Chapter II: Front Matter (2)

_Dioptric System._--The first adaptation of dioptric lenses to
lighthouses is probably due to T. Rogers, who used lenses at one of
the Portland lighthouses between 1786 and 1790. Subsequently lenses by
the same maker were used at Howth, Waterford and the North Foreland.
Count Buffon had in 1748 proposed to grind out of a solid piece of
glass a lens in steps or concentric zones in order to reduce the
thickness to a minimum (fig. 31). Condorcet in 1773 and Sir D.
Brewster in 1811 designed built-up lenses consisting of stepped
annular rings. Neither of these proposals, however, was intended to
apply to lighthouse purposes. In 1822 Augustin Fresnel constructed a
built-up annular lens in which the centres of curvature of the
different rings receded from the axis according to their distances
from the centre, so as practically to eliminate spherical aberration;
the only spherical surface being the small central part or "bull's
eye" (fig. 32). These lenses were intended for revolving lights only.
Fresnel next produced his cylindric refractor or lens belt, consisting
of a zone of glass generated by the revolution round a vertical axis
of a medial section of the annular lens (fig. 33). The lens belt
condensed and parallelized the light rays in the vertical plane only,
while the annular lens does so in every plane. The first revolving
light constructed from Fresnel's designs was erected at the Cordouan
lighthouse in 1823. It consisted of 8 panels of annular lenses placed
round the lamp at a focal distance of 920 mm. To utilize the light,
which would otherwise escape above the lenses, Fresnel introduced a
series of 8 plain silvered mirrors, on which the light was thrown by a
system of lenses. At a subsequent period mirrors were also placed in
the lower part of the optic. The apparatus was revolved by clockwork.
This optic embodied the first combination of dioptric and catoptric
elements in one design (fig. 34). In the following year Fresnel
designed a dioptric lens with catoptric mirrors for fixed light, which
was the first of its kind installed in a lighthouse. It was erected at
the Chassiron lighthouse in 1827 (fig. 35). This combination is
geometrically perfect, but not so practically on account of the great
loss of light entailed by metallic reflection which is at least 25%
greater than the system described under. Before his death in 1827
Fresnel devised his totally reflecting or catadioptric prisms to take
the place of the silvered reflectors previously used above and below
the lens elements (fig. 28). The ray Fi falling on the prismoidal ring
ABC is refracted in the direction i r and meeting the face AB at an
angle of incidence greater than the critical, is totally reflected in
the direction r e emerging after second refraction in a horizontal
direction. Fresnel devised these prisms for use in fixed light
apparatus, but the principle was, at a later date, also applied to
flashing lights, in the first instance by T. Stevenson. Both the
dioptric lens and catadioptric prism invented by Fresnel are still in
general use, the mathematical calculations of the great French
designer still forming the basis upon which lighthouse opticians work.

Fresnel also designed a form of fixed and flashing light in which the
distinction of a fixed light, varied by flashes, was produced by
placing panels of straight refracting prisms in a vertical position on
a revolving carriage outside the fixed light apparatus. The revolution
of the upright prisms periodically increased the power of the beam, by
condensation of the rays emergent from the fixed apparatus, in the
horizontal plane.

The lens segments in Fresnel's early apparatus were of polygonal form
instead of cylindrical, but subsequently manufacturers succeeded in
grinding glass in cylindrical rings of the form now used. The first
apparatus of this description was made by Messrs Cookson of Newcastle
in 1836 at the suggestion of Alan Stevenson and erected at Inchkeith.

In 1825 the French Commission des Phares decided upon the exclusive
use of lenticular apparatus in its service. The Scottish Lighthouse
Board followed with the Inchkeith revolving apparatus in 1835 and the
Isle of May fixed optic in 1836. In the latter instrument Alan
Stevenson introduced helical frames for holding the glass prisms in
place, thus avoiding complete obstruction of the light rays in any
azimuth. The first dioptric light erected by the Trinity House was
that formerly at Start Point in Devonshire, constructed in 1836.
Catadioptric or reflecting prisms for revolving lights were not used
until 1850, when Alan Stevenson designed them for the North Ronaldshay
lighthouse.

_Dioptric Mirror._--The next important improvement in lighthouse
optical work was the invention of the dioptric spherical mirror by Mr
(afterwards Sir) J. T. Chance in 1862. The zones or prisms are
generated round a vertical axis and divided into segments. This form
of mirror is still in general use (figs. 36 and 37).

