upon the height of the object. The relation is
approximately expressed thus,--

Range in nautical miles = 8/7 square root of Height of object in
feet. For example, the top of a tower 100 feet high is visible to an eye
at sea-level a distance of 8/7 square root of 100 = 80/7 = 11.43
miles. Now if the eye is 49 feet above sea-level, a similar computation
will show how far away it may be seen by the original eye at sea-level.
This is 8/7 square root of 49 = 8 miles. Hence an eye 49 feet above
sea-level will be able to see the top of the 100-foot tower at a
distance of 11.43 + 8 or 19.43 nautical miles. Under these conditions an
imaginary line drawn from the top of the tower to the eye will be just
tangent to the spherical surface of the sea at a distance of 8 miles
from the eye and 11.43 miles from the tower.

The luminous intensity of a light-source or of the beam of light is
directly responsible for the range. The luminous intensity of the early
beacon-fires and oil-lamps was equivalent to a few candles. The
improvements in light-sources and also in reflecting and refracting
optical systems have steadily increased the candle-power of the beams,
until to-day the beams from gas-lamps have intensities as high as
several hundred thousand candle-power. The beams sent forth by modern
lighthouses equipped with electric lamps and enormous light-gathering
devices are rated in millions of candle-power. In fact, Navesink Light
at the entrance of New York Bay is rated as high as 60,000,000
candle-power.

Of course, light-production has increased enormously in efficiency in
the past century, but without optical devices for gathering the light,
the enormous beam intensity would not be obtained. For example, consider
a small source of light possessing a luminous intensity of one candle in
all directions. If all this light which is emitted in all directions is
gathered and sent forth in a beam of small angle, say one thousandth of
the total angle surrounding a point, the intensity of this beam would be
1000 candles. It is in this manner that the enormous beam intensities
are built up.

There is an interesting point pertaining to short flashes of light. To
the dark-adapted eye a brief flash is registered as of considerably
higher intensity than if the light remained constant. In other words,
the lookout on a vessel is adapted to darkness and a flash from a beam
of light is much brighter than if the same beam were shining steadily.
T