IN the previous chapters your attention has been directed to the subject of waves on water and waves in air, and we shall now proceed to discuss some of the more difficult matters connected with the production of waves in the æther. We shall find that this portion of our subject makes more demands upon our powers of comprehension, since much that we have to consider is not directly the object of sense perception, and the inferences which we have to make from observed facts are less simple and easy to follow. Nevertheless, I trust that if you have been able to grasp clearly the nature of a surface-wave on water and of a compressional wave in air, you will not readily allow yourselves to be discouraged from encountering a new class of ideas, but will be able to advance still further, and gain a more or less clear conception of the nature of an electric wave in the æther.
In the first place, we must consider the medium in which these waves are created. We can see with our eyes a water-surface, and we are able to understand without much difficulty that the surface can be thrown into humps and hollows, or become wrinkled, and also that these elevations and depressions can change their position, thus creating a surface wave which moves forward. The movement of the water wave is, therefore, only the result of a local elevation of the surface which travels along or takes place progressively at different places on the surface. Then, again, in the case of an air wave, although we cannot see the air, we are able, with some little assistance from experiments, to present to ourselves a clear mental picture of a progressive movement through the air of a region of compression, that is to say, a certain slice, layer, or zone of the air is more compressed than the neighbouring portions, and this region of compression changes its place progressively. It has been carefully explained that the production of a wave of any kind implies, therefore, two things—first, a medium or material in which the wave exists; and, secondly, some kind of periodic change or movement which is experienced by the various portions of this medium at different places successively.
If, therefore, we are given any medium, say water or air, and asked to explain the production of a wave in it, we have first to consider what kind of changes can take place in it, or on it, which can appear progressively at different parts. In the case of the water-surface, some parts may be heaped up higher than the rest, and the heaping up may occur at successive places in such fashion that when it disappears at one place it reappears at a contiguous or neighbouring place. In the case of air, some portion may be compressed more than the rest, and the place of compression may move forward, so that as the compression is released in one place it makes its appearance in an adjacent one. In the first case, we have a wave of elevation on water; in the second case, a wave of compression in the air.
In the next place, let me carry you with me one step more. Here is a glass bulb from which the greater part of the air has been removed. We say, therefore, that there is a vacuum in the bulb. It is impossible for us to remove absolutely every trace of air from the bulb, and so produce what would be called a perfect vacuum; but we can imagine it to be accomplished, and we can picture to ourselves the glass bulb absolutely deprived of every trace of air or other material substance. The question then arises—Is the bulb really empty, or is there still something in its interior?
The same inquiry may be put in another way. The air we breathe forms an atmosphere which surrounds our earth as a garment, but it decreases rapidly in density as we ascend. At a height of about 50 miles above the earth there is reason to believe the air is exceedingly rarefied and, except for the presence of meteoric dust, the space between the sun and the earth, and between the stars and the earth, is in all probability a highly perfect vacuum, in the sense that it is empty of generally diffused matter. The question then arises—Is interstellar space absolutely and completely empty? We know perfectly well that rays of light come to us from the sun and stars through this empty space, and a fact of capital importance is, that these rays of light, swift-footed though they are, take time to travel. It was long ago suspected that this was the case, and the celebrated Galileo made the first experimental attempt to determine the velocity of light. No real knowledge on the subject was gained, however, until after he had made his discovery that the planet Jupiter is accompanied by four moons (a fifth moon has been discovered since), and that these rotate round the planet in definite periods of time, constituting, therefore, the “hands” of a perfect celestial clock. The sunlight, falling on the great globe which forms the body of the planet Jupiter, casts behind it a conical shadow; and the little moons, in their rotation, are plunged into this shadow cone at intervals, and then for a time become invisible, or eclipsed.
