ld be seen in
succession, and then one red; and others, that two red should be
seen, and then one white. When there is a necessity for what is called
a _leading-line_, as a guide for taking some channel, or avoiding some
danger, _double lights_ are exhibited from two towers, one of which is
higher than the other; and when seen in one line, these form a
direction for the course of the shipping.

When the French were recovering from the long night of terror, during
which their commerce had been ruined and their ships disabled, they
directed attention to lighthouses, and resolved to discard the very
imperfect and insignificant reflectors then in use. They investigated
the subject with their usual scientific skill, and the result was the
invention and adoption of the system of lenses instead of reflectors,
known as the Dioptric system.

A transparent lens reduces to parallelism all the luminous rays which
traverse it, whatever be their original amount of divergence, provided
these rays proceed from a point or focus suitably situated. The
substitution of glass lenses for reflectors is not a new idea, since
we find that a proposal to that effect was made by a London optician
to Mr. Smeaton, in 1759, for illuminating the Eddystone lighthouse,
but was not adopted by him. M. Fresnel mentions that lenses had been
used in England so far back as 1789, in the tower light-room at
Portland Island, but from some cause or other were discontinued.

On account of the great loss of light by reflexion at the surface of
mirrors, the French adopted the lenses, and they soon discovered the
source of failure in our use of them; they saw that, in order to
render lenses superior to reflectors, the intensity of the
illuminating flame must be considerably increased, as well as the size
of the lenses; also, that these lenses must have a very short focus;
and that, if constructed by the ordinary rules, their thickness would
be great, their transparency diminished, and their weight far too
great for the safety of the machinery whereby the lights were
revolved. Fresnel therefore adopted the ingenious device proposed by
Condorcet, that of constructing a lens of a number of distinct
pieces. This method was also proposed by Dr. Brewster, in 1811.
Fresnel also invented a lamp, with a number of concentric wicks, the
lustre of which was twenty-five times greater than the best lamps then
existing.

In a lighthouse on the dioptric system, the lantern is c