Washington sentinel. [volume] (Washington [D.C.]) 1853-1855, 1853-10-02
description Of this light to Capt. Mead's own

description Of this light to Capt. Mead's own
notes.
The object to be attained in the use of lenses
in the illuminating apparatus OF lighthouse
is effected by collecting all the rays emanating
from point in the focus, and, after refracting
| or bending them from their original paths, pro-
jecting them forward in beam whose axis CO.
incides with the axis Of the lens.
The earliest notice Of the application of len
ses to lighthouses is found in Smeaton's Mar
rative Of the Eddystone, where he states that a
London optician, in 1850, proposed grinding
the glass of the lantern to radius OF seven
feet six inches. Many attempts were made sub
sequently at different times, butnever succeeded,
owing to the imperfect figure Of the lens, the
impure state Of the glass, and its great thick.
ness, which rendered the lens in its effect in
terior to the refectors then in use. The sug-
gestion Of building the lenses in separate pieces
is due to Condorcet, who, as early as 1878,
| 1911, Sir David Brewster in the "Edinboro'
Cyclopedia," proposed this plan; and in 1922
Augustin Fresnel, Of France, made known the
same ingenious mode Of constructing these in
struments, which he had discovered in 1919, in
ignorance Of the views and labors of his predc-
cessors. To Fresnel belongs the additional and
equally great merit of being the first to execute
as well as design; and, in conjunction with NJ.
NJ Arrago and Matthew, Of placing a powerful
| lamp in the focus Of the lens-in fact applying
it to the practical purpose Of lighthouse.
| Hence the method has been called the Fresnel
system, now introduced into lighthouses all
over the world, and which, within few years
past has been introduced into this country.
1t would be out Of place. indeed impractical
BILE, in an notice of this kind, to enter into all
the details Of the preliminary calculations, and
the different processes Of the manufacture OF
these lenses; only general outline Of the va
rious steps can be given. For a more partcu
lar account the reader is referred to very use
full publication by Alan Stevenson, entitled Gan
Elementary Treatise on the History, Construc
tom. and Illuminating Of Lighthouses, pub
lished by John Weale, London, where will be
found an able and detailed discussion of the
whole subject.
The first point to be effected is the calcula-
ton Of the surfaces of the different parts
These calculations, requiring knowledge of
the higher branches Of mathematics, are ex-
ceedingly minute and compicated-th princ-
pal data on which they arc based being the
focal distance and the retroactive index of the
glass. The elements Of the different parts
being calculated, the glass is cast in moulds
exceeding the intended size Of the finished
| parts. The glass hitherto used in France is
| crown glass, which, though it has a lower re.
ffactive power than flint glass, besides having
| a slight greenish tnge, yet it can more easily
be obtained OF homogeneous quality, and is,
moreover, less sube to deterioration from at
mospheric Influences and therefore peculiarly
suitable for use in the exposed positions gene
rally occupied by lighthouses. The different
parts being cast, they are ground to the pro-
per size by machinery. This process is very
difficult requiring the greatest care, not only to
preserve the optical properties Of the surface,
but to prevent the transparency of the. glass
being affected by scratches. Lastly comes the
adjustment of the different parts in their re.
spcctiye forms, and the union Of the whole so
as to produce the desired effect. The different
parts Of each lens are united by bronze frame,
and, where the glass edges come in contact, by
delicate edges render this operation hazard
Gus one as they are liable to be splintered in
the hands Of unskilfu persons.
There are four different orders Of these in
struments, ranging in size according to the
position they have to occupy and the distances
they are required to be seem Each order is
subdivded into fixed" revolving or flash
ing" and fixed varied BY flashes The
flashing lights can be varied by the duration of
the flash and eclipse, and in some cases colored
flashes have been employed for the sake of dis
tinction.
The apparatus now on exhibition is Of the
first order, and is a revolving or flashing light.
apparel catadioptrique dc lst ordre a
cclipses," Ga cclats sur toute Ia hauteur,) con
structed by NJ. Henri Lepaute, No. 247 Rue st.
Honore, Paris.
It is composed Of movable frame, the
central part Of which is right hollow prism
of twenty-four sides. Each of these sides is
formed Of panel, which is a section of an an
nular plano-convex lens, four feet three inches
in height, (the diameter of the lens,) and nine
inches wide These lenses are constructed as
described above, in separate picces-thc cen.
trail disc being eleven inches in diameter, and
the ann,,lar rings which surround it gradually
decrease in breadth as they recede from their
serrated. thus reducing the thickness Of the
glass, while the optical properties Of the lens
are preserved. The pieces are united together
by fine layers of cement, and the whole are
firmly cemented into u brass frame. All the
light from the lamp in the common focus fall
ing on these lenses is refracted through them,
and thrown out in horizontal beam whose
axis is concident with the axis Of the lens.
There are, therefore, twenty-four beams pro-
jected at the same time from the apparatus
when illuminated, and the revolution causes
succession of brilliant flashes, which gives the
character to the light.
These lenses are larger than any previously
made, being, as stated above, tifty-ouc inches
whereas the usual height of those hitherto
constructed has been about forty inches. There
is limit, however, to the size Of the lens, de-
pendent on the incident ray on its edges, which
must not fall on it at too small IN angle. In
consequence Of this limit to the size Of lenses
y considerable portion Of the light passes
above and below them To prevent its loss
and to cause these rays to contribute their share
to the flashes, the apparatus on exhibition has
below the lenses n series Of four, and above
dome of eleven, catadioptric rings Of glass.
These rings are placed in ~ frames CORES.
bonding with the panels Of the central part, and
are triangular prsms, so constructed and ad-
iusted in their frames that all the rays Of light
impinging on them are, after being refractcd
at the first surface, reflected from the second;
al. q. .~.0