# The wonders of optics/What is light?

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PART II.
THE LAWS OF LIGHT.
WHAT IS LIGHT?
Everybody knows the effects of the action of light, without, however, understanding precisely what constitutes light itself. Any formal definition would rather puzzle than help the student; we must therefore content ourselves with saying that light is that effect of force which causes us to perceive external objects.
A man who was blind from his birth, and upon whom the operation for cataract had been successfully performed, had accustomed himself for a long time to imagine the nature of those unknown phenomena that his affliction had prevented him from observing. He had arranged in his mind the various definitions that had been given to him as to the nature of light, and having combined them, he fancied he had acquired some notion of what the sense of vision really meant. But what was the astonishment of the surgeon who had restored to him his fifth sense, when he asked him to give his opinion upon the effects of light, to see him take up a lump of sugar and reply that it was under that form that he had imagined it to himself.
​As for us who have the happiness of possessing the sense of sight, we know this mysterious agent more by the enjoyment that we have derived from it, than from any analysis we have made of its nature. It is an endless chain that connects us with the entire universe; a bond that laughs at distance and spans the abysses of space. By means of light we can appreciate the beauties of hue and form, and by its power we touch as it were the inaccessible. It constitutes the most intimate connexion between ourselves and external objects—a connexion that seems even to alter our temper, disposition, and character, according to the variations of its intensity. The dull and foggy days of winter, those days when sleet and rain struggle in the atmosphere, spread like a veil over us, and throw a shadow upon our life. The return of the bright spring sun, the reappearance of light and blue sky, on the contrary, open up our hearts and minds, gay nature enchants us once more, and a feeling of fresh happiness prepares us for the coming glories of the newly risen year.
This intimate connexion between the light of heaven and the human mind, hallowed as it is by our desire to rise towards the Source of all light, might be made the subject of many eloquent pages; and it would be an interesting and useful task to show the gradual progress of mankind from those ancient people who trembled at the approach of darkness, and who fervently saluted the dawn with prayers and praises, down to the philosophers of the present age, who investigate its effects with so much reverential joy. But we must cease paying any more attention to the superficial action of this marvellous force which in these latter days has become, in the hands of man, the source of so many illusions and the origin of a complete world of rich and brilliant pictures, but which after all only exist in the imagination.
​It was believed for a long time that light was a compact mass of tiny particles emitted by luminous bodies, which struck our eyes and so produced the phenomenon of vision. These particles or molecules were naturally thought to be extremely minute, and the objects illuminated by them were supposed to throw them off as if they were endowed with elasticity. Under this hypothesis, light was a material body. The illustrious Newton was the first propagator of this theory; the last was M. Biot, a French philosopher, lately dead.
The undulatory theory has now-a-days completely superseded the corpuscular hypothesis. It was first started about the year 1660 by the Dutch philosopher Huyghens, who has left behind him numerous treatises on optics, and the properties of light, as well as a curious account of the inhabitants of the other members of the solar system, including a minute description of the various planetary manners and customs. At the beginning of the present century, Fresnel showed, by the most brilliant discoveries the superiority of this theory, and shortly after Arago confirmed him in his demonstrations. According to the undulatory hypothesis, light is not a mass of molecules emitted by a luminous body, but simply the vibration of an elastic fluid which is conceived to fill the whole of space. A comparative example may assist you in understanding this theory more clearly. If you throw a stone into a smooth piece of water, there will form around the point where the stone fell, a series of circular undulations, starting from the centre and gradually enlarging themselves. If a loud noise is suddenly heard, the same effect is produced round the point from whence the sound proceeds. A series of waves are formed which spread not only horizontally, as on the surface of the water disturbed by the stone, but in every direction. In fact, in the case of sounds, the waves are so many gradually ​increasing spheres. In the case of light, when a luminous body is placed in space, the ether which surrounds it is thrown into a state of vibration, and the motion is immediately propagated in all directions, with extreme velocity. It is these undulations that produce upon our eyes the sensation of light. We may therefore say that light, like sound, is movement, while darkness, like silence, is absolute rest.
Many people still believe that light is propagated instantaneously, and cannot bring themselves to imagine that we do not see a flame the moment we light it, but only an instant after. I have myself spoken to well-educated people possessed of good judgment and a certain amount of elementary knowledge, who could never bring themselves to believe that we see the stars, not as they now exist, but as they appeared at the particular moment when the luminous wave by which we are enabled to perceive them left their surface, and which only reaches us after travelling through space a certain number of years, days, or hours, according to their distance. It is extremely useful and interesting to form a correct idea upon the way in which light is propagated.
