onstructed
with eight sides, which form an octagonal prism around the lamp in the
centre. The centre of each side is occupied by a plano-convex lens,
something similar to a burning-glass, having a diameter of about
fifteen inches. This central lens is not sufficient to cover the
entire side. Indeed, a lens of sufficient size for the purpose would
be very costly and bulky, even supposing it could be manufactured. To
remedy this defect, the central lens is surrounded by a series of
glass rings, the external surface of which is so formed as to have
precisely the same optical effect as the great central lens. A
transverse section of one of these zones or rings presents the form of
a wedge, one side of which is slightly curved.

By this arrangement each lens transmits to all the points of the
horizon in succession a light equivalent to that of from three to four
thousand lamps with double currents, and eight times greater than the
light produced by the silver parabolic reflectors; it is, according to
Arago, the same amount of light as would be obtained if it were
possible to bring together the third of the whole number of gas-lights
which illumine the streets, the shops, and the theatres of Paris; and
this wonderful result is obtained from a single lamp.

This lamp has four concentric burners, which are defended from the
action of the excessive heat produced by their united flames, by means
of a superabundant supply of oil, which is thrown up from the cistern
below by a clock-work movement, and constantly overflows the wicks. A
very tall chimney is necessary in order to supply fresh currents of
air to each wick with sufficient rapidity to support the combustion.
The carbonization of the wicks is not very rapid; and after they have
been burning a long time, the flame is not sensibly diminished, as
the great heat evolved from the mass of flame promotes the rising of
the oil in the cotton.

In the year 1820, in the course of some investigations connected with
the Trigonometrical Survey of Great Britain, and conducted by a
deputation of scientific persons from London and Paris, M. Fresnel
exhibited from the French side of the channel, by means of his lens
and a large lamp, a powerful light which was observed by the English
across the channel. The brilliancy of this light so struck Lieut.-Col.
Colby, of the Royal Engineers, who was engaged in these observations,
that he immediately corresponded with Mr. Stevenson as to its pro