[_Delivered before the Royal Institution on Friday, 9th March, 1860._]
There is no part of my life which gives me more delight than my
connection with the Trinity House. The occupation of nations joined
together to guide the mariner over the sea, to all a point of great
interest, is infinitely more so to those who are concerned in the
operations which they carry into effect; and it certainly has
astonished me, since I have been connected with the Trinity House, to
see how beautifully and how wonderfully shines forth amongst nations
at large the desire to do good; and you will not regret having come
here to-night, if you follow me in the various attempts which have been
made to carry out the great object of guiding in safety all people
across the dark and dreary waste of waters. It is wonderful to think
how eagerly efforts at improvement are made by the various public
bodies--the Trinity House in this country, and Commissions in France
and other nations; and whilst the improvements progress, we come to the
knowledge of such curious difficulties, and such odd modes of getting
over those difficulties, as are not easy to be conceived. I must ask
you this evening to follow me from the simplest possible method of
giving a sign by means of a light to persons at a distance, to the
modes at which we have arrived in the present day; and to consider the
difficulties which arise when carrying out these improvements to a
practical result, and the extraordinary care which those who have to
judge on these points must take in order to guard against the too hasty
adoption of some fancied improvement, thus, as has happened in some few
cases, doing harm instead of good.
If I try to make you understand these things partly by old models, and
partly by those which we have here, it is only that I may the better
be enabled to illustrate that which I look forward to as the higher
mode of lighting, by means of the electric lamp and the lime light.
[Illustration: Fig. 55.]
There is nothing more simple than a candle being set down in a cottage
window to guide a husband to his home; but when we want to make a
similar guide on a large scale, not merely over a river or over a
moor, but over large expanses of sea, how can we then make the signal,
using only a candle? I have shewn in this diagram (fig. 55) what we
may imagine to be the rays of a candle or any other source of light
emanating from the centre of a sphere in all directions round to
infinite distances. After this simple kind of light had been used for
some time--it being found to be liable to be obscured by fogs, or
distance, or other circumstance--there arose the attempt to make larger
lights by means of fires; and after that there was introduced a very
important refinement in the mode of dealing with the light, namely,
the principle of reflection,--for, understand this (which is not
known by all, and not known by many who should know it), that when we
take a source of light--a single candle, for instance, giving off any
quantity of light--we can by no means increase that light: we can make
arrangements around and about the light, as you see here, but we can by
no means _increase the quantity_ of light. The utmost I can do is to
_direct_ the light which the lamp gives me by taking a certain portion
of the rays going off on one side and reflecting them on to the course
of the rays which issue in the opposite direction. First of all, let
us consider how we may gather in the rays of light which pass off from
this candle. You will easily see that if I could take the half-rays
on the one side, and could send them by any contrivance over to the
other side, I should gain an advantage in light on the side to which I
directed them. This is effected in a beautiful manner by the parabolic
mirror, by means of which I gather all that portion of the rays which
are included in it--upwards, downwards, sideways, anywhere within its
sphere of action: they are all picked up and sent forward. You thus
see what a beautiful and important invention is that of the parabolic
reflector for throwing forward the rays of light.
[Illustration: Fig. 56.]
Before I go further into the subject of reflection, let me point out a
further mode of dealing with the direction of the light. For instance,
here is a candle, and I can employ the principle of _refraction_ to
bend and direct the rays of light; and if I want to increase the light
in any one direction, I must either take a reflector or use the
principle of refraction. I will place this lens (fig. 56) in front of
the candle, and you will easily see that by its means I can throw on
to that sheet of paper a great light; that is to say, that instead of
the light being thrown all about, it is _refracted_ and concentrated
on to that paper. So here I have another means of bending the light
and sending it in one direction; and you see above a still better
arrangement for the same purpose,--one which comes up to the maximum, I
may say, of the ability of directing light by this means. You are aware
that without that arrangement of glass the light would be dispersed in
all directions; but the lens being there, all the light which passes
through it is thrown into parallel beams and cast horizontally along.
