Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/626

606 LIGHT Inter ference. Tides of Batsha. Young s dis- most brilliant success a guess of Hooke s (of date 1672), that the vibrations of light in an isotropic medium are perpendicular to the direction of the ray. Taking the undulatory theory as the only one left possible by the experiments of Foucault, we will now con sider the explanation it offers of various phenomena. It will be remembered that u<e have as yet made no assumption ivhatever as to the precise nature of a wave ; and it will be found that a large class of important phenomena can be explained by it without our making any such assumption, but that other classes of phenomena compel us to adopt certain limitations of the very general hypothesis with which we started. As long as we deal with the first class of phenomena, we may take for granted those properties which are common to all ordinary forms of wave-motion, such as those in water or air. In ordinary water-waves the motion of a particle is partly to and fro in the direction in which a wave is travelling, partly up and down and therefore perpendicular to that direction. This is obvious to every one who watches a floating cork. In sound-waves, whether in air or in water, the displacement of each particle of the medium is wholly in the direction in which the wave is travelling. Directly connected with this there is another distinction between these classes of waves. In ordinary water-waves the water-elements change only their form as the wave passes ; in sound-waves there is change of volume also. A third distinction, also directly connected with the first, is that sound-waves in water travel at a much greater rate than the swiftest, i.e., the longest, of surface waves. But, in either case, when two similar and equal series of waves arrive at a common point they interfere, as it is called, with one another, so that the actual disturbance of the medium at any instant is the resultant of the disturb ances which it would have suffered at that instant from the two series separately. Thus if crests, and therefore troughs, arrive simultaneously from the two series, the result is a doubled amount of disturbance. If, on the contrary, a crest of the first series arrive along with a trough of the second, the next trough of the first series will arrive along with the next crest of the second, and so on. One series is then said to be half a wave-length behind the other. In this case, the portion of the medium considered will remain undisturbed. Thus, at the port of Batsha in Tong-king, the ocean tide-wave arrives by two different channels, one part being nearly six hours, or half a wave-length, behind the other. As a result, there is scarcely any noticeable tide at Batsha itself, though at places not very far from it the rise and fall are consider able. This was known to Newton, and is noticed by him in the Priiicipia, iii. 24. See also Phil. Trans., vol. xiv. p. 677, for the observed facts and Halley s comments. Thus also (see ACOUSTICS) two sounds of the same wave length and of equal intensity produce silence if they reach the external ear with an interval of half a wave-length, or any odd multiple of half a wave-length. It is not remarkable that Young s Bakerian Lecture (1801), in which the principle of interference is for the first time described and applied, should consist in great part of extracts from the Prindpia. For there are many passages in Newton s works which might have been written by an upholder of the wave-theory. Unaccountably, how ever, Newton in the context almost always brings in a reference to the "rays of light" as something different from tlio vibrations of the ether, yet capable of being acted on by them so as to be put into " fits of easy reflexion or of easy transmission." These allusions are the most obscure parts of all Newton s scientific writings ; and it is very difficult to form a precise conception of what he meant to express in them. The following passage, extracted from Young s temperate reply (Works, vol. i. p. 202) to the violent but ignorant assault on him by Lord Brougham in the Edinburgh Review, is chosen as showing his own estimate of his own work and of its relation to what was already known : "It was in May 1801 that I discovered, by reflecting on the beautiful experiments of Newton, a law which appears to me to account for a greater variety of interesting phenomena than any other optical principle that has yet been made known. I shall endeavour to explain this law by a comparison. " Suppose a number of equal waves of water to move upon the surface of a stagnant lake, with a certain constant velocity, and to enter a narrow channel leading out of the lake. Suppose then another similar cause to have excited another equal series of waves, which arrive at the same channel, with the same velocity, and at the same time with the first. Neither series of waves will destroy the other, but their effects will be combined : if they enter the channel in such a manner that the elevations of one series coincide with those of the other, they must together produce a series of greater joint elevations ; but if the elevations of one series are so situated as to correspond to the depressions of the other, they must exactly fill up those depressions, and the surface of the water must remain smooth ; at least I can discover no alternative, either from theory or from experiment. Now I maintain that similar effects take place whenever two por tions of light are thus mixed ; and this I call the general law of the interference of light. I have shown that this law agrees, most accurately, with the measures recorded in Newton s Optics, relative to the colours of transparent substances, observed under circum stances which had never before been subjected to calculation, and with a great diversity of other experiments never before explained. This, I assert, is a most powerful argument in favour of the theory which I had before revived : there was nothing that could have led to it in any author with whom I am acquainted, except some imperfect hints in those inexhaustible but neglected mines of nascent inventions, the works of the great Dr Robert Hooke, which had never occurred to me at the time that I discovered the law ; and except the Newtonian explanation of the combinations of tides in the port of Batsha." Young s first application of the principle of interference inter- was made to the colours of striated surfaces, the next to ferena the colours of thin plates. These, however, are not so ex r en easily intelligible as the application to an experiment devised by Fresnel several years later. We therefore commence with Fresnel s experiment, which gives the most simple arrangement yet contrived, but it must be under stood that the explanation is really due to Young. BCD (fig. 31) is an isosceles prism of glass, with the angle at C very little less than two right angles. A luminous point is placed at O, in the plane through the obtuse edge of the prism and perpendicular to its base. If homogeneous light be used, the light which passes through the prism will consist of two parts, diverging as if from points Oj and 0, symmetrically situated on opposite sides of the line CO. Suppose a sheet of paper to be placed at A with its plane perpendicular to the line OCA, and let us consider what illumination will be produced at different parts of this paper. As O l and O 2 are images of O, crests of waves Fringe must be supposed to start from them simultaneously, in hoa Hence they will arrive simultaneously at A, which isj> eilt , OT equidistant from them, and there they will reinforce one lg another. Thus there will be a bright band on the paper parallel to the edges of the prism. If P a be chosen so that the difference between P^ and P 1 1 is half a wave length (i.e., half the distance between two successive crests), the two streams of light will constantly meet in such relative conditions as to destroy one another. Hence there will be a line of darkness on the paper, through P 1? parallel to the edges of the prism. At P 2 , where 2 P 2 exceeds OjP 2 by a whole wave-length, we have another