_Azimuthal Condensing Prisms._--Previous to 1850 all apparatus were
designed to emit light of equal power in every azimuth either
constantly or periodically. The only exception was where a light was
situated on a stretch of coast where a mirror could be placed behind
the flame to utilize the rays, which would otherwise pass landward,
and reflect them back, passing through the flame and lens in a seaward
direction. In order to increase the intensity of lights in certain
azimuths T. Stevenson devised his azimuthal condensing prisms which,
in various forms and methods of application, have been largely used
for the purpose of strengthening the light rays in required directions
as, for instance, where coloured sectors are provided. Applications of
this system will be referred to subsequently.

_Optical Glass for Lighthouses._--In the early days of lens lights the
only glass used for the prisms was made in France at the St Gobain and
Premontré works, which have long been celebrated for the high quality
of optical glass produced. The early dioptric lights erected in the
United Kingdom, some 13 in all, were made by Messrs Cookson of South
Shields, who were instructed by Léonor Fresnel, the brother of
Augustin. At first they tried to mould the lens and then to grind it
out of one thick sheet of glass. The successors of the Cookson firm
abandoned the manufacture of lenses in 1845, and the firm of
Letourneau & Lepaute of Paris again became the monopolists. In 1850
Messrs Chance Bros. & Co. of Birmingham began the manufacture of
optical glass, assisted by M. Tabouret, a French expert who had been a
colleague of Augustin Fresnel himself. The first light made by the
firm was shown at the Great Exhibition of 1851, since when numerous
dioptric apparatus have been constructed by Messrs Chance, who are, at
this time, the only manufacturers of lighthouse glass in the United
Kingdom. Most of the glass used for apparatus constructed in France is
manufactured at St Gobain. Some of the glass used by German
constructors is made at Rathenow in Prussia and Goslar in the Harz.

The glass generally employed for lighthouse optics has for its
refractive index a mean value of µ = 1.51, the corresponding critical
angle being 41° 30'. Messrs Chance have used dense flint glass for the
upper and lower refracting rings of high angle lenses and for dioptric
mirrors in certain cases. This glass has a value of µ = l.62 with
critical angle 38° 5'.

_Occulting Lights._--During the last 25 years of the 19th century the
disadvantages of fixed lights became more and more apparent. At the
present day the practice of installing such, except occasionally in
the case of the smaller and less important of harbour or river lights,
has practically ceased. The necessity for providing a distinctive
characteristic for every light when possible has led to the conversion
of many of the fixed-light apparatus of earlier years into occulting
lights, and often to their supersession by more modern and powerful
flashing apparatus. An occulting apparatus in general use consists of
a cylindrical screen, fitting over the burner, rapidly lowered and
raised by means of a cam-wheel at stated intervals. The cam-wheel is
actuated by means of a weight or spring clock. Varying characteristics
may be procured by means of such a contrivance--single, double, triple
or other systems of occultation. The eclipses or periods of darkness
bear much the same relation to the times of illumination as do the
flashes to the eclipses in a revolving or flashing light. In the case
of a first-order fixed light the cost of conversion to an occulting
characteristic does not exceed £250 to £300. With apparatus
illuminated by gas the occultations may be produced by successively
raising and lowering the gas at stated intervals. Another form of
occulting mechanism employed consists of a series of vertical screens
mounted on a carriage and revolving round the burner. The carriage is
rotated on rollers or ball bearings or carried upon a small mercury
float. The usual driving mechanism employed is a spring clock. "Otter"
screens are used in cases when it is desired to produce different
periods of occultations in two or more positions in azimuth in order
to differentiate sectors marking shoals, &c. The screens are of sheet
metal blacked and arranged vertically, some what in the manner of the
laths of a venetian blind, and operated by mechanical means.

_Leading Lights._--In the case of lights designed to act as a lead
through a narrow channel or as direction lights, it is undesirable to
employ a flashing apparatus. Fixed-light optics are employed to meet
such cases, and are generally fitted with occulting mechanism. A
typical apparatus of this description is that at Gage Roads,
Fremantle, West Australia (fig. 38). The occulting bright light covers
the fairway, and is flanked by sectors of occulting red and green
light marking dangers and intensified by vertical condensing prisms. A
good example of a holophotal direction light was exhibited at the 1900
Paris Exhibition, and afterwards erected at Suzac lighthouse (France).
The light consists of an annular lens 500 mm. focal distance, of 180°
horizontal angle and 157° vertical, with a mirror of 180° at the back.
The lens throws a red beam of about 4½° amplitude in azimuth, and
50,000 candle-power over a narrow channel. The illuminant is an
incandescent petroleum vapour burner. Holophotal direction lenses of
this type can only be applied where the sector to be marked is of
comparatively small angle. Silvered metallic mirrors of parabolic form
are also used for the purpose. The use of single direction lights
frequently renders the construction of separate towers for leading
lights unnecessary.

