Light

Front matter

Southern Branch of the

Los Angeles

FormLl

Contents

I. RADIATION 13

II. WONDER AND MYSTERY 21

III. THE VELOCITY OF LIGHT 30

Chapter I. Radiation

LIGHT proceeding from a luminous object tends to radiate in all directions. If the luminous object is a candle, the rays can only diffuse themselves upward and on the sides, those tending downward being intercepted by the candlestick, the table, and the ground.

If the candle, so shining, is supposed to be at the surface of the earth, or upon any horizontal plane, and there is nothing to intercept its rays upward or on any side, then it is plain that the space which the rays illuminate will be of the form of a hemisphere, with a radius equal to the distance through the air to which the light could penetrate. The base of the hemisphere would coincide with the ground, or the horizontal plane, whatever it might be, on which the candle was placed, while the spherical surface of it would extend into the air, forming a great dome over and around the candle, like a kind of lower sky.

Let us suppose that the atmosphere at the time in question is so clear that the light of such a candle would be visible for a distance of half a mile. Then the radius of the hemisphere in the atmosphere which would be illuminated— that is, the distance from the centre to the outer boundary of it, would be half a mile.

14 RADIATION.

But, now, if we suppose that instead of a candle at the surface of the earth we have a flame, or other incandescent object, of the same size, and of precisely the same power to radiate light, in the air half a mile above the surface of the ground, then the hemisphere that was illuminated would become a sphere or globe, the diameter of which would be a mile, the distance from the centre to the circumference being on every side half a mile.

The light of this candle, or of the luminous object, whatever it might be, so placed, would be barely visible to any one on the earth looking upward, for, by the supposition, half a mile is the limit of the distance to which the rays could penetrate through the atmosphere and retain sufficient force to produce their proper eifect on the human eye. An eye placed any where else, also, on the margin of the illuminated sphere, and directed toward the centre, would see the light. So, also, if an eye were placed any where within the outer boundary of the sphere, and were directed toward the centre of it, it would see the light, the impression being the more vivid as the eye making the observation moved in from the outer boundary toward the centre.

We must not suppose, however, that such an illuminated sphere as we have described would have any precise or definite boundary. Some human eyes are much more sensitive than others, and can see a much fainter light, or, in other words, can see a given luminous object at a much greater distance than others. The eyes of some animals, such as insects, night birds, or beasts of prey, are probably more sensitive than any human eyes. And even beyond the limit at which the light would cease to produce an effect upon any organ of vision, some of its radiations may penetrate and produce other effects of which we have no cognizance. So that the magnitude of the sphere which

AN ILLUMINATED SPHERE. 15

would be occupied by the radiance would be estimated very differently according to the different tests of the presence of light which we might apply. Still the portion of space that the radiation would fill would in all cases be a sphere, with the luminous source itself in the centre of it, since the limits would be at an equal distance from the centre on every side, whatever might be the test by which the limits were determined.

Thus every luminous point, the radiation from which is not interrupted on any side, is the centre of an illuminated sphere — illuminated in a certain sense, as will be presently explained — which sphere is larger or smaller according to the intensity of the light and the transparency of the medium surrounding it. In the case of a common candle, and in an ordinary condition of the atmosphere, this sphere might perhaps be a mile in diameter, supposing the limits of it to be determined by the powers of human vision.

The sphere thus surrounding the luminous point is filled with light — that is, filled in a certain sense, which will also, like the sense in which it is illuminated, be presently explained. A light bright enough to be seen at a distance oftfive miles would in the same manner, if its radiance were not obstructed in any direction, form the centre of an illuminated sphere ten miles in diameter.

The sense in which this sphere is illuminated or filled with light is this, namely, that if an eye is placed any where within it, and is turned toward the centre, it will see the light — that is, there is no part of it in which there would be found any space as large as the pupil of the human eye, and probably not any as large as the area included by the eye of the smallest insect, that would not furnish rays enough to form an image upon the retina so as to produce vision.

And here I must pause a moment to explain how it is

] Q RADIATION.

that an image is formed upon the retina of the eye so as to produce vision.

