to send the light equally towards the horizon in all directions at once,
the problem is capable of solution, either by a proper form of glass
apparatus or by a proper form of mirrors. Suppose the section, _e c f_,
Fig. 305, to revolve about a vertical axis passing through the lamp, it
would sweep out a form which, when executed in glass, would spread out
all the light falling upon it into one horizontal sheet. Fresnel was
obliged to content himself with an approximation to this shape, formed
by a prismatic frame of many sides, containing straight horizontal bars
of glass, having the section _e c f_. The light is not quite uniformly
distributed by such apparatus, but the difficulty and expense attending
the formation of prismatic rings were very great when Fresnel
constructed this apparatus. Such rings can now be produced economically
and accurately, and therefore the fixed-light apparatus is now
constructed of circular glass rings, mounted in sections in such a
manner that a vertical section through the axis of the apparatus would
cut them in the form represented at _e c f_. Instead of forming the
metal framework in which the glass is mounted with vertical ribs, it is
made with the ribs placed somewhat diagonally, in order that the dark
sectors which would be produced by the shadows of upright ribs may be
avoided. It should be understood that the forms of the glass in each
side of the octagonal apparatus represented in the figure are produced
by the revolution of the same section, _e c f_, about the horizontal
axis, _d g_.
An ingenious promoter of the catoptric system has contrived to solve the
same problem by mirrors. The form of these may be understood by the aid
of Fig. 306, which, however, relates to another contrivance. Suppose
that the lines A B, A´ B´, are turned about C D as an axis, all three
preserving their relative positions, A B and A´ B´ would sweep out two
parabolic cones, which would have the property of reflecting in a
horizontal direction all rays falling upon them from a lamp placed at L.
But glass, as a material for lighthouse apparatus, has so many
advantages over metal that it is probable that metallic reflectors will
soon be entirely obsolete. The polish of the metal is very readily
destroyed, and as it is constantly liable to be tarnished, the frequent
cleaning required is apt to produce a scratched state of the surface,
even when great care is used. Far greater accuracy of form can be
imparted to glass than to metal reflectors. And then there is the great
loss of light occurring at even the most highly polished surfaces of
metal: a loss which is far greater than that occasioned by the
refraction and reflections of the glass apparatus. There are cases,
however, in which it is desirable to throw the whole of the light into
one beam, and this cannot be done without reflecting the light from one
side. Mr. Alan Stevenson contrived an excellent apparatus for this
purpose, and the diagram, Fig. 306, will explain its nature. L is a
point representing the source of light, A B, B´ A´, a parabolic metallic
mirror. All the rays between L A and L B, and all between L A´ and L
B´—that is, all those which fall upon the mirror—will be reflected
parallel to L G; but those between L B and L B´ would escape from the
mouth of the mirror, B B´, as a diverging cone. This is prevented by
placing the lens, H I, the focus of which is at L, so as to convert the
diverging cone, I L H, into the cylindrical beam, E H I F; and thus half
the light emitted from the luminous point is sent in one direction. A
hemispherical reflector, C K D, of which L is the centre, receives the
other half, which is thus thrown back through L, and then follows the
same course as the direct rays. For the metallic reflector, C K D, Mr.
Stevenson afterwards substituted a system of glass zones; of which O P Q
represents the sections. These had the same effect as the metallic
reflectors, without the loss of light occasioned by the latter. The
inner surface of the glass, C K D, is hemispherical, and the prismatic
zones are such as would be produced by turning the section about L K (or
C D) as an axis. The dotted lines show the course of a ray of light, L
_m_, which, meeting the hemispherical surface perpendicularly, passes
straight through it, and is totally reflected at _m_ by the inclined
surface, and again at _n_, so that it returns to L by the path _n_ L.
Reflecting glass prisms were also substituted for the metallic mirror, A
B, B´ A´, and thus the use of metal has been entirely dispensed with in
this apparatus. This light has been termed by Mr. Stevenson the
_holophotal_ (ὁλο, _entire_, φως, _light_). Such an apparatus will form
the intensest beam that a given source of illumination can yield. On the
other hand, when a fixed light is distributed to the whole horizon
simultaneously, the illuminating power of the source is taxed to the
utmost. These two cases may be considered the extreme modes of disposing
of the light, while the parcelling of it into several beams, as effected
by the apparatus represented in Fig. 305, is an intermediate mode.
[Illustration:
FIG. 306.—_Stevenson’s Holophotal Light._
]
It may be interesting to mention that the holophotal light at Baccalieu,
in Newfoundland, is visible in clear weather from another point 40 miles
distant. So long a range as this is seldom possible at sea, on account
of the rounded form of the earth rendering it necessary to raise the
light nearly 1,000 ft. above the water, if it is required to be visible
at 40 miles’ distance. A shorter distance generally suffices for the
requirements of the navigator; and therefore lighthouse towers rising
from the water are seldom carried to a greater height than something
between 100 ft. and 150 ft. A light elevated 100 ft. above the water
would be seen from the deck of a vessel 14 miles distant, and from the
masthead a much greater distance.