If two distinct lights are employed to indicate the line of navigation
through a channel or between dangers they must be sufficiently far
apart to afford a good lead, the front or seaward light being situated
at a lower elevation than the rear or landward one.

_Coloured Lights._--Colour is used as seldom as possible as a
distinction, entailing as it does a considerable reduction in the
power of the light. It is necessary in some instances for
differentiating sectors over dangers and for harbour lighting
purposes. The use of coloured lights as alternating flashes for
lighthouse lights is not to be commended, on account of the unequal
absorption of the coloured and bright rays by the atmosphere. When
such distinction has been employed, as in the Wolf Rock apparatus, the
red and white beams can be approximately equalized in initial
intensity by constructing the lens and prism panels for the red light
of larger angle than those for the white beams. Owing to the
absorption by the red colouring, the power of a red beam is only 40%
of the intensity of the corresponding white light. The corresponding
intensity of green light is 25%. When red or green sectors are
employed they should invariably be reinforced by mirrors, azimuthal
condensing prisms, or other means to raise the coloured beam to
approximately the same intensity as the white light. With the
introduction of group-flashing characteristics the necessity for using
colour as a means of distinction disappeared.

_High-Angle Vertical Lenses._--Messrs Chance of Birmingham have
manufactured lenses having 97° of vertical amplitude, but this result
was only attained by using dense flint glass of high refractive index
for the upper and lower elements. It is doubtful, however, whether the
use of refracting elements for a greater angle than 80° vertically is
attended by any material corresponding advantage.

_Group Flashing Lights._--One of the most useful distinctions consists
in the grouping of two or more flashes separated by short intervals of
darkness, the group being succeeded by a longer eclipse. Thus two,
three or more flashes of, say, half second duration or less follow
each other at intervals of about 2 seconds and are succeeded by an
eclipse of, say, 10 seconds, the sequence being completed in a period
of, say, 15 seconds. In 1874 Dr John Hopkinson introduced the very
valuable improvement of dividing the lenses of a dioptric revolving
light with the panels of reflecting prisms above and below them,
setting them at an angle to produce the group-flashing characteristic.
The first apparatus of this type constructed were those now in use at
Tampico, Mexico and the Little Basses lighthouse, Ceylon (double
flashing). The Casquets apparatus (triple flashing) was installed in
1877. A group-flashing catoptric light had, however, been exhibited
from the "Royal Sovereign" light-vessel in 1875. A sectional plan of
the quadruple-flashing first order apparatus at Pendeen in Cornwall
is shown in fig. 39; and fig. 55 (Plate 1.) illustrates a double
flashing first order light at Pachena Point in British Columbia.
Hopkinson's system has been very extensively used, most of the
group-flashing lights shown in the accompanying tables, being designed
upon the general lines he introduced. A modification of the system
consists in grouping two or more lenses together separated by equal
angles, and filling the remaining angle in azimuth by a reinforcing
mirror or screen. A group-flashing distinction was proposed for gas
lights by J. R. Wigham of Dublin, who obtained it in the case of a
revolving apparatus by alternately raising and lowering the flame. The
first apparatus in which this method was employed was erected at
Galley Head, Co. Cork (1878). At this lighthouse 4 of Wigham's large
gas burners with four tiers of first-order revolving lenses, eight in
each tier, were adopted. By successive lowering and raising of the gas
flame at the focus of each tier of lenses he produced the
group-flashing distinction. The light showed, instead of one prolonged
flash at intervals of one minute, as would be produced by the
apparatus in the absence of a gas occulter, a group of short flashes
varying in number between six and seven. The uncertainty, however, in
the number of flashes contained in each group is found to be an
objection to the arrangement. This device was adopted at other
gas-illuminated stations in Ireland at subsequent dates. The
quadriform apparatus and gas installation at Galley Head were
superseded in 1907 by a first order bi-form apparatus with
incandescent oil vapour burner showing five flashes every 20 seconds.

