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 distinguish 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.