this field the significance of
light-signals is based almost universally on color. The setting of a
switch is indicated by the color of the light that it shows. With the
introduction of the semaphore system, in which during the day the
position of the arm is significant, colored glasses were placed on the
opposite end of the arm in such a manner that a certain colored glass
would appear before the light-source for a certain position of the arm.
A kerosene flame behind a glass lens was the lamp used, and, for
example, red meant "Stop," green counseled "Caution," and clear or white
indicated "All clear." For many years the kerosene lamp has been used,
but recently the electric filament lamp is being installed to some
extent for this purpose. In fact, on one railroad at least, tungsten
lamps are used for light-signals by day as well as by night. Three
signals--red, green, and white--are placed in a vertical line and behind
each lens are two lamps, one operating at high efficiency and one at low
efficiency to insure against the failure of the signal. The normal
daylight range is about three thousand feet and under the worst
conditions when opposed to direct sunlight, the range is not less than
two th