_Flashing Lights indicating Numbers._--Captain F. A. Mahan, late
engineer secretary to the United States Lighthouse Board, devised for
that service a system of flashing lights to indicate certain numbers.
The apparatus installed at Minot's Ledge lighthouse near Boston
Harbour, Massachusetts, has a flash indicating the number 143, thus: -
---- ---, the dashes indicating short flashes. Each group is separated
by a longer period of darkness than that between successive members of
a group. The flashes in a group indicating a figure are about 1½
seconds apart, the groups being 3 seconds apart, an interval of 16
seconds' darkness occurring between each repetition. Thus the number
is repeated every half minute. Two examples of this system were
exhibited by the United States Lighthouse Board at the Chicago
Exhibition in 1893, viz. the second-order apparatus just mentioned and
a similar light of the first order for Cape Charles on the Virginian
coast. The lenses are arranged in a somewhat similar manner to an
ordinary group-flashing light, the groups of lenses being placed on
one side of the optic, while the other is provided with a catadioptric
mirror. This system of numerical flashing for lighthouses has been
frequently proposed in various forms, notably by Lord Kelvin. The
installation of the lights described is, however, the first practical
application of the system to large and important coast lights. The
great cost involved in the alteration of the lights of any country to
comply with the requirements of a numerical system is one of the
objections to its general adoption.

FIG. 54.--FASTNET LIGHTHOUSE--FIRST ORDER SINGLE-FLASHING BIFORM
APPARATUS.

FIG. 55.--PACHENA POINT LIGHTHOUSE, B.C.--FIRST ORDER DOUBLE-FLASHING
APPARATUS.]

FIG. 56.--OLD EDDYSTONE LIGHTHOUSE.

FIG. 57.--EDDYSTONE LIGHTHOUSE.

FIG. 58.--ILE VIERGE LIGHTHOUSE.

FIG. 59.--MINOT'S LEDGE LIGHTHOUSE.]

_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.

_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:--

TABLE II.

+-------------+---------+-------------------------------------+
| | | Vertical Angles of Optics. |
| | | (Ordinary Dimensions.) |
| | Focal +-------------+-----------------------+
| Order. |Distance,| | Holophotal Optics. |
| | mm. | Dioptric +-------+-------+-------+
| | | Belt only. | Lower | Lens. | Upper |
| | | |Prisms.| |Prisms.|
+-------------+---------+-------------+-------+-------+-------+
| Hyper-Radial| 1330 | 80° | 21° | 57° | 48° |
| 1st order | 920 |92°, 80°, 58°| 21° | 57° | 48° |
| 2nd " | 700 | 80° | 21° | 57° | 48° |
| 3rd " | 500 | 80° | 21° | 57° | 48° |
| Small 3rd | | | | | |
| order | 375 | 80° | 21° | 57° | 48° |
| 4th order | 250 | 80° | 21° | 57° | 48° |
| 5th " | 187.5 | 80° | 21° | 57° | 48° |
| 6th " | 150 | 80° | 21° | 57° | 48° |
+-------------+---------+-------------+-------+-------+-------+

Lenses of small focal distance are also made for buoy and beacon
lights.

_Light Intensities._--The powers of lighthouse lights in the British
Empire are expressed in terms of standard candles or in "lighthouse
units" (one lighthouse unit = 1000 standard candles). In France the
unit is the "Carcel" = .952 standard candle. The powers of burners and
optical apparatus, then in use in the United Kingdom, were carefully
determined by actual photometric measurement in 1892 by a committee
consisting of the engineers of the three general lighthouse boards,
and the values so obtained are used as the basis for calculating the
intensities of all British lights. It was found that the intensities
determined by photometric measurement were considerably less than the
values given by the theoretical calculations formerly employed. A
deduction of 20% was made from the mean experimental results obtained
to compensate for loss by absorption in the lantern glass, variations
in effects obtained by different men in working the burners and in the
illuminating quality of oils, &c. The resulting reduced values are
termed "service" intensities.

As has been explained above, the effect of a dioptric apparatus is to
condense the light rays, and the measure of this condensation is the
ratio between the vertical divergence and the vertical angle of the
optic in the case of fixed lights. In flashing lights the ratio of
vertical condensation must be multiplied by the ratio between the
horizontal divergence and the horizontal angle of the panel. The loss
of light by absorption in passing through the glass and by refraction
varies from 10% to 15%. For apparatus containing catadioptric elements
a larger deduction must be made.

The intensity of the flash emitted from a dioptric apparatus, showing
a white light, may be found approximately by the empirical formula I =
PCVH/vh, where I = intensity of resultant beam, P = service intensity
of flame, V = vertical angle of optic, v = angle of mean vertical
divergence, H = horizontal angle of panel, h = angle of mean
horizontal divergence, and C = constant varying between .9 and .75
according to the description of apparatus. The factor H/h must be
eliminated in the case of fixed lights. Deduction must also be made in
the case of coloured lights. It should, however, be pointed out that
photometric measurements alone can be relied upon to give accurate
values for lighthouse intensities. The values obtained by the use of
Allard's formulae, which were largely used before the necessity for
actual photometric measurements came to be appreciated, are
considerably in excess of the true intensities.

