stars out, and those none of the freshest-looking;
you stare through the thick air and
mist towards the horizon, which is all gloom.
Presently there emerges from this vaporous
gloom, at a great distance, a faint red orb,
which gradually attains its full disk, and then
declines, and disappears. It has been visible
half a minute. All is darkness again for
about two minutes— and then it appears again,
and disappears, with the same planetary
effect, as before. Sometimes a revolving
"light " alternates a red " light " and a white
one; sometimes two red and one white,—
and so on.

The machinery for the revolving " lights,"
as may readily be supposed, is far more
complicated than that which exhibits the fixed
"light "; but all these differences fade to
nothing, in comparison with the scientific
complexities of the different optical systems
employed in obtaining the largest amount and
the purest degree of light. A very concise
account of these, simplified as much as
possible, is all we can venture to offer.

A lamp constructed of a simple burner,
would send forth scattered rays, illumining
parts not needed, and wasting many rays,
upward, into the sky, and also below the plane
of vision of those out at sea. To correct this,
reflectors are employed, by means of which,
according to laws which cannot here be
discussed, the rays are concentrated and thrown
in the greatest strength towards given points
on the horizon. The amount of luminous
effect, produced by reflectors in fixed " lights,"
has been ascertained to be about three
hundred and fifty times greater than . can be
obtained from the unassisted flame; and for
those larger ones, which are used in revolving
"lights," about four hundred and fifty times
greater. These calculations, however, will
vary with the distances at which the observations
are taken.

The manufacture of these reflectors is a very
nice and curious process. It is described at
considerable length, and with great clearness,
by the engineer to the Board of Northern
Lighthouses (Mr. Alan Stevenson), to whose
excellent Rudimentary Treatise we are much
indebted.

In Lighthouses of the first class, the
reflectors are made of sheet copper, thickly
plated inside with silver. They are moulded
to a paraboloïdal form by a delicate and
laborious process of beating with mallets and
hammers of various forms and materials.
They are frequently tested during the operation,
by a gauge applied to the back, and
another to the concave face. After the face
has received its last polish, it is tested by
placing a burner in the focus, and measuring
the strength of the light at various points of
the reflected conical beam. The flame
generally used is derived from an Argand fountain-
lamp.

The system of lights just described is called
the " catoptric." It includes nine distinctions
viz., the fixed; the revolving white; the
revolving white and red; the revolving red
with two whites; the revolving white with
two reds; the flashing; the intermittent; the
double fixed light; and double revolving white
light.

The first exhibits a fixed and uniform
appearance. The revolving light is produced by
the revolution of a frame with three or four
sides, having reflectors ranged on each side.
The succession of red and white lights is
obviously caused by the revolution of a frame
presenting different lights on each side, according
to arrangement— and carefully marked on
the sailors' charts. The flashing light is
produced in the same manner as the revolving;
but having a different disposition of the
mirrors, and a greater quickness of the
revolution, which produces the effect of a flash in
the darkness once in about five seconds of
time. It is very useful, from its striking
difference to other lights. The intermittent
light bursts suddenly into view, continues
steady for a short time, and then suddenly
disappears. The different colours are
produced by interposing coloured media, which
generally absorbing nearly five-sevenths of
the whole light, they cannot be seen from so
great a distance as the white. The French
tried red glass, which absorbs only four-
sevenths of the light; but a greater improvement
has been made in the Scotch Lighthouses,
by using chimneys of red glass
instead of placing coloured media in front of
the reflectors.

Coloured lights are only used for the
necessity of variety. Next to the white in
power is the red. The green and the blue are
seldom used, except as pier and harbour lights,
where distance is not required, because these
colours absorb nearly all the rays.

But the different effects previously enumerated,
are not accomplished by one system of
optics. Great and various have been the
studies of lighthouse opticians, and innumerable
the experiments that have been made to
produce all the varieties now in use. We
have spoken of the catoptric, or reflecting
system; but to Condorcet belongs the merit
of having suggested the dioptric, or refracting,
system, which was afterwards reduced to
practice by Fresnel. By the adoption of
lenses, he not only accomplished the effect of
flashing lights, but finally devised a new
distinction of light, viz. a fixed light, varied by
flashes. The great lamp, which has four
concentric burners, is also of very peculiar
construction, and was made to burn in the
great Lighthouse, called the Tour de Corduan,
during seven hours, without snuffing or having
the wicks raised. Subsequently, the same
kind of lamp has been known to burn,
untouched, in some of the Scotch Lighthouses,
for the extraordinary period of seventeen
hours.

Into the intense complexities of catoptric,
dioptric, dia-catoptric, and cata-dioptric