« AnteriorContinuar »
All, absolutely, that we know of a star is built upon a Ray of Light.
Other senses fail us here. We cannot touch, we cannot hear, those distant orbs. No sound from them of roaring flames can ever reach our ears ; for sound is carried by air ; and a few miles, or at most a few hundred miles from Earth's surface, air ceases. In solemn silence—silence, so far as human powers of hearing are concerned-century after century the countless Suns of the Universe roll onward. Sight alone speaks to us of their wonders ; and Light alone can speak to Sight.
To some extent we may say the same of our sun; but there we have bodily consciousness of heat, as well as of light; and the power of the sun is visible everywhere about us, in an infinitude of ways. Even the paler and more insignificant moon makes herself felt as well as seen in our lives. Every time that we hear ocean's tides pouring in or out, we hear the utterance of moon-power. Moreover, in the case of sun, moon, and planets, the rays of light are many, the surfaces to be examined are more or less extended. The sun has his spots and streaks; the moon has her mountains and hollows; the planets have their various characteristics of polar spots, or bands, or diverse hues.
With the stars all such landscape features are lacking. Each one, whether the brightest or the dimmest, has for us no surface, no disc, no shape, no apparent size ; nothing but light, brought down through exceeding distance to a slim indivisible shaft of radiance. Indivisible in one sense ; not in another. More of this later. Our whole knowledge of that faroff sun is founded upon one continuous beam of brightness. Where sight fails, the stars for us cease to exist, except in imagination. We do not feel their heat, or see upon earth results of their power.
In trying to picture to ourselves what is meant by LIGHT, two sides of the matter claim attention: the ray of light itself, and the eye which receives that ray.
We are apt to think of light as being instantaneously everywhere ; but this is error. Light is given out by a bright bodywhether candle or lamp, sun or star-in what we are pleased to call rays or beams; and those rays or beams travel at a certain rate of speed. A ray starting from the sun, or from a star, is not immediately here, any more than a train starting from London is immediately at Liverpool. Time is occupied in either journey.
The speed of light has been repeatedly measured in divers modes ; one result proving another, as when we add up a sum in two different ways, to see if the answers agree. Slight corrections have been made from time to time, due to increased delicacy of instruments, and increased accuracy of observation. Here is the result : that light journeys at the rate of 186 thousands of miles each second; or, roughly, at the rate of 600 billions of miles each year.
If you have seen a gun fired, or a rock blasted, at some distance, you must have noticed that the flash was first visible. Then, , after a brief pause, followed the bang of sound. So, too, in a thunderstorm : first the lightning is seen, then the thunder is heard.
The reason for this is that, while sound and light both require time to travel, sound is very much slower than light. They start on their way at the same instant, but sound lags behind, and light speeds forward. Sound travels at the rate of 1140 feet in one second ; and light travels at the rate of 186,000 miles in one
. second. A very considerable difference! No wonder the light of an explosion reaches our eyes before the sound of it can reach our ears.
Nevertheless, extraordinarily rapid as light is in motion, it does journey, and it does occupy time in journeying.
Light coming from the sun by no means touches earth at the identical instant that it quits yonder blazing surface. A ray of sunlight, which reaches London at precisely mid-day, left the sun at close upon nine minutes to twelve of London time. Through those nine minutes the ray has flashed through space at the rate of over eleven millions of miles each minute, till arrested by earth.
A soft moonbeam, touching your face or mine, quitted the moon less than one minute and a half ago. One might suppose that light from the radiant sun would journey faster than a pale weak moonbeam. But, no! Light, whether it spring from sun or moon, lamp or candle, whether direct from a blazing body, or only reflected from a dark body, travels always at the same speed.
When we seek to realise the distances of the stars, no better measuring-line can well be found than this of light-speed. Nine minutes' journey from the sun means what we count to be an enormous space. Yet this great dividing gap sinks into a mere rift, beside the stupendous chasm which divides the SOLAR SYSTEM from the stars. Here we take for our 'yard-measure,' not the speed of light each second,—186,000 miles, but the speed of light each year,—600 billions of miles,—and we reckon the distances of the stars in so many years of light-journeying.
Viewed thus, we find that while the moon is less than one and a half minute distant, and the sun less than nine minutes distant, the very nearest known to us of all the fixed stars, Alpha Centauri by name, lies FOUR YEARS AND FOUR MONTHS AWAY.
Think what this implies. We sometimes say, speaking of a town, 'Oh, it is one hour away !'--meaning one hour of railway travelling, at perhaps forty miles an hour, with stoppages. In like manner one might say, 'Oh, the moon is not one minute and a half away!'or, “Oh, the sun is nearly nine minutes off !' as if that were a slight matter. But of the stars, in a more awe-struck tone we must say, 'The nearest is almost four years and a half away!' And this nearest--the very closest neighbour known to us in the Universe, outside our Solar family—this Alpha Centauri, lies about ten billions of miles nearer than any other star, the distance of which has yet been measured.