_Optical Calculations._--The mathematical theory of optical apparatus
for lighthouses and formulae for the calculations of profiles will be
found in the works of the Stevensons, Chance, Allard, Reynaud, Ribière
and others. Particulars of typical lighthouse apparatus will be found
in tables VI. and VII.

4. ILLUMINANTS.--The earliest form of illuminant used for lighthouses was a fire of coal or wood set in a brazier or grate erected on top of the lighthouse tower. Until the end of the 18th and even into the 19th century this primitive illuminant continued to be almost the only one in use. The coal fire at the Isle of May light continued until 1810 and that at St Bees lighthouse in Cumberland till 1823. Fires are stated to have been used on the two towers of Nidingen, in the Kattegat, until 1846. Smeaton was the first to use any form of illuminant other than coal fires; he placed within the lantern of his Eddystone lighthouse a chandelier holding 24 tallow candles each of which weighed 2/5 of a lb. and emitted a light of 2.8 candle power. The aggregate illuminating power was 67.2 candles and the consumption at the rate of 3.4 lb. per hour.

_Oil._--Oil lamps with flat wicks were used in the Liverpool
lighthouses as early as 1763. Argand, between 1780 and 1783, perfected
his cylindrical wick lamp which provides a central current of air
through the burner, thus allowing the more perfect combustion of the
gas issuing from the wick. The contraction in the diameter of the
glass chimney used with wick lamps is due to Lange, and the principle
of the multiple wick burner was devised by Count Rumford. Fresnel
produced burners having two, three and four concentric wicks. Sperm
oil, costing 5s. to 8s. per gallon, was used in English lighthouses
until 1846, but about that year colza oil was employed generally at a
cost of 2s. 9d. per gallon. Olive oil, lard oil and coconut oil have
also been used for lighthouse purposes in various parts of the world.

_Mineral Oil Burners._--The introduction of mineral oil, costing a
mere fraction of the expensive animal and vegetable oils,
revolutionized the illumination of lighthouses. It was not until 1868
that a burner was devised which successfully consumed hydrocarbon
oils. This was a multiple wick burner invented by Captain Doty. The
invention was quickly taken advantage of by lighthouse authorities,
and the "Doty" burner, and other patterns involving the same
principle, remained practically the only oil burners in lighthouse use
until the last few years of the 19th century.

The lamps used for supplying oil to the burner are of two general
types, viz. those in which the oil is maintained under pressure by
mechanical action and constant level lamps. In the case of single
wick, and some 2-wick burners, oil is supplied to the burner by the
capillary action of the wick alone.

The mineral oils ordinarily in use are petroleum, which for lighthouse
purposes should have a specific gravity of from .820 to .830 at 60° F.
and flashing point of not less than 230° F. (Abel close test), and
Scottish shale oil or paraffin with a specific gravity of about .810
at 60° F. and flash point of 140° to 165° F. Both these varieties may
be obtained in England at a cost of about 6½d. per gallon in bulk.

_Coal Gas_ had been introduced in 1837 at the inner pier light of
Troon (Ayrshire) and in 1847 it was in use at the Heugh lighthouse
(West Hartlepool). In 1878 cannel coal gas was adopted for the Galley
Head lighthouse, with 108-jet Wigham burners. Sir James Douglass
introduced gas burners consisting of concentric rings, two to ten in
number, perforated on the upper edges. These give excellent results
and high intensity, 2600 candles in the case of the 10-ring burner
with a flame diameter at the focal plane of 5(5/8) in. They are still
in use at certain stations. The use of multiple ring and jet gas
burners is not being further extended. Gas for lighthouse purposes
generally requires to be specially made; the erection of gas works at
the station is thus necessitated and a considerable outlay entailed
which is avoided by the use of oil as an illuminant.

_Incandescent Coal Gas Burners._--The invention of the Welsbach mantle
placed at the disposal of the lighthouse authorities the means of
producing a light of high intensity combined with great focal
compactness. For lighthouse purposes other gaseous illuminants than
coal gas are as a rule more convenient and economical, and give better
results with incandescent mantles. Mantles have, however, been used
with ordinary coal gas in many instances where a local supply is
available.

