ousand 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.

It has been shown that the visibility of point sources of white light in
clear atmosphere, for distances up to a mile at least, is proportional
to their candle-power and inversely proportional to the square of the
distance. Apparently the luminous intensities of signal-lamps required
in clear weather in order that they may be visible must be 0.43 candles
for one nautical mile, 1.75 candles for two nautical miles, and 11
candles for five nautical miles. From the data available it appears that
a red or a white signal-light will be easily visible at a distance in
nautical miles equal to the square root of its candle-power in that
direction. The range in nautical miles of a green light apparently is
proportional to the cube root of the candle-power. Whether or not these
relations between the range in miles and the luminous intensity in
candles hold for greater distances than those ordinarily encountered has
not been determined, but it is interesting to note that the square root
of the luminous intensity of the Navesink Light at the entrance to New
York Harbor is about 7000. Could this light be seen at a distance of
seven thousand miles through ordinary atmosphere?

The most distinctive colored lights are red, yellow, green, and blue. To
these white (clear) and purple have been added for signaling-purposes.
Yellow is intense, but it may be confused with "white" or clear. Blue
and purple as obtained from the present practicable light-sources are of
low intensity. This leaves red, green, and clear as the most generally
satisfactory signal-lights.

There are numerous other applications, especially indoors. Some of these
have been devised for special needs, but there are many others which are
general, such as for elevators, telephones, various call