obstacle which stops part of them, bend afterwards and dilate themselves gradually into the quiet water behind the obstacle. The waves, pulses, or vibrations of the air, wherein sounds consist, bend manifestly, though not so much as the waves of water. For a bell or a cannon may be heard beyond a hill which interrupts the sight of the sounding body; and sounds are propagated as readily through crooked pipes as straight But light is never known to follow crooked passages, nor to bend into the shadow. For the fixed stars, by the interposition of any of the planets, cease to be seen, and so do the parts of the sun, by the interposition of the moon, Mercury, or Venus." a

ones.

Young combats these conclusions on several grounds. He doubts the validity of the demonstration of the proposition upon which they are chiefly founded, though his objections refer less to the reasoning by which it is supported than to the consequences to which it leads; asserting, also, in opposition to Newton-as the result of later and more extended observations— that sound is not equally loud when diffused in spaces which are perfectly free and when passing round corners and obstacles; and that its easy propagation through bent tubes is rather due to reflexions on their surfaces than to the perfect facility of its diffusion. If, therefore, any sensible reduction of this diffusive tendency is observable in the undulations of a medium like air, how much greater must it be in the ether, whose undulations constitute light, the elasticity of which is incomparably greater!

This was the conclusion at which Huygens arrived, in a work replete with remarkable anticipations of the great truths which form the basis of the undulatory theory, upon grounds which will be afterwards more

• Newton's Optics, query 28, quoted by Dr. Young, Works, vol. i. p. 153. b Works, vol. i. p. 74.

с

Théorie de la Lumière, written in 1678 and published in 1790.

particularly noticed, and which enabled Fresnel, when combined with the principle of the interference of light, to give a complete answer to this the most popular and the most potent of all the objections which have been made to this theory. The suggestion and establishment of this second principle was almost entirely due to Dr. Young," though he failed to perceive its full bearing upon the question which we are now considering.

The following is the account which Young himself has given of the train of reasoning by which he was led to the recognition of this important principle :

"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

a

"A doctrine" (the interference of light), says Sir John Herschel, "which we owe almost entirely to the ingenuity of Dr. Young, though some of its features may be pretty distinctly traced in the writings of Hooke (the most ingenious man perhaps of his age), and though Newton himself occasionally indulged in speculations bearing a certain relation to it. But the unpursued speculations of Newton, and the aperçus of Hooke, however distinct, must not be put in competition, and indeed ought scarcely to be mentioned, with the elegant, simple, and comprehensive theory of Young,-a theory which, if not founded in nature, is certainly one of the happiest fictions that the genius of man has yet invented to group together natural phenomena, as well as the most fortunate in the unexpected support it has received from all classes of new phenomena, which, at their first discovery, seemed in irreconcilable opposition to it: it is, in fact, with all its applications and details, a succession of felicities, insomuch that we may be almost induced to say, if it be not true, it deserves to be so.”—Optics, Encyc. Metrop., art. 595. There is now no sufficient ground even for the fragment of doubt which is here insinuated: the evidence upon which this theory rests, though inferior in completeness, is hardly so in force to that which exists for the theory of gravitation. b Supra, p. 71. Works, vol. i., p. 132.

!

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 portions of light are thus mixed; and this I call the general law of the interference of light."

a

The law itself is enunciated in the following form, in the first of the three Memoirs which we are now considering:

"When two undulations from different origins coincide, either perfectly or very nearly in direction, their joint effect is a combination of the motions belonging to each."b

All movements in the same elastic medium, of whatever nature, whether gentle or violent, small or great, are known to be propagated with the same velocity; and all vibrations whether great or small, produced by a force varying as the distance from a central point, provided it be the same at the same distance, will be completed in the same time. It will follow, therefore, that the undulations will be of the same length, when the vibrations are isochronous, but different for those vibrations, which, though following the same law, are not completed in the same time. It will be found that the first is the case with light which is homogeneous

a

Reply to the Edinburgh Reviewers, Works, vol. i. p. 202. b Works, vol. i. p. 157.

but the times of vibration and therefore the lengths of the undulation of light of different colours, will be found not to be the same but to vary from red at one extremity of the prismatic spectrum to violet at the other, very nearly in the proportion of 5 to 3.

The most important consequences will be found to follow from these different lengths of the undulations of light of different colours, giving rise to phenomena of endless variety and beauty. We shall afterwards have occasion to refer to some of the remarkable observations by which those lengths admit of being measured with an accuracy which is not surpassed by any other determination in philosophy.

The amplitude of an undulation is measured by the greatest excursion of a vibrating particle from its initial position or place of rest, considered with reference to the vibrating particles themselves: its phase by the time elapsed from the beginning of the vibration, or more commonly, by the angle which would be described in that time by a radius whose extremity describes a circle uniformly in the whole time of an undulation. The movement of a vibrating particle is positive or progressive during the first half of an undulation, or between the phases 0 and 180°, and negative or retrograde during the second half, or between the phases 180° and 360° or 0° its greatest positive or negative velocity is at the end of one quarter or of three quarters of an undulation, and in all other positions it is proportional to the product of the amplitude of the vibration and the sine of its phase.

:

The intensity of a vibration, or its light-producing power, must be assumed, upon dynamical considerations (the conservation of the vis viva), to be proportional, not to the simple power, but to the square, of its ampli

tude. Light may be extinguished, but can never become negative.

With these preliminary explanations, we shall be in a condition to enter upon the consideration of some of the more simple cases of the interference of such undulations. Such are those where they are of the same length and intensity, and in complete accordance or discordance with each other, that is, in the same or opposite phases. Other cases would require for their explanation the use of analytical processes, which, though not in themselves difficult, could not easily be rendered intelligible in ordinary language.

Let there be assumed, for convenience of illustration, two sources of homogeneous light of equal intensity; and let us consider the undulations which they transmit, in the same direction, in the line which passes through them ; and let us farther assume that the undulations which emanate from both these sources are equal in length, in intensity, and, therefore, also in amplitude, and that when they issue from them they are in the same phases of their vibrations. If under such circumstances we suppose the sources of light to be distant from each other by an exact multiple of the length of one of those undulations, they will be found, when they concur, in exact accordance with each other; every particle of the medium being agitated by a double force and producing a single vibration of double amplitude and quadruple intensity, performed in the same time, and therefore transmitted by an undulation of the same length as before. In such a case the separate undulations are said to interfere, producing the maximum effect by their corroborative action.

Again, if we consider the undulations transmitted from two such sources of light, not upon the line

LIFE.

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