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 systems, and traffic signals.  Light has the advantages of being silent and controllable as to position and direction, and of being a visible signal at night.  Thus, in another field artificial light has responded to the demands of civilization.

XVI

THE COST OF LIGHT

Artificial light is so superior to natural light in many respects that mankind has acquired the habit of retiring many hours after darkness has fallen, a result of which has brought forth the issue known as “daylight saving.”  Doubtless, daylight should be used whenever possible, but there are two sides to the question.  In the first place, it costs something to bring daylight indoors.  The architectural construction of windows and skylights increases the cost of daylight.  Light-courts, by sacrificing valuable floor-area, add to the expense.  The maintenance of windows and sky lights is an appreciable item.  Considering these and other factors, it can be seen that daylight indoors is expensive; and as it is also undependable, a supplementary system of artificial lighting is generally necessary.  In fact, it is easy to show in some cases that artificial lighting is cheaper than natural lighting.