As soon, however, as these eclipses of Jupiter’s moons began to be regularly observed, it was found that the intervals of time between two eclipses of any one moon were not equal, but exhibited a progressive variation in magnitude, and were longer by about 16 minutes and 26 seconds at one time of the year than at the other. The astronomer Roemer, in the year 1675, correctly concluded that this difference must be due to the fact that rays of light take time to traverse the earth’s orbit, and not to any want of regularity in the operation of this celestial timepiece. Hence, although the eclipses do happen at equal intervals of time, our information about them is delayed by the time taken for the ray of light to travel over the variable distance between Jupiter and our earth. These observations, critically considered, led, therefore, to the conclusion that the speed of light rays is about 186,500 miles a second. By means which it would take too long to describe here, experimental measurements of the velocity of light have been made many times since by various investigators by methods which do not involve astronomical observations, and the result has been to confirm the above value, and to give us a very exact knowledge of the speed with which rays of light travel through space. It is as shown in the table below:—
| The Velocity of Light. | |||||||
| Miles per second. | |||||||
| From observations on Jupiter’s satellites (Roemer) | 186,500 | ||||||
| ” | experimental measurements by | Foucault | (1862) | 185,177 | |||
| ” | ” | ” | ” | Cornu | (1874) | 185,487 | |
| ” | ” | ” | ” | ” | (1878) | 186,413 | |
| ” | ” | ” | ” | Michelson | (1879) | 186,364 | |
| ” | ” | ” | ” | ” | (1882) | 186,328 | |
| ” | ” | ” | ” | Newcomb | (1882) | 186,333 | |
When anything takes time to travel from one place to another, it can only be one of two things. It must either be an actual object which is transferred bodily from place to place, like a letter sent by post or a bullet fired from a gun, or else it must be a wave-motion created in a medium of some kind which fills all space. The illustrious Newton suggested an hypothesis or supposition as to the nature of light, viz. that it consists of small corpuscles shot out violently from every luminous body. It is a wonderful testimony to Newton’s exalted powers of thought, that the most recent investigations show that hot and luminous bodies, such as the sun and a lamp, are in fact projecting small bodies called corpuscles into space, but there is abundant proof that these are not the cause of light. Subsequently to the date of Newton’s speculations on the nature of light, the alternative hypothesis was developed, viz. that it consists in a wave-motion in a universally diffused medium called the æther. A great gulf, however, separates mere conjecture and speculation from that accumulation of rigid proof which scientific investigation demands, and hence, although this conception of an æther had arisen as an hypothesis in the minds of Huyghens, Descartes, and many other philosophers, it was not accepted by Newton, and the general assent of scientific investigators to the hypothesis of a universal æther was long deferred. The philosopher to whom we owe the crucial demonstration of the validity of, and indeed necessity for, this assumption was Dr. Thomas Young, the first Professor of Natural Philosophy in the Royal Institution of London. Young was a man whose exalted intellectual powers were not properly appreciated by the world until after his decease. His researches in physical optics alone are, however, epoch-making in character. He it was who first gave a proof that under some circumstances it is possible for two rays of light to destroy each other, and thus produce darkness. Briefly described, the experiment is as follows: If a beam of light of one colour, say red, proceeding from a single source of light, falls upon a screen in which are two small holes very near together, we shall obtain from these holes two streams of light originating, as it were, from closely contiguous sources. If we then hold a white screen not far from these holes, and receive on it the light proceeding from them, we shall find that the screen is marked with alternate bands of red light and black bands. If we cover up one of the small holes, the black bands vanish and the screen is uniformly illuminated. Young pointed out that this effect was due to interference, and that the difference of the distances from any black band to the two holes was an exact odd multiple of a certain small distance called the wave-length of the light. If light is a substance, no possible explanation can be given which will enable us to account for the combination of two rays of light producing darkness at their meeting-point. If, on the other hand, rays of light consist of waves of some kind in a medium, then, as we have seen in the case of water ripples and air waves, it is quite possible for two wave-trains to annihilate each other’s effect at a certain point, if a hollow of one wave-train reaches that place coincidently with a hump belonging to the other.
Accordingly, the experiment of producing interference between two sets of light rays so that they destroy each other is a strong argument in favour of the view that light must consist in some kind of wave-motion existing in a medium susceptible of supporting it, filling all space, and existing in all transparent bodies. This medium we call the luminiferous æther.
The term “æther,” or “ether,” has been in use for many centuries to express the idea of something more rare, tenuous, or refined than ordinary matter.