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 the 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 slight reflection on such an analogy will suffice to generalise it, so as to convey a tolerably exact idea of the case of the interference of two rays of light. We have only to imagine in like manner two sets of waves propagated through an ethereal medium and coinciding in direction, when it will be easily apparent that, just as in the case of the supposed canal, they may have their waves either conspiring or counteracting, and consequently giving a point of brightness or darkness accordingly.

This would directly apply to the thin films. A ray impinging would be partly reflected at the first surface of the thin film, partly entering it would be reflected internally at its second surface, and emerge coinciding in direction with the first, but retarded behind it in its undulations either by a whole or a halfundulation, or some multiples of these; thus combining with the first, and giving either a point of brightness or one of darkness accordingly; or by some intermediate fraction, giving an intermediate shade. And this would go on alternately at successively greater thicknesses of the film, giving a succession of such points or bands.

If two rays or sets of waves, instead of being exactly superimposed, be supposed to meet, inclined at a very acute angle, in a somewhat similar way, they would, across the whole space which the united rays would occupy, alternately conspire, or clash, with each other, thus giving rise to a series of bright and dark points, the assemblage of which would produce bands or stripes on a screen intercepting the rays. Now, as to actual experimental cases, it was in the application of this latter theoretical idea that the invention of Dr. Young was peculiarly displayed. The former case was that alone which seems to have occurred to

* Works, vol. i. p. 202.

Hooke in reference to the colours of thin plates; and even this was in his mind but a very indefinite conception; nor did it seem at first sight readily comparable with such cases as the diffraction-fringes, or still less with the internal bands in a shadow observed by Grimaldi. Young, however, in following out his comprehensive theory to the case last supposed, conceived a community of principle applying to both these as well as to other classes of phenomena.

He repeated and extended Grimaldi's experiment; using narrow slips of card to intercept the ray, in which case dark and bright stripes parallel to the sides internally marked the whole shadow longitudinally, while the external fringes appeared on the outside at each edge.

In theory, originating at the small aperture, a series of waves was propagated onward, till, on reaching the card, they were broken up into two new sets of waves, and spread in circles round each edge as a new centre; while part of the original set continued to pass on at each side. On the principle just mentioned these would interfere with the new portions on the outside; and the two new portions would interfere with each other in the inside of the shadow; in either case giving stripes or bands. To complete the proof, -when an opaque screen was placed so as to intercept the rays on one side, though abundance of light was present on the other, yet all the internal bands immediately disappeared; demonstrating that the effect was due solely to the concurrence of the light from both sides.

Young's investigations in fact extended to a number of minor points; we have here merely indicated the leading and essential results: they were also long complicated by various considerations which it required time and experience to show were erroneous or superfluous. But on these we need not dwell.

When the public mind is generally ill-informed on any subject, it is not surprising that any one writer, especially if of some previous reputation, should be able to assert an absolute command over public opinion, whether in favour or in disparagement of a particular invention or a particular investigator of that subject. Such was the case with Dr. Young and his theory of light, which, in his own country, was neglected and disparaged to an extent explicable only from the apathy even of the scientific world, more especially when acted upon by the acrimonious and overbearing censures cast upon that theory, by a critic known to be conversant with the subject, and whose acknowledged powers as a writer gave immense influence to the bitter sarcasms and violent abuse and ridicule with which he assailed the discoveries of Dr. Young and the whole theory of waves. Those who have read the articles alluded to in the Edinburgh Review of that date,-well

known to have been the productions of Lord Brougham,-will not be at a loss to comprehend the crushing effect with which they must have acted on a discovery at the time new, unsupported, open to many confessed difficulties, and opposed to the prepossessions of a large portion even of the scientific world.

From the interruption of communication with the continent during the war, it happened that, many years after the date of Young's researches, M. Fresnel, entirely ignorant of what had been done in this country, followed up, independently, the very same investigations; and in a Memoir on Diffraction presented to the Academy of Sciences in 1815, detailed all the same, or similar experiments, leading to the establishment of the same explanation of the phenomena. Fresnel, however, pursued the subject to a greater extent, and carried out the mathematical development by a more elaborate analysis.

