his is a physiological phenomenon which operates in favor of the
flashing light.

[Illustration: A. A COMPLETED LIGHTHOUSE LENS]

[Illustration: B. TORRO POINT LIGHTHOUSE, PANAMA CANAL]

[Illustration: AMERICAN SEARCH-LIGHT POSITION ON WESTERN FRONT IN 1919]

[Illustration: AMERICAN STANDARD FIELD SEARCH-LIGHT AND POWER UNIT]

Doubtless, the reader has noted that reliability, simplicity, and low
cost of operation are the primary considerations for light-sources used
as aids to navigation. This accounts for the continued use of oil and
gas. From an optical standpoint the electric arc-lamps and
concentrated-filament lamps are usually superior to the earlier sources
of light, but the complexity of a plant for generating electricity is
usually a disadvantage in isolated places. The larger light-ships are
now using electricity generated by apparatus installed in the vessels.
There seems to be a tendency toward the use of more buoys and fewer
lighthouses, but the beam-intensities of the latter are increasing.

In the hundred years since the Boston Light was built the same great
changes wrought by the development of artificial light in other
activities of civilization have appeared in the beacons of the mariner.
The development of these aids to navigation has been wonderful, but it
must go on and on. The surface of the earth comprises 51,886,000 square
statute miles of land and 145,054,000 square miles of water. Three
fourths of the earth's surface is water and the oceans will always be
highways of world commerce. All the dangers cannot be overcome, but
human ingenuity is capable of great achievements. Wreckage will appear
along the shore-lines despite the lights, but the harvest of the shoals
has been much reduced since the time described by Robert Louis
Stevenson, when the coast people in the Orkneys looked upon wrecks as a
source of gain. He states:

It had become proverbial with some of the inhabitants to
observe that "if wrecks were to happen, they might as well be
sent to the poor island of Sanday as anywhere else." On this
and the neighboring island, the inhabitants have certainly had
their share of wrecked goods. On complaining to one of the
pilots of the badness of his boat's sails, he replied with some
degree of pleasantry, "Had it been His [God's] will that you
come na here wi these lights, we might a' had better sails to
our boats and more o' other things."