ry. They were at first ground from a solid piece of
glass, in concentric zones, in order to reduce the thickness. They were
similar in principle to some of the tail-light lenses used at present on
automobiles. Later the lenses were built up by means of separate annular
rings. The name of Fresnel is permanently associated with lighthouse
lenses because in 1822 he developed an elaborate built-up lens of
annular rings. The centers of curvature of the different rings receded
from the axis as their distance from the center increased, in such a
manner as to overcome a serious optical defect known as spherical
aberration. Fresnel devised many improvements in which he used
refracting and reflecting prisms for the outer elements.

The optical apparatus of lighthouses usually aims (1) to concentrate the
rays of light into a pencil of light, (2) to concentrate them into a
belt of light, or (3) to concentrate the rays over a limited azimuth. In
the first case a single lens or a parabolic reflector suffices, but in
the second case a cylindrical lens which condenses the light vertically
into a horizontal sheet of light is essential. The third case is a
combination of the first two. The modern lighthouse lenses are very
elaborate in construction, being built up by means of many elements into
several sections. For example, the central section may consist of a
spherical lens ground with annular rings. In the next section refracting
prisms may be used and in the outer section reflecting glass prisms are
employed. The various elements are carefully designed according to the
laws of geometrical optics.

The flashing light has such advantages over the fixed that it is
generally used for important beacons. A variety of methods of obtaining
intermittent light have been employed, but they are not of particular
interest. Sometimes the lens or reflector is revolved and in other types
an opaque screen containing slits is revolved. In the larger lighthouses
the optical apparatus and its structure sometimes weigh several tons.
When it is necessary to revolve apparatus of this weight, the whole
mechanism is floated upon mercury contained in a cast-iron vessel of
suitable size, and by an ingenious arrangement only a small portion of
mercury is required.

The characteristics of navigation lights are varied considerably in
order to enable the mariner to distinguish them and thereby to learn
exactly where he is. The fixed light is liable to be confuse