If you examine one of the glasses of a pair of spectacles such as are used by elderly persons, and sometimes, indeed, by persons who are still young, but not near-sighted, you will see that the glass is thicker in the middle than at the edges. Such a glass is called a convex lens. If, now, in the evening, you remove or extinguish all the lights in the room but one, and put that light at one side, or in one corner, and then proceed to the opposite corner, or into the darkest part of the room, and there hold a small piece of white paper against the wall, and one of the glasses of the spectacles between it and the light at the proper distance, you will find that an image of the candle, inverted, will bo formed upon the paper or card. The image may be small, but if the experiment is carefully performed it will be beautifully distinct and clear. The lens collects and concentrates the light, and forms an image of the candle upon the paper or the card, which serves as a screen to receive it. Of course this experiment can be performed on a larger scale, and in a much more satisfactory manner, with a proper lens and other convenient apparatus, as shown in the following engraving.

Now in the eye there is just such a lens and just such a screen — that is, just such in respect to function. The lens is in the front part of the eye, and the screen, which is called the retina, is in the back part ; and it is by means of this image on the retina that the picture of the outward object is conveyed to the mind.

Now when it is said that the whole of the illuminated sphere surrounding a source of light as described is, in a certain seme, filled with light, the meaning is that there is no part of the whole space where an opening no larger than the pupil of the human eye will not take in enough to

EXPLANATION OF THE EYE.

THE INVESTED IMAGE.

form, by their concentration upon the retina, an image of the luminous point from which they proceed, just as a lens, held in the manner I have described, will gather rays enough coming from the candle on the other side of the room to form an image of the candle on the paper screen.

"Lawrence," said John, one day, as he had been reading about this in a book, " here's a nice experiment for me to try, if I only had a pair of spectacles."

" Would my eye-glass answer the purpose ?" asked Lawrence.

" No," replied John, " I think not. 1 suppose your eye-glass is concave, and what I require is a convex lens. Let me take it a moment, and I can soon tell."

Lawrence was lying, or, rather, reclining on a sofa in the corner of the room near a window, with his head toward the window, so that the light fell fair upon the page of the book which he was reading. John was sitting at a table near. Lawrence unhooked his glass from the cord to which jg RADIATION.

it was attached, and handed it to John, saying at the same

time,*

" There it is ; but find out whether it is convex or concave with your eyes, if you can, and not with your fingers."

" Why not with my fingers ?" asked John.

" You can feel of it if you find it necessary," said Lawrence, "but the less we touch polished glass with our hands the better. There are always particles of dust floating in the air, and these alight on our fingers and on the glass, and when we rub our fingers over the glass we rub the surface with these."

"And does that do any harm?" asked John.

" It depends upon what the particles of dust are composed of," replied Lawrence. " Some of them are minute fragments of cotton or woolen fibres worn off" from clothes. They would not do much harm. Some are minute spores of plants."

" What are spores ?" asked John.

" A kind of seeds," said Lawrence. " They are from such plants as form mould and mildew ; and some smaller still — so small, indeed, that the plants themselves can not be seen except with a microscope ; and you can judge how small the seeds must be. These would not do much harm any more than the woolen and cotton abrasions. But there is another kind of dust which comes from the road, and which consists of minute scales of iron, from the shoes of the horses, and the tires of the wheels, or, what is still worse, of fragments of stone from the pavements, some of which are siliceous — that is, of the nature of flint, and are exceedingly hard. When you rub these over the glass with your fingers, or with a cloth, or a piece of leather, although no one rubbing produces any perceptible effect, after a time the fine polish begins to be dimmed. * See Frontispiece.

TAKING CAKE OF LENSES. 19

" So that, if you are going to study optics," continued Lawrence, " and are to have any nice lenses and prisms to make experiments with, I advise you to be very careful how you rub them with dusty fingers or dusty cloths."

"Yes, I will," said John.

" A good lens," said Lawrence, " is a -very delicate thing, and sometimes a very costly thing. It requires, in the first place, a very nice preparation and mixture of the materials out of which the glass is made, and great care in the making of it, to secure its being uniform and homogeneous throughout, so as to act upon the light in the same way in every part. Then it is a very nice operation to grind it precisely to the true form, and to polish it perfectly. So that, when you get a good lens, if you ever do get one, you can't be too careful of it."

While Lawrence had been saying these things, John had been attentively examining the eye-glass, without, however, touching the glass at all.

" Yes," said he, " yours is a concave lens ; it is thinner in the middle than at the edges. I want one which is thicker in the middle than at the edges."

