[Illustration: FIG. 40.--Sule Skerry Apparatus.]
_Hyper-radial Apparatus._--In 1885 Messrs Barbier of Paris constructed
the first hyper-radial apparatus (1330 mm. focal distance) to the
design of Messrs D. and C. Stevenson. This had a height of 1812 mm. It
was tested during the South Foreland experiments in comparison with
other lenses, and found to give excellent results with burners of
large focal diameter. Apparatus of similar focal distance (1330 mm.)
were subsequently established at Round Island, Bishop Rock, and Spurn
Point in England, Fair Isle and Sule Skerry (fig. 40) in Scotland,
Bull Rock and Tory Island in Ireland, Cape d'Antifer in France, Pei
Yu-shan in China and a lighthouse in Brazil.
The light erected in 1907 at Cape Race, Newfoundland, is a fine
example of a four-sided hyper-radial apparatus mounted on a mercury
float. The total weight of the revolving part of the light amounts to
7 tons, while the motive clock weight required to rotate this large
mass at a speed of two complete revolutions a minute is only 8 cwt.
and the weight of mercury required for flotation 950 lb. A similar
apparatus was placed at Manora Point, Karachi, India, in 1908 (fig.
41).
The introduction of incandescent and other burners of focal
compactness and high intensity has rendered the use of optics of such
large dimensions as the above, intended for burners of great focal
diameter, unnecessary. It is now possible to obtain with a
second-order optic (or one of 700 mm. focal distance), having a
powerful incandescent petroleum burner in focus, a beam of equal
intensity to that which would be obtained from the apparatus having a
10-wick oil burner or 108-jet gas burner at its focus.
_Stephenson's Spherical Lenses and Equiangular Prisms._--Mr C. A.
Stephenson in 1888 designed a form of lens spherical in the horizontal
and vertical sections. This admitted of the construction of lenses of
long focal distance without the otherwise corresponding necessity of
increased diameter of lantern. A lens of this type and of 1330 mm.
focal distance was constructed in 1890 for Fair Isle lighthouse. The
spherical form loses in efficiency if carried beyond an angle
subtending 20° at the focus, and to obviate this loss Mr Stephenson
designed his equiangular prisms, which have an inclination outwards.
It is claimed by the designer that the use of equiangular prisms
results in less loss of light and less divergence than is the case
when either the spherical or Fresnel form is adopted. An example of
this design is seen (fig. 40) in the Sule Skerry apparatus (1895).
_Fixed and Flashing Lights._--The use of these lights, which show a
fixed beam varied at intervals by more powerful flashes, is not to be
recommended, though a large number were constructed in the earlier
years of dioptric illumination and many are still in existence. The
distinction can be produced in one or other of three ways: (a) by the
revolution of detached panels of straight condensing lens prisms
placed vertically around a fixed light optic, (b) by utilizing
revolving lens panels in the middle portion of the optic to produce
the flashing light, the upper and lower sections of the apparatus
being fixed zones of catadioptric or reflecting elements emitting a
fixed belt of light, and (c) by interposing panels of fixed light
section between the flashing light panels of a revolving apparatus. In
certain conditions of the atmosphere it is possible for the fixed
light of low power to be entirely obscured while the flashes are
visible, thus vitiating the true characteristic of the light. Cases
have frequently occurred of such lights being mistaken for, and even
described in lists of light as, revolving or flashing lights.
_"Cute" and Screens._--Screens of coloured glass, intended to
distinguish the light in particular azimuths, and of sheet iron, when
it is desired to "cut off" the light sharply on any angle, should be
fixed as far from the centre of the light as possible in order to
reduce the escape of light rays due to divergence. These screens are
usually attached to the lantern framing.
_Divergence._--A dioptric apparatus designed to bend all incident rays
of light from the light source in a horizontal direction would, if the
flame could be a point, have the effect of projecting a horizontal
band or zone of light, in the case of a fixed apparatus, and a
cylinder of light rays, in the case of a flashing light, towards the
horizon. Thus the mariner in the near distance would receive no light,
the rays, visible only at or near the horizon, passing above the level
of his eye. In practice this does not occur, sufficient natural
divergence being produced ordinarily owing to the magnitude of the
flame. Where the electric arc is employed it is often necessary to
design the prisms so as to produce artificial divergence. The measure
of the natural divergence for any point of the lens is the angle whose
sine is the ratio of the diameter of the flame to the distance of the
point from centre of flame.
In the case of vertical divergence the mean height of the flame must
be substituted for the diameter. The angle thus obtained is the total
divergence, that is, the sum of the angles above and below the
horizontal plane or to right and left of the medial section. In fixed
dioptric lights there is, of course, no divergence in the horizontal
plane. In flashing lights the horizontal divergence is a matter of
considerable importance, determining as it does the duration or length
of time the flash is visible to the mariner.