The determination of the prodigious quickness with which the waves of light move through space, says Arago, is undoubtedly one of the happiest results of modern astronomy. The ancients believed that it moved with infinite velocity, and their view of the subject was not, like so many of the questions relating to physics, a mere opinion without proof; for Aristotle, in mentioning it, brings forward the apparently instantaneous transmission of daylight. This notion was disputed by Alhazen, in his Treatise on Optics, but only by meta-physical weapons, which were again opposed by several very worthless arguments, by his commentator, Porta, although he admitted the immateriality of light. Galileo seems to have been the first amongst modern philoso-​phers who endeavoured to determine the velocity of light by experiment. In the first of his dialogues, Delle Scienze Nuove, he announces by the mouth of Salviati, one of the speakers present, the ingenious means he had employed, and which he thought quite sufficient to solve the question. Two observers with lights were placed at the distance of one mile from each other; one of them extinguished his light, and the other as soon as he perceived it extinguished his. But as the first observer saw the second light disappear the instant he had extinguished his own, Galileo concluded that light was propagated instantaneously through a distance double that which separated the two observers. Certain analogous experiments that were made by the members of the Academy Del Cimento, but at three times the distance, led to precisely the same conclusions.
These attempted proofs seem at first sight to be absurd, when we think of the vastness of the problem to be solved; but we must judge these experiments with less severity, when we consider that almost at the same epoch, men of such well-deserved repute as Lord Bacon believed that the velocity of light, like that of sound, was sensibly altered by the force and direction of the wind.
Descartes, whose theories upon light had so much analogy with those known under the name of the undulatory hypothesis, believed that light was transmitted instantaneously throughout any distance, and endeavours to prove his position by proofs that he thought he had obtained whilst observing an eclipse of the moon. It must be acknowledged, however, that his very ingenious train of reasoning proves that whether the transmission of light is instantaneous or not, it is at least too considerable to be determined by experiments made on the earth, like those of Galileo, and which he vainly hoped would have solved the question.
​The frequent occultations of the first satellite of Jupiter, the discovery of which was almost consequent upon that of lenses, furnished Römer with the first means of demonstrating that light was propagated by perceptible degrees.
In tracing out the history of human knowledge, says Dr. Lardner, we have frequently to point out with some little surprise, joined to a feeling of profound humility, the important part played by chance in the advancement of science. In searching zealously after mere trifles which, when found, are of no consequence, we frequently lay our hands on inestimable treasures. The frequency of this fact impresses the mind with the notion that some secret and unceasing power exists, in accordance with which human knowledge and science are continually progressing. It is in physical, as in moral philosophy. In our ignorance—like the dog mentioned by Æsop, which, seeing in the water the reflection of the prey it held in its mouth, dropped the substance and tried to seize the shadow—we are continually searching after trifles; but, more fortunate than the animal of whom we have been speaking, the shadow that we try to seize is often transformed into a rich treasure. We can say with every confidence that "the Providence which shapes our ends," knows our wants better than we do ourselves, and bestows on us the things we ought to have asked for instead of those we have asked for. We shall find a very simple proof of this in the history of the discovery of the velocity of light.
A short time after the invention of the telescope and the consequent discovery of Jupiter's satellites, Römer, a celebrated Danish astronomer, was engaged in a series of observations, the object of which was to determine the time which one of these bodies took to revolve round its planet. The method employed by Römer was to observe the successive occultations of the satellite, ​and to notice the interval that elapsed between each of them. But it at last happened that the interval between the two occultations, which was about forty-five hours, became prolonged by periods of 8, 13, and 16 minutes, during that half of the year when the earth was receding from the planet, while it became proportionally cut short during the rest of the year. Römer was struck by a happy idea; he suspected instantly that the moment when he remarked the disappearance of the satellite was not always coincident with the instant when it really took place, but that it sometimes appeared to happen later—that is to say, after an interval of time sufficiently long to allow the light that had left the satellite immediately after its disappearance, to reach the eye of the observer. Hence it became evident that the farther off the earth was from the satellite, the longer was the interval of time between its disappearance and that of the arrival of the last portions of its light upon the earth; but that the moment of the disappearance of the satellite is that of the commencement of the occultation, and that the moment of the arrival of the last portions of light is that when the commencement of the occultation is observed.
It was thus that Römer explained the difference between the calculated and observed time of the occultation, and he saw that he was on the threshold of a great discovery. In a word, he saw that light propagated itself through space with a certain velocity, and that the fact we have just mentioned furnished the precise means of measuring it.
Thus the occultation of the satellite was retarded one second for every 185,000 miles that the earth is distant from Jupiter; the reason being, that a ray of light takes a second to travel this distance, or, in other words, because the velocity of light is at the rate of 185,000 miles per second.