"Perhaps the landlady will lend you her spectacles," said Lawrence.

Lawrence and John were at this time in lodgings in London. The keeper of the lodging-house where they had taken their rooms was quite an elderly woman, and very soon after he had given Lawrence's eye-glass back to him, John heard her footsteps in his bedroom, which was a small room adjoining their sitting-room. John went in immediately, and asked her if she used spectacles. She said she did sometimes. John asked her if she was willing to lend him her spectacles a few minutes; he wished to make an experiment on light with them.

" Certainly," she said. He could have them as well as

20 RADIATION.

not ; but they were a poor old pair, very loose in the joints, and she was afraid they would hardly be of any use to him.

John said that the looseness of the joints would not be of any consequence. So the old lady went out and pretty soon returned with the spectacles.

John's plan was to go into his own room to try his experiment, as it would be necessary to darken the room, he said, and he did not wish to interrupt his cousin's reading. But Lawrence said he would like to see the experiment himself. So John lighted a candle and closed the shutters, following the directions given in his book. Then, placing the candle on one corner of the mantel-piece, and going to the farther corner of the room, with the spectacles and one of Lawrence's cards in his hand, he attempted to form an image on the paper in the manner we have already described.

JOHN'S EXPERIMENT. 21

CHAPTER H

WONDER AND MYSTERY.

JOHN was quite surprised at one phenomenon which presented itself to his attention in performing his experiment. While he was making his preparations, Lawrence remained on the sofa, intending, as soon as he found that John had succeeded in bringing the light to a focus and producing an image of the candle, to go and see it. But John seemed to encounter some difficulty, and presently he said that he could not manage the spectacles, for he could not keep the bow out of the way.

" If I take them by one of the glasses," he said, " and hold the other glass up for the light to shine through, the upper bow falls down over it, the joints are so loose."

" Never mind," replied Lawrence ; " let it fall."

" Then that will make a blank bar across my picture," said John.

" No," replied Lawrence ; " try it and see."

So John held one of the glasses up, with the bow belonging on that side hanging down over it, and then, placing his card against the wall, he moved the glass to and fro, so as to find the right distance for producing a distinct image. He expected, of course, that the shadow of the bow would be seen extending from above down over the picture, if he succeeded in producing any picture.

But, quite to his surprise, he soon obtained a very excellent image of the candle, and without any shadow, or dark bar, or any other indication of the bow at all, to disfigure it. The image was reversed, it is true — that is, it was upside down — but it was very distinct and very beautiful.

22 WONDER AND MYSTEKT.

There was the flame perfectly formed, though pointing downward, and the wick (which appeared like a slender black line in the middle of it), and the top of the candle (which was rendered bright for a little distance by the translucency of the wax at the margin), all plainly to be seen.

John was very much pleased to find his experiment so successful, and he called Lawrence to come and see it. Lawrence came, and he showed John how he could vary the effect by changing the distance of the lens from the candle, though this made it necessary also to change the distance from the lens to the screen. The nearer the candle was to the lens on one side, the farther it was necessary to place the screen on the other, in order to bring the rays to a, focus, as it is called— that is, to make the image distinct.

John was, however, very much surprised to find that there was no dark line across the picture of the candle corresponding to the bar formed by the bow of the spectacles. Lawrence told him it would be the same with any opaque substance at the surface of the glass. He might put a patch directly upon the glass itself, and it would not show as a spot of shadow on his picture.

John tried this experiment. He cut out a small round piece of paper about as large as the section of a pea, and then, wetting it to make it adhere, he put it on the glass. Notwithstanding what Lawrence had told him, he could not help expecting to see it produce a round black spot upon the image of the candle. But it did not do so. The image became somewhat less bright than before, it is true, but there was no appearance upon it of any shadow, either from the bar or from the paper patch.

Lawrence explained to him how this was, and I intend to repeat the explanation in a future chapter ; but now I

DIVERGENCE OP KAYS. 23

must return to the illumined sphere, which, I have said, always surrounds every luminous point, so far as there is no object intervening to intercept the rays.

This sphere, as has already been said, is filled with light in the sense that in no part of it can an eye be placed where there will not be rays enough, if the eye is turned toward the luminous point, to enter the pupil and form an image of the source of the light on the retina, as John formed an image of the candle on his card by the lens. And it is in this sense only that the space within the sphere is illumined, namely, that it is completely filled with rays of light proceeding in close proximity to each other from the centre to the circumference.

