d with
others, so it has now become a minor light. Flashes of short duration
followed by longer periods of darkness are extensively used. The mariner
by timing the intervals is able to recognize the light. This method is
extended to groups of short flashes followed by longer intervals of
darkness. In fact, short flashes have been employed to indicate a
certain number so that a mariner could recognize the light by a number
rather than by means of his watch. However, a time element is generally
used. A combination of fixed light upon which is superposed a flash or a
group of flashes of white or of colored light has been used, but it is
in disrepute as being unreliable. A type known as "occulating lights"
consists of a fixed light which is momentarily eclipsed, but the
duration of the eclipse is usually less than that of the light.
Obviously, groups of eclipses may be used. Sometimes lights of different
colors are alternated without any dark intervals. The colored ones used
are generally red and green, but these are short-range lights at best.
Colored sectors are sometimes used over portions of the field, in order
to indicate dangers, and white light shows in the fairway. These are
usually fixed lights for marking the channel.

The distance at which a light may be seen at sea depends upon its
luminous intensity, upon its color or spectral composition, upon its
height and that of the observer's eyes above the sea-level, and upon the
atmospheric conditions. Assuming a perfectly clear atmosphere, the
visibility of a light-source apparently depends directly upon its
candle-power. The atmosphere ordinarily absorbs the red, orange, and
yellow rays less than the green, blue, and violet rays. This is
demonstrated by the setting sun, which as it approaches closer to the
horizon changes from yellow to orange and finally to red as the amount
of atmosphere between it and the eye increases. For this reason a red
light would have a greater range than a blue light of the same luminous
intensity.

Under ordinary atmospheric conditions the range of the more powerful
light-sources used in lighthouses is greater than the range as limited
by the curvature of the earth. For the uncolored illuminants the range
in nautical miles appears to be at least equal to the square root of the
candle-power. A real practical limitation which still exists is the
curvature of the earth, and the distance an object may be seen by the
eye at sea-level depends