There is consequently no loss of light--the beam goes forward of the
same dimensions, and will consequently continue to go forward for five
or ten miles, or so long as the imperfection of the atmosphere does
not absorb it: and see, what a glorious power that is, to be able to
convert what was just now darkness on that paper into brilliant light!
Whenever we have refraction of this sort, we are liable to an evil
consequent upon the necessary imperfections in the form of the lens;
and Dr. Tyndall will take this lens, and will shew you even in this
small and perfect apparatus what is the evil of spherical aberration
with which we have to fight. This can be illustrated by means of the
electric lamp: if you look at the screen, you will see produced, by
means of this lens, a figure of the coal points. This image is produced
by the rays which pass through the _middle_ of the lens, a piece of
card with a hole in the centre being placed in front; but if, keeping
the rest of the apparatus in the same position, I change this card for
another piece which will only allow the rays to pass through the _edge_
of the lens, you observe how inferior the image will be. In order to
get it distinct, I have to bring the screen much nearer the lamp; and
so, if I take the card away altogether, and allow the light to pass
through all parts of the lens, we cannot get a perfect image, because
the different parts of the lens are not able to act together. This
spherical aberration is, therefore, what we try to avoid by building
up compound lenses in the manner here shewn (fig. 58). Look at this
beautiful apparatus--is it not a most charming piece of workmanship?
Buffon first, and Fresnel afterwards, built up these kind of lenses,
ring within ring, each at its proper adjustment, to compensate for the
effects of spherical aberration. The ring round that centre lens is
ground so as to obviate what would otherwise give rise to spherical
aberration; and the next ring being corrected in the same manner, you
will perceive, if you look at the disc of light thrown by the apparatus
upstairs, that there is nothing like the amount of aberration that
there would have been if it had been one great bull’s-eye. Here is
one of Fresnel’s lamps of the fourth order so constructed (fig. 57):
observe the fine effect obtained by these different lenses, as you see
them revolve before you, and understand that all this upper part is
made to form part of the lens, each prism throwing its rays to increase
the effect; and although you may think it is imperfect, because, if
you happen to sit below or above the horizontal line, you perceive but
little if any of the light, yet you must bear in mind that we want the
rays to go in a straight line to the horizon. So that all that building
up of rings of glass is for the purpose of producing one fine and
glorious lens of a large size, to send the rays all in one direction.
Here is another apparatus used to pull the rays down to a horizontal
sheet of light, so that the mariner may see it as a constant and
uniform fixed light. The former lamp is a revolving one, and the light
is seen only at certain times, as the lenses move round, and these are
the points which make them valuable in their application.
[Illustration: Fig. 57.]
There are various orders and sizes of lights in light-houses, to shine
for twenty or thirty miles over the sea, and to give indications
according to the purposes for which they are required; but suppose
we want more effect than is produced by these means, how are we to
get more light? Here comes the difficulty. We cannot get more light,
because we are limited by the condition of the burner. In any of these
cases, if the spreading of the ray, or _divergence_, as it is called,
is not restrained, it soon fails from weakness; and if it does not
diverge at all, it makes the light so small, that perhaps only one in
a hundred can see it at the same time. The South Foreland light-house
is, I think, 300 or 400 feet above the level of the sea; and therefore
it is necessary to have a certain divergence of the beam of light, in
order that it may shine along the sea to the horizon. I have drawn
here two wedges--one has an angle of 15°, and shews you the manner in
which the light opens out from this reflector, seen at the distance
of half-a-mile or more; the other wedge has an angle of 6°, which is
the beautiful angle of Fresnel. When the angle is less than 6°, the
mariner is not quite sure that he will see the light--he may be beneath
or above it; and, in practice, it is found that we cannot have a larger
angle than 15°, or a less one than 6°. In order, therefore, to get
more light, we must have more combustion, more cotton, more oil; but
already there are in that lamp four wicks, put in concentric rings,
one within the other; and we cannot increase them much more, owing to
the divergence which would be caused by an increase in the size of the
light--the more the divergence, the more the light is diffused and
lost. We are therefore restrained, by the condition of the light and
the apparatus, to a certain sized lamp. At Teignmouth, some of the
revolving lights have ten lamps and reflectors, all throwing their
light forward at once. But even with ten lamps and reflectors, we do
not get sufficient light; and we want, therefore, a means of getting a
light more intense than a candle in the space of a candle--not merely
an accumulation of candle upon candle, but a concentration, into the
space of a candle, of a greater amount of light; and it is here that
the electric light comes to be of so much value.