These rays, it is true, diminish in intensity, in some mysterious way, as they proceed from the centre to the circumference ; but, in whatever way this diminution of intensity is eifected, it is not done by a separation of the rays from each other as they diverge, so as to leave some parts of the space empty.

Radiation from a luminous point is, indeed, often in books represented by lines diverging from each other as tliey recede from the centre, and this, however closely the lines are together in the centre, gives us the idea of a necessary separation between them toward the outer portions. But we must not imagine that the diminution in the intensity of light, as the distance from the source increases, is produced by any separation of the rays. Exactly how we are to picture this diminution of intensity to our minds it is difficult to say, but it is certain that it is not the result of the separation of lines of radiance from each other as they recede, leaving intervals between them dark.

In describing the phenomena we use the word rays, and we represent the radiation by lines ; but we must conceive of it, so far as we can, as homogeneous throughout, and as

24 WONDER AND MYSTERY.

diminishing in intensity, when it does so diminish, without the least interruption of continuity.

The sphere thus is illuminated only in this sense, that an eye in any part of it, turned toward the centre, would see the light ; looking in any other direction through the sphere, it would see nothing. We may, however, conceive such a sphere to be illuminated in another sense, as follows :

If, for example, the spherical space were filled with dust or smoke, or any other substance consisting of fine particles, and if the light in the centre wrere increased in intensity just enough to make up for the loss that would be occasioned by the intercepting of the light by sucli particles, then the space included would be illuminated in another way. The sphere itself would then become visible, just as the sunbeams do when shining through a crevice into a dusty or smoky room, or the rays of the sun when they illuminate the mistiness floating among the clouds at evening in the western sky, and which people call " drawing water," under the erroneous idea that those lines of light are streams of vapor ascending into the air. The effect is produced by the rays of light passing through the interstices in the clouds, and then shining upon and being reflected by the particles of mist which they meet with on the way. It is true that the direction of the illuminated lines is generally downward, as there is usually more mistiness in the atmosphere near the earth than above, though they are sometimes seen ascending as well as descending, as is represented in the following engraving.

If now the sphere surrounding the luminous point which I have been describing were illuminated in this manner — that is, by having particles floating in the air to receive and reflect portions of the light — with a sufficiently increased intensity at the source to just make up for the loss, the form and the extent of it — that is, of the whole

UNSEEN LIGHT.

ILLUMINATED VAPOE8.

sphere— would be visible to the eye, appearing like a vast ball of light a mile in diameter, bright at the centre, and gradually diminishing in brightness from the centre to the outer surface, where the light, by insensible gradations, without any definite boundary, would melt into the darkness and disappear.

It must be understood, however, that this sphere would be thus visible to us, not by means of any of the rays of light which were passing out from the centre to the circumference of the sphere on their regular course, but only by means of that portion of them which was intercepted on the way and reflected to the eye by the solid particles.

And this brings to our minds a principle of fundamental importance, namely, that no light produces any effect upon our vision except such as passes into the eye. It may pass before us or across our field of view in any quantity and of any intensity without being perceptible to us at all. It is only when it enters the eye, and falls upon the screen called the retina, in the back part of it, that we can have any consciousness of its presence.

Thus, if such a candle as we have supposed were sur- B

26 WONDER AXD 3IYSTEKY.

rounded by an atmosphere so far transparent as not to contain any substances capable of sensibly reflecting the light, the light would go on diminishing in intensity as it receded from the centre until it disappeared, and in that sense the whole sphere would be illuminated — that is, it would befitted with a radiation of light; but in looking toward it we should see nothing except the luminous point in the centre, and we should not see that unless our eyes were turned directly toward it.

And now let us suppose that, instead of one luminous point or candle flame, there were two, and that they were placed at the distance of half a mile from each other; the two illuminated spheres would then interpenetrate each other, so to speak, to the extent of half their diameters. The rays from A, proceeding in the direction toward B, would encounter those of B coming toward A. The encounter would be direct on the line joining the two central points, and in all other parts it would be indirect, and the crossing would be at various angles.

LT.CMINATEB SPHEEE8.