The optical apparatus of a lighthouse is protected by an outer metal
framework glazed with thick plate glass. This framework is made of iron,
or of gun-metal—the latter being preferred on account of the frequent
painting which iron needs in order to preserve it from corrosion. The
glass is carefully fitted into the framework, so as to avoid exposure to
strains from the shocks and vibrations to which a lighthouse is exposed.
The keepers are always provided with a store of panes of glass, ready
for fitting into their places in case of accidents. Sometimes the glass
is broken by large sea-birds dashing against it, and by pebbles which
are thrown up by the waves, or driven by the wind against the panes. It
is the interior of this lantern which forms the light-room already
spoken of. Great pains have been bestowed on the proper ventilation of
these light-rooms, as not only must the air have access to the lamp to
supply the flame, but the carbonic acid which escapes from the chimney
of the lamp must be promptly removed. Another serious inconvenience of
an ill-ventilated light-room would be the condensation, in the inner
surface of the plate glass, of the aqueous vapour, which is also a
product of the combustion.
The lenses and circular prisms for lighthouses are usually made of crown
glass, and are ground by fixing them on a large revolving iron table, on
which they are bedded in plaster of Paris and cemented by pitch—great
care being taken to place them in the exact position required, for only
about one-eighth of an inch is allowed for grinding down to shape the
glass as it comes from the moulds. Sand, emery, and finally rouge, are
used with water for the grinding and polishing processes. The cost of
the optical apparatus alone of a light of the first order, like that
shown in Fig. 305, amounts to upwards of £1,500. The lenses and prisms
are very carefully adjusted in their framework after this has been
fixed, and no plan of testing the adjustment has been found more
efficient than that of viewing the sea horizon through them from the
position which the flame will occupy.
The men to whom the charge of a lighthouse is confided undertake a duty
involving the gravest responsibilities, and demanding unremitting care.
In those lighthouses where a number of reflectors are hung upon a
revolving frame, the extinction of one lamp may not be a matter of much
consequence; but where only one lamp is used, life and death depend upon
its burning. To isolated lighthouses—such as those of Skerryvore and the
Bell Rock—four keepers are appointed, and one of these is always on
shore on leave, so that the men may be relieved at intervals; for it has
been found that a residence in these lonely towers cannot be continued
long together without bad effects. The duties of the lighthouse-keepers
must be performed with the greatest regularity. The glasses of the
light-room and the optical apparatus are carefully cleaned every
morning; the lamps are supplied with oil, the wicks trimmed or renewed,
the machinery oiled and adjusted, and everything prepared in readiness
for the evening. At sunset the lamps are lighted, and one keeper takes
his watch until midnight, when he is relieved by another, who maintains
the vigil till sunrise, when the lamps are extinguished.
The expediency of the regulation appointing three men to be always at
the lighthouse may be illustrated by an incident which occurred about
the beginning of the present century at the lighthouse on the “Smalls,”
a rock in the Bristol Channel. Two keepers held watch over the light on
that rock, which for months together is sometimes cut off from all
communication with the shore. At the time alluded to, after the weather
had for two weeks prevented access to the lighthouse, it was rumoured
among the seafaring men of the neighbouring ports that something was
wrong at the “Smalls,” for a signal of distress had been observed; but
the boats could not go within speaking distance, although many attempts
were made to reach the rock. The relatives of the men became anxious,
and night after night watched for the light. But the light never failed
to appear at the proper hour. After four months came calmer weather, and
then a boat brought to shore one lightkeeper alive, the other dead. What
the former felt when he found his comrade to be dying in their dreadful
isolation, or what his emotions were when he found himself there alone
with the lifeless body, is not recorded. But the thought occurred to him
that he must not commit the body to the waves, lest any suspicion of
foul play might fall upon himself. He therefore contrived a sort of
coffin for the dead man, and dragging it up to the gallery of the
lighthouse, tied it there. Punctually and faithfully for four long
months did he perform all the duties of his position, keeping watch from
twilight till dawn in that lonely light-room, while his ghastly charge
remained there within sight. But he came on shore strangely altered—a
sad, silent, gloomy, worn man—so that even his intimate friends hardly
knew him.
Here we close this brief account of the modern lighthouse, and of its
beautiful appliances, by which Science “has given new securities to the
mariner,” in addition to those with which she furnished him when she
showed him the use of the compass, supplied him with the chronometer,
and placed the sextant in his hands. How anxiously must the seaman who
has been prevented by unfavourable skies from ascertaining his exact
position, and has been trusting to the log and the compass to work his
reckoning, scan the horizon for the first glimpse of the hospitable
light beacon, which seems to say that the country he is approaching has
been watching for his coming, and welcomes him to its shores.