​It must be remembered when considering this subject, that in any system of undulations or vibrations, no matter through what medium they are propagated, their movement is simply a change of form, and not a transmission of matter. The waves which spread round a central point when a stone is thrown into the water, give one the idea that the water which forms the wave really moves towards the observer. But it is not so, as may be readily proved by placing on the surface a floating body, which we shall find is but little, if at all, influenced by the undulations of the water. The appearance of rolling waves given on the stage by means of a painted cloth, to which an undulatory motion is given, is an instance of this apparent movement. In the case of the floating body, which would follow the movements of the water, we shall find that wave after wave rolls to the shore, in the same way as the painted marks on the imitation sea keep their place, although the cloth itself undulates. The waves of the sea even appear to the eye to be endowed with a progressive motion, but an instant's observation will convince us of our error; for if such were the case, every object floating on the ocean would be gradually carried on shore. A vessel floating on the waves is not carried along by them, at least not until it reaches within a few yards of the shore, where the water is really in motion; but out in the open sea a floating body will alternately rise on their crests, and fall into the valleys that separate them. The same effect may be observed with any object floating on the water. If, however, in addition to being in a state of undulation the sea is really in motion from the effects of a current, or from any other cause, the floating object will of course be carried along by it—in fact, the two movements are quite independent of each other, and may take place in similar or contrary directions. It is very important that we should be able to distin-​guish at an early period the exact difference between true movement and mere undulation; and we must remember that although the waves of light are propagated at the rate of 185,000 miles a second, still there is no transmission of any material substance at this marvellous rate. The same observation applies to sonorous vibrations transmitted through the air.
Thus we are constrained to admit peaceably the truth of the undulatory hypothesis as compared with the corpuscular theory. I say peaceably, because I am forcibly reminded by the contrast I have made between the two theories of an anecdote related of one of the greatest monsters who ever walked this earth, but who was afterwards struck down in the midst of his power by the hand of a weak girl. I allude to the infamous Marat, who one day presented himself at the house of Dr. Charles, a celebrated natural philosopher, of the time of the first French Republic, in order to advance certain notions of his own against the optical principles that Newton has left behind in his Principia, and other works—also, to oppose certain theories connected with electrical science. Dr. Charles, who did not approve of Marat's wild notions, undertook to convince him of his errors. But instead of discussing the matter peaceably, Marat allowed himself to be carried away by his temper, which was naturally very violent. Every argument advanced by his antagonist seemed to increase his rage, until at last he lost all control over himself, drew his small sword, and rushed upon his opponent. The doctor, who was unarmed, had to exercise all his powers to prevent himself from being wounded, and being much more stoutly built than Marat, he at last succeeded in throwing him down, and wresting his sword from him, which he immediately took care to break. Whether it was the violence of the fall, the shame he felt at being doubly beaten, or the effects of his fit of passion, does not ​appear, but Marat fainted. Assistance was called, and he was carried home to his house, his offence against all the laws of propriety being forgiven by his more talented and better-tempered adversary.
There are many persons, no doubt, whom we should astonish, and possibly enrage, by asserting positively that we could cause darkness by means of light, that silence could be produced by sound, or cold by heat. These are daring paradoxes, and at first sight appear almost as reasonable as that of Anaxagoras, a Greek philosopher, who asserted that snow was black. But as I hope that most of my readers do not possess the passionate temper of the French tribune, I will confide to them a little secret that will make these paradoxes plain. It is called by natural philosophers the theory of interference.
The experiments connected with this subject are exceedingly difficult to perform, and require the aid of apparatus far beyond the reach of the ordinary student. It is a case where theory and description are much easier than practice.
If a ray of electric light is thrown upon a screen, it is possible to direct another ray upon the same spot in such a manner that they will extinguish each other mutually. The reason of this phenomenon may be understood, if we remember that light is caused by undulatory movement, and that by opposing two series of waves to each other in such a manner that their vibrations coming in contact produce rest, we can easily see how the waves of light of one ray may be stopped by those of a second.
Going back to our illustration of the eddies on a pool of water, it is easy to prove that by throwing a second stone into the water we form another series of undulations; which are mutually destroyed when they encounter each other. It is the same with the ​peculiar fluid which, existing throughout space, is thrown in a state of undulation by incandescent bodies; by opposing one set of waves to another we obtain rest as a result.
This fact was first observed by Grimaldi in 1665, and Dr. Thomas Young was the first to offer an explanation. Fresnel used it with great success at the beginning of the century to demonstrate the truth of the undulatory theory, by showing that it could not be explained by any other.