Let me now shew you what are the properties of that light which make
it useful for light-house illumination, and which has been brought
to a practical condition by the energy and constancy of Professor
Holmes. I will, first of all, shew you the image of the charcoal points
on the screen, and draw your attention to the spot where the light
is produced. There are the coal points. The two carbons are brought
within a certain distance; the electricity is being urged across by the
voltaic battery, and the coal points are brought into an intense state
of ignition. You will observe that the light is essentially given by
the carbons. You see that one is much more luminous than the other, and
that is the end which principally forms the spark. The other does not
shine so much, and there is a space between the two, which, although
not very luminous, is most important to the production of the light.
Dr. Tyndall will help me in shewing you that a blast of wind will blow
out that light--the electric light can, in fact, be blown out easier
than a candle. We have the power of getting our light where we please.
If I cause the electricity to pass between carbon and mercury, I get a
most intense and beautiful light--most of it being given off from the
portion of the mercury between the liquid and the solid pole. I can
shew you that the light is sometimes produced by the vapour between
the two poles better, if I take silver, than when I use mercury. Here
is the carbon pole, there is the silver, and there is the beautiful
green light, which comes from the intervening portions. Now, that light
is more easily blown out than the common lamp, the slightest puff of
wind being sufficient to extinguish it, as you will see if Dr. Tyndall
breathes upon it.
You see, therefore, how we are able, by using this electric spark, to
get, first of all, the light into a very small space. That oil-lamp has
a burner 3¾ inches in diameter. Compare the size of the flame with the
space occupied by this electric light. Next, compare the intensity
of this light with any other. If I take this candle, and place it by
the side, I actually seem to put out the candle. We are thus able to
get a light which, while it surpasses all others in brilliancy, is
at the same time not too large; for I might put this light into an
apparatus not larger than a hat, and yet I could count upon the rays
being useful. Moreover, when such large burners are used in a lantern,
we have to consider whether the bars of the window do not interfere
to throw a shadow or otherwise; but with this light there will be no
difficulty of that sort, as a single small speculum, no larger than a
hat, will send it in any direction we please; and it is wonderful what
advantages, by reason of its small bulk, we have in the consideration
of the different kinds of apparatus required, reflecting or refracting,
irrespective of other reasons for using the electric light. And it is
these kind of things which make us decide most earnestly and carefully
in favour of the electric light.
[Illustration: Fig. 58.]
I am going to shew you the effect that will take place with that
large lens, when we throw the oil-lamp out of action, and put the
electric light into use. It is astonishing to find how little the
eye can compare the relative intensities of two lights. Look at that
screen, and try to recollect the amount of light thrown upon it from
the 3¾ inch lamp of Fresnel; and, now, when we shift the lens sideways,
look at the glorious light arising from that small carbon point (fig.
58)--see how beautifully it shines in the focus of that lens, and
throws the rays forward. At present, the electric light is put at
just the same distance as the oil light; and therefore, being in the
focus of the lens, we have parallel rays which are thrown forward in
a perfectly straight line--as you will see by comparing the size of
the lens with that of the light thrown on the screen. You will now see
how far we can affect this beam of light by increasing or diminishing
the distance of the lamp. We are able, by a small adjustment, to get a
beam of a large or small angle; and observe what power I have now over
it,--for if I want to increase the degrees of divergence, I am limited
by the power of light, in the case of the oil-lamp; but, with the
electric light, I can make it spread over any width of the horizon by
this simple adjustment. These, then, are some of the reasons which make
it desirable to employ the electric light.