[Illustration]
[Illustration:
FIG. 307.
]
PHOTOGRAPHY.
No other of our nineteenth century inventions is at once so beautiful,
so precious, so popular, so appreciated as photography. It is exercising
a beneficial influence over the social sentiments, the arts, the
sciences of the whole world—an influence not the less real because it is
wide-spread and unobtrusive. The new art cherishes domestic and friendly
feelings by its ever-present transcripts of the familiar faces, keeping
fresh the memory of the distant and the dead; it keeps alive our
admiration of the great and the good by presenting us with the
lineaments of the heroes, the saints, the sages of all lands. It
gratifies, by faithful portrayals of scenes of grandeur and beauty, the
eyes of him who has neither wealth nor leisure for travel. It has
improved pictorial art by sending the painter to the truths of nature;
it has reproduced his works with marvellous fidelity; it has set before
the multitude the finest works of the sculptor. It is lending invaluable
aid to almost every science. The astronomer now derives his mathematical
data from the photograph; by its aid the architect superintends the
erection of distant buildings, the engineer watches over the progress of
his designs in remote lands, the medical man amasses records of morbid
anatomy, the geologist studies the anatomy of the earth, the ethnologist
obtains faithful transcripts of the features of every race. To the mind
of an intelligent reader numberless instances will present themselves,
not only of the utility of photography in the narrower sense of the
term, but of its higher utility in ministering to our love of the
beautiful in art and in nature.
Effects produced by chemical changes to which the rays of the sun give
rise are matters of common observation. The fading of the colour in the
portions of a fabric which are exposed to the light is a familiar
instance; and the bleaching of linen under the influence of sunshine in
the presence of moisture is a well-known operation. Decompositions
produced by light in certain compounds of silver soon attracted the
attention of chemists, and the remarkable activity of the solar rays in
causing the combination of hydrogen and chlorine gases has been even
made the means of measuring the intensity of light. When equal volumes
of these two gases are mixed together in the dark, they may be kept for
an indefinite period without change, provided only that the mixture be
preserved from access of light. But the instant it is exposed to the
direct rays of the sun, or to an intense light, such as that of burning
magnesium, the two gases suddenly unite with a loud explosion, in which
the glass vessel containing them is shattered into atoms. The product is
an intensely acid invisible gas, called hydrochloric acid; and if the
mixture is exposed to the diffused light of day, instead of the direct
rays of the sun, then the production of hydrochloric acid will take
place gradually, and with a rapidity depending on the intensity of the
light.
Of vastly more importance than the small operations of the laboratory
and the bleach-field are the changes which the sun’s rays silently and
unobtrusively effect in the vegetable world. The chemical effect of
light here appears to reside in its power of separating oxygen from
substances with which it is combined. The green parts of plants absorb
from the atmosphere the carbonic acid gas, which is constantly produced
by the respiration of men and animals, and by combustion, and other
processes. Under the influence of sunshine, this carbonic acid is
decomposed within the tissues of the plant; the oxygen is restored to
the atmosphere; the carbon with which it was united is retained to build
up the structure of the plant. In a similar manner light separates the
oxygen from the hydrogen of water, and the former gas is given off by
the leaves, while the hydrogen enters into the composition of the plant.
The carbon, which forms so large an element in the food of plants, is
chiefly obtained in this way; and the abundance of the supply of oxygen
thus thrown into the atmosphere may be inferred from the fact that a
single leaf of the water-lily will in the course of one summer give off
nearly eleven cubic feet of oxygen. But for this continual restoration
of oxygen to the atmosphere, animal life would soon disappear from the
face of the earth. It is the office of the vegetable world not only to
furnish a supply of organic matter as food for animals, but when the
materials of that food have been converted into oxidized products in the
animal system, and returned to the atmosphere as carbonic acid and
aqueous vapour, the sunshine, acting on the vegetable structure (chiefly
on the delicate tissue of the leaf), tears apart the oxygen and the
other substance. These are, therefore, once more capable of combination,
by which they may again supply the animal with heat and the other
energies of life.
Those actions of light which have been last referred to are called by
the chemist _reducing actions_, a term which he applies to the cases in
which a compound is made to part with its oxygen or other similar
element: when the remaining ingredient is a metal, the operation by
which the other has been removed is always called _reduction_. On the
other hand, the inverse operations by which oxygen, chlorine, &c., are
fixed upon other bodies, are distinguished as processes of _oxidation_.