_Incandescent Mineral Oil Burners._--Incandescent lighting with
high-flash mineral oil was first introduced by the French Lighthouse
Service in 1898 at L'Île Penfret lighthouse. The burners employed are
all made on the same principle, but differ slightly in details
according to the type of lighting apparatus for which they are
intended. The principle consists in injecting the liquid petroleum in
the form of spray mixed with air into a vaporizer heated by the mantle
flame or by a subsidiary heating burner. A small reservoir of
compressed air is used--charged by means of a hand pump--for providing
the necessary pressure for injection. On first ignition the vaporizer
is heated by a spirit flame to the required temperature. A reservoir
air pressure of 125 lb. per sq. in. is employed, a reducing valve
supplying air to the oil at from 60 to 65 lb. per sq. in. Small
reservoirs containing liquefied carbon dioxide have also been employed
for supplying the requisite pressure to the oil vessel.

The candle-power of apparatus in which ordinary multiple wick burners
were formerly employed is increased by over 300% by the substitution
of suitable incandescent oil burners. In 1902 incandescent oil burners
were adopted by the general lighthouse authorities in the United
Kingdom. The burners used in the Trinity House Service and some of
those made in France have the vaporizers placed over the flame. In
other forms, of which the "Chance" burner (fig. 44) is a type, the
vaporization is effected by means of a subsidiary burner placed under
the main flame.

Particulars of the sizes of burner in ordinary use are given in the
following table.

+--------------------+------------------+-------------------+
| Diameter of Mantle.|Service Intensity.|Consumption of oil.|
| | | Pints per hour. |
+--------------------+------------------+-------------------+
| 35 mm. | 600 candles. | .50 |
| 55 mm. | 1200 " | 1.00 |
| 85 mm. | 2150 " | 2.25 |
|Triple mantle 50 mm.| 3300 " | 3.00 |
+--------------------+------------------+-------------------+

The intrinsic brightness of incandescent burners generally may be
taken as being equivalent to from 30 candles to 40 candles per sq. cm.
of the vertical section of the incandescent mantle.

In the case of wick burners, the intrinsic brightness varies,
according to the number of wicks and the type of burner from about 3.5
candles to about 12 candles per sq. cm., the value being at its
maximum with the larger type of burner. The luminous intensity of a
beam from a dioptric apparatus is, _ceteris paribus_, proportional to
the intrinsic brightness of the luminous source of flame, and not of
the total luminous intensity. The intrinsic brightness of the flame of
oil burners increases only slightly with their focal diameter,
consequently while the consumption of oil increases the efficiency of
the burner for a given apparatus decreases. The illuminating power of
the condensed beam can only be improved to a slight extent, and, in
fact, is occasionally decreased, by increasing the number of wicks in
the burner. The same argument applies to the case of multiple ring and
multiple jet gas burners which, notwithstanding their large total
intensity, have comparatively small intrinsic brightness. The economy
of the new system is instanced by the case of the Eddystone bi-form
apparatus, which with the concentric 6-wick burner consuming 2500
gals. of oil per annum, gave a total intensity of 79,250 candles.
Under the new régime the intensity is 292,000 candles, the oil
consumption being practically halved.

_Incandescent Oil Gas Burners._--It has been mentioned that
incandescence with low-pressure coal gas produces flames of
comparatively small intrinsic brightness. Coal gas cannot be
compressed beyond a small extent without considerable injurious
condensation and other accompanying evils. Recourse has therefore been
had to compressed oil gas, which is capable of undergoing compression
to 10 or 12 atmospheres with little detriment, and can conveniently be
stored in portable reservoirs. The burner employed resembles the
ordinary Bunsen burner with incandescent mantle, and the rate of
consumption of gas is 27.5 cub. in. per hour per candle. A reducing
valve is used for supplying the gas to the burner at constant
pressure. The burners can be left unattended for considerable periods.
The system was first adopted in France, where it is installed at eight
lighthouses, among others the Ar'men Rock light, and has been extended
to other parts of the world including several stations in Scotland and
England. The mantles used in France are of 35 mm. diameter. The 35 mm.
mantle gives a candle-power of 400, with an intrinsic brightness of 20
candles per sq. cm.

The use of oil gas necessitates the erection of gas works at the
lighthouse or its periodical supply in portable reservoirs from a
neighbouring station. A complete gas works plant costs about £800. The
annual expenditure for gas lighting in France does not exceed £72 per
light where works are installed, or £32 where gas is supplied from
elsewhere. In the case of petroleum vapour lighting the annual cost of
oil amounts to about £26 per station.