A light-ray, whether from sun or star, world or moon, lamp or candle, or any kind of body, either giving out or reflecting brightness, travels always in straight lines, unbending and unbroken, so long as it continues in one medium, or in different mediums of exactly the same density. But if it passes out of one medium, and enters obliquely another of different density--as in passing out of air into water, or out of glass into air-it is bent or REFRACTED into a new direction. Water is much more dense in make than air ; therefore, a ray of light, passing obliquely out of air into water, or out of water into air, is sharply refracted out of its former course in the act. Glass is much more dense than air ; and a ray of light, passing obliquely from the one to the other, shows the same result.
Drop a penny into a cup, and stand so that the coin is just hidden from your eyes by the cup-rim. Then, without moving your head, pour, or get some one to pour, water slowly into the cup. The penny will quietly rise to view, even while the rim of china is still interposed between it and your eyes.
For the rays of light, which proceed from the surface of the coin, are bent out of their straight course at the moment of quitting water for air, and thus they reach your eyes. So here you have a case of
reflection and refraction ; the penny reflecting sunlight, and the
; reflected sunlight being refracted on its road to you.
When we look on the sun, at the moment of his setting, we see his body after it has actually passed below our horizon. The rays of sunlight, as they journey from the ether of space into our denser atmosphere, or from a less to a more dense layer of air, are bent or refracted out of their course, so that they reach our eyes over the rising surface of earth, which otherwise would hide the sun from sight.
It is well to understand that whatever we see, we see by virtue of its own brightness—of the light which it gives forth. That light may be either intrinsic or reflected. A sun, a star, a lamp, a candle, shine by their own radiance. A planet, a world, a mirror, and all conceivable bodies or surfaces which have no light of their own, shine by borrowed brightness. Rays of light proceeding from a shining body fall upon a dark body, and rebounding thence enter your eyes, rendering the object visible to you.
If you wish to see a pond, a house, or a tree, it is not in the least necessary that the sun itself should shine upon your face ; such shining, in fact, would only hinder sight. It is only needful that the sun should shine on pond, or house, or tree, and that reflected sun-rays, rebounding thence, should carry its outlines to your eyes. If you wish to read a book at night, it is quite needless for the lamp to cast its glare upon you. The lamprays must fall upon the book; and those rays, rebounding, have to reach your eyes. The less you see of the lamp itself, the better.
Even so, the sun shines upon the moon's dark body and lights it up. And we see the moon best when the sun is not visible. When he is, the glare of sunlight commonly prevents our seeing the moon at all.
Thus, upon whatever object light-rays fall, from any bright body, that object is made visible by means of the light which it gives forth again. Some substances reflect much more light than others, and consequently they become more distinct, even more brilliant. But if a body gave forth no light whatever, our eyes could not detect its presence.
A looking-glass in full sunlight will flash radiant beams, while a heap of earth in a dark cellar can barely be seen at all. Yet both are visible from the same cause ; only in the one case we have abundance of light and a good reflecting surface; in VOL. 85 (V.- NEW SERIES). 35
the other case we have very little light thrown back from the surface.
When we talk of a ray of light journeying, or being reflected, we are using a convenient term to help our understandings; but in strict truth light no more comes in separate rays than a river flows in separate drops. The ideal drop of water is infinitesimally small; and the ideal ray of light is infinitesimally slender. The smallest surface which shines at all may be said to send millions of light-rays to the eye, to make its presence and appearance known. By magnifying that object, the so-called ‘rays' can be multiplied and divided to almost any extent.
As light journeys through space it is invisible. It becomes visible only when arrested by some object in its path. Many people fail to grasp this fact, and picture the vast reaches of space as everywhere lighted up by countless brilliant suns, like an enormous room lighted by countless lamps and candles. Such a notion is wrong. Not only is the hugest room ever built a mere speck compared with heavenly space ; but also in that room is something which does not exist in space. I mean, the atmosphere.
Air carries and spreads about light. Each particle of air to some extent arrests and reflects sunlight. In the wide reaches of space we do, indeed, believe that something exists, which has been named 'ether,' something exceedingly attenuated, something not to be compared with air in density, something without which light probably could not travel at all. But although ether may be a means of light passing from one object to another, it cannot stop the rays of light, as even one particle of air can do, causing them to give forth their brightness.
Through every part of space countless billions of light-rays are ever darting on interminable journeys from multitudinous suns; yet the vast gaps between those suns are dark, lighted only here and there probably by some world or comet or meteorite, which intercepts the hidden rays, and makes them visible.
Our sun gives out an enormous amount of heat and brightness. With only his present expenditure of both, he could light and warm and keep in life no less than two thousand two hundred millions of little worlds such as ours.
If all the planets of the Solar System, all the moons, all the comets, all the meteors and meteorites, were brought together, and the whole supply of solar heat and light received by them were subtracted, it would be found to be the merest fraction compared