Light is the means of determining each of these kinds of changes,
according to the conditions and the nature of the substances exposed to
its action. Thus moist chloride of silver will retain its white colour
if preserved in the dark; but if exposed to sunlight, it quickly
acquires a violet tint, which deepens in intensity until it has become
black. The dark matter was formerly admitted to be silver; for it was
known that the finely divided metal has this appearance, that during the
process the compound gives off chlorine, and that when nitric acid is
poured upon the darkened matter, reddish fumes are given off, exactly as
when the acid acts upon pure silver. The use of silver nitrate as a
marking-ink for linen depends upon a similar alteration of the salt
within the fibres; and the same reduction takes place when to a solution
of the nitrate in water organic matter is added. If a piece of white
silk be dipped into a solution of chloride of gold, and exposed to the
sun’s rays while still wet, the silk becomes first green, then purple,
and finally a film of metallic gold will be found overspreading its
surface. Many other chlorides and analogous compounds are similarly
affected by sunlight. On the other hand, chlorides, as we have already
seen, and oxygen, fix on hydrogen and on organic substances with greater
energy under the influence of light. A large series of chemical
compounds are obtained by means of the augmented affinity of chlorine
for hydrogen induced by the rays of the sun.
It was in availing himself of an action of the latter class that, in
1813, Joseph Nicéphore Niepce[11] established photography; for he was
the first to obtain a permanent sun-picture. Twelve years before this,
Wedgwood and Davy had copied paintings made on glass, and the profiles
of objects, the shadows of which were projected upon a piece of white
paper, or white leather, saturated with a solution of nitrate of silver.
The images so obtained could not be fixed, as no means was then known of
removing the silver salts which had not been acted upon during the
exposure; and the pictures soon blackened in every part when exposed to
the light. The application of the _camera obscura_, and the fixing of
the image so obtained, define the commencement of the art of
photography. The process of Niepce, which was termed _heliography_, was
conducted by smearing a highly polished metallic plate with a certain
resinous substance known as “bitumen of Judæa,” and this was exposed to
the image formed in the camera for some hours. The action of the light
was such, that the resin, which before exposure was soluble in oil of
lavender, became insoluble in that substance. Hence, on treating the
plate after exposure with that solvent, only the deep shadows dissolved
away, the lights being represented by the undissolved resin. The
brightly polished parts of the plate, which were uncovered by the
removal of the resin, appeared dark when made to reflect dark objects,
while the resin remaining unchanged on the plate appeared light in
comparison.
Footnote 11:
Born at Chalon-sur-Saône, died 1833.
In 1826 a French artist, named Daguerre,[12] who had already made some
reputation as a painter of dioramas, entered into a sort of partnership
with Niepce, into whose process he introduced some improvements; but,
dissatisfied with the slowness of this proceeding, he invented a process
of his own, by which pictures of great beauty could be produced with all
the shadows, lights, and half-tints faithfully rendered; while the time
of exposure in the camera was reduced to twenty minutes. In this process
the burnished surface of silver formed the shadows. A plate of copper,
coated with pure silver, had the silvered surface polished to the
highest degree, and it was then exposed to the vapour of iodine until a
thin yellow film had been produced uniformly over the silver. It was
then placed in the camera; and, although when withdrawn no image was
perceptible, a latent image was nevertheless present; for when the plate
was exposed to the vapour of mercury, that substance attached itself to
the parts of the plate in proportion as they had been acted upon by the
light. Means were adopted by Daguerre for fixing the picture; and after
his processes had been made public in 1839, several important
improvements were proposed by other persons. By using bromine as well as
iodine the sensitiveness of the plates was so much increased that the
time required for exposure was reduced to two minutes, so that about the
year 1841 portraits began to be taken by this process.
Footnote 12:
L. J. M. Daguerre, born 1787, died 1851.
The world at large, which profits most by great inventions, has little
idea at what cost of intense application, concentrated thought, and
heroic perseverance, such discoveries are made. What his discovery must
have cost Daguerre may be inferred from an anecdote related by J.
Baptiste Dumas, the distinguished French chemist and statesman. At the
close of one of his popular lectures in 1825–-_fourteen years before
Daguerre had perfected his process_—a lady came up to him and said,
“Monsieur Dumas, I have to ask you a question of vital importance to
myself. I am the wife of Daguerre, the painter. He has for some time let
the idea possess his mind that he can fix the images of the camera. Do
you, as a man of science, think it can ever be done, or is my husband
mad?” “In the present state of our knowledge we are unable to do it,”
replied Dumas; “but I cannot say it will always remain impossible, or
set down as mad the man who seeks to do it.” The French Government, with
an honourable recognition of the merits of Daguerre, and of Niepce who
had passed away poor and almost unknown, awarded to the former a pension
of 6,000 francs (£240), and to Isidore Niepce, the son of the latter, a
pension of 4,000 francs, one-half to be continued to their widows.