_Acetylene._--The high illuminating power and intrinsic brightness of
the flame of acetylene makes it a very suitable illuminant for
lighthouses and beacons, providing certain difficulties attending its
use can be overcome. At Grangemouth an unattended 21-day beacon has
been illuminated by an acetylene flame for some years with
considerable success, and a beacon light designed to run unattended
for six months was established on Bedout Island in Western Australia
in 1910. Acetylene has also been used in the United States, Germany,
the Argentine, China, Canada, &c., for lighthouse and beacon
illumination. Many buoys and beacons on the German and Dutch coasts
have been supplied with oil gas mixed with 20% of acetylene, thereby
obtaining an increase of over 100% in illuminating intensity. In
France an incandescent burner consuming acetylene gas mixed with air
has been installed at the Chassiron lighthouse (1902). The French
Lighthouse Service has perfected an incandescent acetylene burner with
a 55 mm. mantle having an intensity of over 2000 candle-power, with
intrinsic brightness of 60 candles per sq. cm.

_Electricity._--The first installation of electric light for
lighthouse purposes in England took place in 1858 at the South
Foreland, where the Trinity House established a temporary plant for
experimental purposes. This installation was followed in 1862 by the
adoption of the illuminant at the Dungeness lighthouse, where it
remained in service until the year 1874 when oil was substituted for
electricity. The earliest of the permanent installations now existing
in England is that at Souter Point which was illuminated in 1871.
There are in England four important coast lights illuminated by
electricity, and one, viz. Isle of May, in Scotland. Of the former St
Catherine's, in the Isle of Wight, and the Lizard are the most
powerful. Electricity was substituted as an illuminant for the then
existing oil light at St Catherine's in 1888. The optical apparatus
consisted of a second-order 16-sided revolving lens, which was
transferred to the South Foreland station in 1904, and a new second
order (700 mm.) four-sided optic with a vertical angle of 139°,
exhibiting a flash of .21 second duration every 5 seconds substituted
for it. A fixed holophote is placed inside the optic in the dark or
landward arc, and at the focal plane of the lamp. This holophote
condenses the rays from the arc falling upon it into a pencil of small
angle, which is directed horizontally upon a series of reflecting
prisms which again bend the light and throw it downwards through an
aperture in the lantern floor on to another series of prisms, which
latter direct the rays seaward in the form of a sector of fixed red
light at a lower level in the tower. A somewhat similar arrangement
exists at Souter Point lighthouse.

The apparatus installed at the Lizard in 1903 is similar to that at St
Catherine's, but has no arrangement for producing a subsidiary sector
light. The flash is of .13 seconds duration every 3 seconds. The
apparatus replaced the two fixed electric lights erected in 1878.

The Isle of May lighthouse, at the mouth of the Firth of Forth, was
first illuminated by electricity in 1886. The optical apparatus
consists of a second-order fixed-light lens with reflecting prisms,
and is surrounded by a revolving system of vertical condensing prisms
which split up the vertically condensed beam of light into 8 separate
beams of 3° in azimuth. The prisms are so arranged that the apparatus,
making one complete revolution in the minute, produces a group
characteristic of 4 flashes in quick succession every 30 seconds (fig.
45). The fixed light is not of the ordinary Fresnel section, the
refracting portion being confined to an angle of 10°, and the
remainder of the vertical section consisting of reflecting prisms.

In France the old south lighthouse at La Hève was lit by electricity
in 1863. This installation was followed in 1865 by a similar one at
the north lighthouse. In 1910 there were thirteen important coast
lights in France illuminated by electricity. In other parts of the
world, Macquarie lighthouse, Sydney, was lit by electricity in 1883;
Tino, in the gulf of Spezia, in 1885; and Navesink lighthouse, near
the entrance to New York Bay, in 1898. Electric apparatus were also
installed at the lighthouse at Port Said in 1869, on the opening of
the canal; Odessa in 1871; and at the Rothersand, North Sea, in 1885.
There are several other lights in various parts of the world
illuminated by this agency.

Incandescent electric lighting has been adopted for the illumination
of certain light-vessels in the United States, and a few small harbour
and port lights, beacons and buoys.

Table VI. gives particulars of some of the more important electric
lighthouses of the world.

