On the various forces of nature and their relations to each other

by Michael Faraday

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.

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.

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.