In those parts of the space common to both spheres which are equidistant from the two centres, the two radiances encountering each other would be equal. In those parts which were nearer to one than to the other, the light

EMANATIONS OF LIGHT. 27

coming from the nearest point would be strongest. Thus the radiances issuing from the two centres would encounter each other, in the space common to both, at every possible angle and with every possible disparity of force.

And yet, so far as we can discover, such rays do not interfere with each other in the least ; for, wherever you put your eye within the portion traversed by the light from both the centres, if you turn your eye to either, you have its image as clearly and distinctly painted on your retina as if the other did not exist ; that is to say, the radiance which comes from one of the points, though its track has been crossed all the way by the emanation from the other — both filling the space completely — is not disturbed or interfered with by the other in the slightest degree.

This is wonderful, and it is for the purpose of furnishing a clear, simple, and precise idea of the nature of this mystery, as the foundation of a right understanding of what is to follow, that I have made these suppositions of candles in the air.

If you have followed what I have said closely enough to have received distinctly an idea of the nature of this wonder of the non-interference, in the ordinary sense, of luminous emanations moving in contrary directions, and crossing each other at every possible angle and on the same track, you are prepared to appreciate in some degree the amazing magnitude and extent of it. Every star in the sky is the centre of a sphere that is illuminated by its radiation in the manner I have explained — a sphere, too, which is so enormous in extent that its magnitude and grandeur surpass all human conception.

Light is proved to move at the rate of between one and two hundred thousand miles in a second, which is sufficient to carry it round the earth in the seventh part of a second, and there are stars so remote that it would require Hun-

28 WONDER AND MYSTERY.

dreds of years for their light, moving at that inconceivably great velocity, to reach the earth ! Think of the enormous magnitude of a sphere traversed by the radiance of such a star ! Now every one of the millions of stars — and if we include, as we ought to do, with those that can be seen by the naked eye, those which are brought to view by the tel escope, either as single stars or are resolved from nebulae, the number is to be reckoned at thousands of millions — is surrounded by a sphere of radiance which must extend to us, and all these spheres occupy the same space, entering into, crossing, and interpenetrating each other in every direction, so that, when you hold up a needle in the evening air on a clear night, there are millions upon millions of distinct radiations passing through the eye of it in both directions, encountering each other at every possible angle and with every conceivable disparity of force. And yet each one of these radiations maintains its way so entirely uninterrupted and undisturbed by the rest, that you can select any one of them you choose, and, by conducting a sufficient portion of it from the space around, by means of the telescope, to your eye, you can there produce a picture of its source upon the retina as clear, and distinct, and as sharply defined as if its own radiance was the only one emitted, and had the entire and exclusive occupation of the field.

There is no way of escaping from or diminishing the unutterable wonderfulness of these facts. I have only called the emission a radiance — that is, something radiated — without intending to say in what it consists. It has been thought to consist of streams of infinitely minute particles of matter. It is now generally considered as an undulation or vibration in some extremely subtle medium diffused through space, to which the name luminiferous ether has been given, which phrase means, simply, the unknown

SYMBOLS FOB DESIGNATING LIGHT. 29

something which transmits the light. It is supposed that one or the other of these two suppositions must be correct, because these are the only two ways in which we can conceive that action of any kind can be conveyed through space. But whether there may not be modes of transmission for force that man, with his present mental constitution, can not conceive of, is a grave question.

At any rate, it is now universally agreed among scientific men to regard light as transmitted by a series of undulatory movements in an intervening medium, and all the calculations and all the language used in describing the phenomena are based at the present day on this hypothesis. In all the drawings, however, and other illustrations intended to represent the action of light to the eye, the radiation is represented by lines, which are more appropriate to the idea of a progressive motion of streams of particles than to that of undulations. We are obliged to use both these modes, as the best symbols of thought at our command ; but when we attempt to pass from these symbols to the realities which they are intended to represent, we are lost in wondering what the actual nature of emanations can be that can thus meet, and cross, and encounter each other in every imaginable way — in such countless numbers, within such inconceivably narrow limits, and at such inconceivably rapid rates of motion — and each of the millions of separate motions pursue its own way without being in the least degree deranged, disturbed, or interfered with by the rest. We ask ourselves in amazement, What can the emanation be, in its intrinsic nature, that can exist under such conditions as these? What is light ? We can not tell. We can really know nothing of its essential nature. We can only study such of its modes of action and such of its effects as come within the reach of our senses and of our half-developed reasoning powers.