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 thousand feet.  It is said that these lights are seen more easily than semaphore arms under all circumstances and that they show two or three times as far as the latter during a snow-storm.

The standard colors for light-signals as adopted by the Railway Signal Association are red, yellow, green, blue, purple, and lunar white.  These are specified as to the amount of the various spectral colors which they transmit when the light-source is the kerosene flame.  Obviously, the colors generally appear different when another illuminant is used.  The blue and purple are short-range signals, but the effective range of the best railway signal employing a kerosene flame is only about four miles.