_Electric Lighthouse Installations in France._--A list of the thirteen
lighthouses on the French coast equipped with electric light
installations will be found in table VI. It has been already mentioned
that the two lighthouses at La Hève were lit by electric light in 1863
and 1865. These installations were followed within a few years by the
establishment of electricity as illuminant at Gris-Nez. In 1882 M.
Allard, the then director-general of the French Lighthouse Service,
prepared a scheme for the electric lighting of the French littoral by
means of 46 lights distributed more or less uniformly along the
coast-line. All the apparatus were to be of the same general type, the
optics consisting of a fixed belt of 300 mm. focal distance, around
the outside of which revolved a system of 24 faces of vertical lenses.
These vertical panels condensed the belt of fixed light into beams of
3° amplitude in azimuth, producing flashes of about ¾ sec. duration.
To illuminate the near sea the vertical divergence of the lower prisms
of the fixed belt was artificially increased. These optics are very
similar to that in use at the Souter Point lighthouse, Sunderland. The
intensities obtained were 120,000 candles in the case of fixed lights
and 900,000 candles with flashing lights. As a result of a nautical
inquiry held in 1886, at which date the lights of Dunkerque, Calais,
Gris-Nez, La Canche, Baleines and Planier had been lighted, in
addition to the old apparatus at La Hève, it was decided to limit the
installation of electrical apparatus to important landfall lights--a
decision which the Trinity House had already arrived at in the case of
the English coast--and to establish new apparatus at six stations
only. These were Créac'h d'Ouessant (Ushant), Belle-Île, La Coubre at
the mouth of the river Gironde, Barfleur, Île d'Yeu and Penmarc'h. At
the same time it was determined to increase the powers of the existing
electric lights. The scheme as amended in 1886 was completed in
1902.[2]

All the electrically lit apparatus, in common with other optics
established in France since 1893, have been provided with mercury
rotation. The most recent electric lights have been constructed in the
form of twin apparatus, two complete and distinct optics being mounted
side by side upon the same revolving table and with corresponding
faces parallel. It is found that a far larger aggregate candle-power
is obtained from two lamps with 16 mm. to 23 mm. diameter carbons and
currents of 60 to 120 amperes than with carbons and currents of larger
dimensions in conjunction with single optics of greater focal
distance. A somewhat similar circumstance led to the choice of the
twin form for the two very powerful non-electric apparatus at Île
Vierge (figs. 43 and 43A) and Ailly, particulars of which will be seen
in table VII.

Several of the de Meritens magneto-electric machines of 5.5 K.W., laid
down many years ago at French electric lighthouse stations, are still
in use. All these machines have five induction coils, which, upon the
installation of the twin optics, were separated into two distinct
circuits, each consisting of 2½ coils. This modification has enabled
the old plants to be used with success under the altered conditions of
lighting entailed by the use of two lamps. The generators adopted in
the French service for use at the later stations differ materially
from the old type of de Meritens machine. The Phare d'Eckmühl
(Penmarc'h) installation serves as a type of the more modern
machinery. The dynamos are alternating current two-phase machines, and
are installed in duplicate. The two lamps are supplied with current
from the same machine, the second dynamo being held in reserve. The
speed is 810 to 820 revolutions per minute.

The lamp generally adopted is a combination of the Serrin and Berjot
principles, with certain modifications. Clockwork mechanism with a
regulating electromagnet moves the rods simultaneously and controls
the movements of the carbons so that they are displaced at the same
rate as they are consumed. It is usual to employ currents of varying
power with carbons of corresponding dimensions according to the
atmospheric conditions. In the French service two variations are used
in the case of twin apparatus produced by currents of 60 and 120
amperes at 45 volts with carbons 14 mm. and 18 mm. diameter, while in
single optic apparatus currents of 25, 50 and 100 amperes are utilized
with carbon of 11 mm., 16 mm. and 23 mm. diameter. In England fluted
carbons of larger diameter are employed with correspondingly increased
current. Alternating currents have given the most successful results
in all respects. Attempts to utilize continuous current for lighthouse
arc lights have, up to the present, met with little success.

The cost of a first-class electric lighthouse installation of the most
recent type in France, including optical apparatus, lantern, dynamos,
engines, air compressor, siren, &c., but not buildings, amounts
approximately to £5900.

_Efficiency of the Electric Light._--In 1883 the lighthouse
authorities of Great Britain determined that an exhaustive series of
experiments should be carried out at the South Foreland with a view to
ascertaining the relative suitability of electricity, gas and oil as
lighthouse illuminants. The experiments extended over a period of more
than twelve months, and were attended by representatives of the chief
lighthouse authorities of the world. The results of the trials tended
to show that the rays of oil and gas lights suffered to about equal
extent by atmospheric absorption, but that oil had the advantage over
gas by reason of its greater economy in cost of maintenance and in
initial outlay on installation. The electric light was found to suffer
to a much larger extent than either oil or gas light per unit of power
by atmospheric absorption, but the infinitely greater total intensity
of the beam obtainable by its use, both by reason of the high luminous
intensity of the electric arc and its focal compactness, more than
outweighed the higher percentage of loss in fog. The final conclusion
of the committee on the relative merits of electricity, gas or oil as
lighthouse illuminants is given in the following words: "That for
ordinary necessities of lighthouse illumination, mineral oil is the
most suitable and economical illuminant, and that for salient
headlands, important landfalls, and places where a very powerful light
is required electricity offers the greater advantages."