_Feux-Éclairs or Quick Flashing Lights._--One of the most important
developments in the character of lighthouse illuminating apparatus
that has occurred in recent years has been in the direction of
reducing the length of flash. The initiative in this matter was taken
by the French lighthouse authorities, and in France alone forty lights
of this type were established between 1892 and 1901. The use of short
flash lights rapidly spread to other parts of the world. In England
the lighthouse at Pendeen (1900) exhibits a quadruple flash every 15
seconds, the flashes being about ¼ second duration (fig. 39), while
the bivalve apparatus erected on Lundy Island (1897) shows 2 flashes
of 1/3 second duration in quick succession every 20 seconds. Since
1900 many quick flashing lights have been erected on the coasts of the
United Kingdom and in other countries. The early _feux-éclairs_,
designed by the French engineers and others, had usually a flash of
(1/10)th to (1/3)rd of a second duration. As a result of experiments
carried out in France in 1903-1904, 3/10 second has been adopted by
the French authorities as the minimum duration for white flashing
lights. If shorter flashes are used it is found that the reduction in
duration is attended by a corresponding, but not proportionate,
diminution in effective intensity. In the case of many electric
flashing lights the duration is of necessity reduced, but the greater
initial intensity of the flash permits this loss without serious
detriment to efficiency. Red or green requires a considerably greater
duration than do white flashes. The intervals between the flashes in
lights of this character are also small, 2½ seconds to 7 seconds. In
group-flashing lights the intervals between the flashes are about 2
seconds or even less, with periods of 7 to 10 or 15 seconds between
the groups. The flashes are arranged in single, double, triple or even
quadruple groups, as in the older forms of apparatus. The _feu-éclair_
type of apparatus enables a far higher intensity of flash to be
obtained than was previously possible without any corresponding
increase in the luminous power of the burner or other source of light.
This result depends entirely upon the greater ratio of condensation of
light employed, panels of greater angular breadth than was customary
in the older forms of apparatus being used with a higher rotatory
velocity. It has been urged that short flashes are insufficient for
taking bearings, but the utility of a light in this respect does not
seem to depend so much upon the actual length of the flash as upon its
frequent recurrence at short intervals. At the Paris Exhibition of
1900 was exhibited a fifth-order flashing light giving short flashes
at 1 second intervals; this represents the extreme to which the
movement towards the reduction of the period of flashing lights has
yet been carried.
_Mercury Floats._--It has naturally been found impracticable to
revolve the optical apparatus of a light with its mountings, sometimes
weighing over 7 tons, at the high rate of speed required for
_feux-éclairs_ by means of the old system of roller carriages, though
for some small quick-revolving lights ball bearings have been
successfully adopted. It has therefore become almost the universal
practice to carry the rotating portions of the apparatus upon a
mercury float. This beautiful application of mercury rotation was the
invention of Bourdelles, and is now utilized not only for the
high-speed apparatus, but also generally for the few examples of the
older type still being constructed. The arrangement consists of an
annular cast iron bath or trough of such dimensions that a similar but
slightly smaller annular float immersed in the bath and surrounded by
mercury displaces a volume of the liquid metal whose weight is equal
to that of the apparatus supported. Thus a comparatively insignificant
quantity of mercury, say 2 cwt., serves to ensure the flotation of a
mass of over 3 tons. Certain differences exist between the type of
float usually constructed in France and those generally designed by
English engineers. In all cases provision is made for lowering the
mercury bath or raising the float and apparatus for examination.
Examples of mercury floats are shown in figs. 41, 42, 43 and Plate I.,
figs. 54 and 55.
[Illustration: FIG. 41.--Manora Point Apparatus and Lantern.]
_Multiform Apparatus._--In order to double the power to be obtained
from a single apparatus at stations where lights of exceptionally high
intensity are desired, the expedient of placing one complete lens
apparatus above another has sometimes been adopted, as at the Bishop
Rock (fig. 13), and at the Fastnet lighthouse in Ireland (Plate I.,
fig. 54). Triform and quadriform apparatus have also been erected in
Ireland; particulars of the Tory Island triform apparatus will be
found in table VII. The adoption of the multiform system involves the
use of lanterns of increased height.
_Twin Apparatus._--Another method of doubling the power of a light is
by mounting two complete and distinct optics side by side on the same
revolving table, as I shown in fig. 43 of the Île Vierge apparatus.
Several such lights have been installed by the French Lighthouse
Service.
_Port Lights._--Small self-contained lanterns and lights are in common
use for marking the entrances to harbours and in other similar
positions where neither high power nor long range is requisite. Many
such lights are unattended in the sense that they do not require the
attention of a keeper for days and even weeks together. These are
described in more detail in section 6 of this article. A typical port
light consists of a copper or brass lantern containing a lens of the
fourth order (250 mm. focal distance) or smaller, and a single wick or
2-wick Argand capillary burner. Duplex burners are also used. The
apparatus may exhibit a fixed light or, more usually, an occulting
characteristic is produced by the revolution of screens actuated by
spring clockwork around the burner. The lantern may be placed at the
top of a column, or suspended from the head of a mast. Coal gas and
electricity are also used as illuminants for port lights when local
supplies are available. The optical apparatus used in connexion with
electric light is described below.
_"Orders" of Apparatus._--Augustin Fresnel divided the dioptric
lenses, designed by him, into "orders" or sizes depending on their
local distance. This division is still used, although two additional
"orders," known as "small third order" and "hyper-radial" respectively
are in ordinary use. The following table gives the principal
dimensions of the several sizes in use:--