[Illustration: Fig. 59.]
By means of a magnet, and of motion, we can get the same kind of
electricity as I have here from the battery; and, under the authority
of the Trinity House, Professor Holmes has been occupied in introducing
the magneto-electric light in the light-house at the South Foreland;
for the voltaic battery has been tried under every conceivable
circumstance, and, I take the liberty of saying, it has hitherto proved
a decided failure. Here, however, is an instrument wrought only by
mechanical motion. The moment we give motion to this soft iron in
front of the magnet, we get a spark. It is true, in this apparatus
it is very small, but it is sufficient for you to judge of its
character. It is the _magneto-electric_ light; and an instrument has
been constructed, as there shewn (fig. 59), which represents a number
of magnets placed radially upon a wheel--three wheels of magnets and
two sets of helices. When the machine, which is worked by a two-horse
power engine, is properly set in motion, and the different currents
are all brought together, and thrown by Professor Holmes up into the
lantern, we have a light equal to the one we have been using this
evening. For the last six months the South Foreland has been shining
by means of this electric light--beyond all comparison, better than
its former light. It has shone into France, and has been seen there
and taken notice of by the authorities, who work with beautiful accord
with us in all these matters. Never for once during six months has it
failed in doing its duty--never once--more than was expected by the
inventor. It has shone forth with its own peculiar character, and this
even with the old apparatus; for, as yet, no attempt has been made to
construct special reflectors or refractors for it, because it is not
yet established. I will not tell you that the problem of employing the
magneto-electric spark for light-house illumination is quite solved
yet, although I desire it should be established most earnestly (for I
regard this magnetic spark as one of my own offspring). The thing is
not yet decidedly accomplished, and what the considerations of expense
and other matters may be, I cannot tell. I am only here to tell you as
a philosopher, how far the results have been carried; but I do hope
that the authorities will find it a proper thing to carry out in full.
If it cannot be introduced at all the light-houses--if it can only be
used at one--why, really, it will be an honour to the nation which can
originate such an improvement as this--one which must of necessity be
followed by other nations.
You may ask, what is the use of this bright light? It would not be
useful to us, were it not for the constant changes which are taking
place in the atmosphere, which is never pure. Even when we can see
the stars clearly on a bright night, it is not a pure atmosphere. The
light of a light-house, more than any other, is liable to be dimmed
by vapours and fogs; and where we most want this great power, is not
in the finest condition of the atmosphere, but when the mariner is in
danger--when the sleet and rain are falling, and the fogs arise, and
the winds are blowing, and he is nearing coasts where the water is
shallow, and abounds with rocks,--then is his time of danger, when he
most wants this light. I am going to shew you how, by means of a little
steam, I can completely obscure this glorious sun, this electric light
which you see. The cloud now obscuring the light on the screen is only
such a cloud as you see when sitting in a train on a fine summer’s day.
You may observe that the vapour passing out of the funnel casts as deep
a shadow on the ground as the black funnel; the very sun itself is
extinguished by the steam from the funnel, so that it cannot give any
light; and the sun itself, if set in the light-house, would not be able
to penetrate such a vapour.
Now, the haze of this cloud of steam is just what we have to overcome,
and the electric light is as soon, proportionally, extinguished by an
obstruction of this kind as any other light. If we take two lights,
one four times the intensity of the other, and we extinguish half of
one by a vapour, we extinguish half of the other--and that is a fact
which cannot be set aside by any arrangement. But, then, we fall back
upon the _amount_ of light which the electric spark does give us in
aid of the power of penetrating the fog; for the light of the electric
spark shines so far at times, that even before it has arisen above the
horizon, twenty-five miles off, it can be seen. This intense light has,
therefore, that power which we can take advantage of,--of bearing a
great deal of obstruction, before it is entirely obscured by fogs or
otherwise.
Taking care that we do not lead our authorities into error by the
advice given, we hope that we shall soon be able to recommend the
Trinity House, from what has passed, to establish either one or more
good electric lights in this country.