In the year 1816 Young's researches first became known to the French savans, on the occasion of a visit made by Arago and Gay-Lussac to England, when they sought the acquaintance of Dr. Young, already known to them by his general reputation, and found him at Worthing, whither he had gone for the summer, where a memorable conversation took place, the account of which we will give in Arago's words:

"In the year 1816 I visited England, accompanied by my scientific friend Gay-Lussac. Fresnel had then just entered on his career of science in the most brilliant manner by his Memoir on Diffraction. This work, which, in our opinion, contained a capital experiment irreconcilable with the Newtonian theory of light, became naturally the first subject of our conversation with Dr. Young. We were surprised at the numerous qualifications which he put upon our commendations of it; when at length he declared that the experiment which we thought so important was to be found published as long ago as 1807 in his lectures on Natural Philosophy. This assertion seemed to us questionable; and a long and minute discussion followed. Mrs. Young was present, but did not take any part in the conversation, as we thought from the fear of being supposed liable to the ridiculous sobriquet of bas-bleu, which makes English ladies reserved on such subjects in the presence of strangers. Our want of tact did not strike us until Mrs. Young abruptly quitted the room. We were beginning to apologise to her husband, when she returned with a large 4to volume under her It was the first volume of the Natural Philosophy. She placed it on the table, opened the book, without saying a word, at p. 787,*

arm.

*We must here notice a trifling inaccuracy into which Dr. Peacock has fallen. The reference given in the above passage to Young's Natural Philosophy is stated by Arago correctly, p. 787, as we have copied it. Dr. Peacock has, by some inadvertence, made it p. 387, and thus finds a difficulty in reconciling the narrative with the book. At p. 787 will, however, be found a description referring to a diagram in the plates (fig. 445), which entirely agrees with the description. (Life of Young, p. 389.)

and pointed to a diagram where the curvilinear course of the diffracted bands, which was the subject of the discussion, is theoretically established."*

In a word, the claims of Young were at once recognised by Arago; and by none more freely than by Fresnel himself; and between these two eminent fellow-labourers a correspondence and even intimacy was soon established. They constantly communicated their ideas as new points of investigation pressed on their notice, and each contributed, in some instances, to clear up the difficulties which presented themselves to the other. Yet the announcement of Fresnel's researches excited violent hostility among a considerable section of the savans of the Institute, consisting of devoted adherents to Laplace, who continued to uphold the theory of emission; and this great mathematician in particular used his utmost influence to discourage and suppress the new doctrine.

The fate of the undulatory theory on its first announcement was thus remarkably and similarly unfortunate, from different causes, in England and in France; and considering the very abstract nature of the subject, and how apparently remote it is from any thing which could be supposed to involve the interests or passions of men, it is not less singular to observe the bitterness and acrimony of the hostility which it had to encounter. The British reviewer and the French academician seemed to vie with each other in malignity; and the two great discoverers, Young and Fresnel, were both for a long time destined to the mortification of neglect and discouragement. The only cheering feature was certainly in favour of British science; the several papers of Young, successively communicated to the Royal Society, having been not only printed in its Transactions, but also in two instances selected as the Bakerian lectures; a compliment which the council has the power of bestowing annually, from a small bequest, on one of the papers communicated to the Society.

In 1809 Young published an elaborate refutation of Laplace's memoir on double refraction; showing that its laws could be much more satisfactorily explained on the wave system. This may be regarded as the most signal blow directed against the lingering credit of the molecular theory. It stood its ground among the French academicians longer than elsewhere, from the predominant influence of Laplace, who, with an obstinacy which formed part of his character, continued to the last to cling to the doctrine he had so long upheld, in the face of all the new facts and reasonings which were now enlightening the world of science. It has been well observed, that "simplicity is not always a

* Arago's Biog. Notice of Young, Euvres, tom. i. p. 292.

fruit of the first growth;" and accordingly some of the earliest of Young's researches were complicated by unnecessary conditions afterwards easily removed, but which in some instances continued long to embarrass the subject and furnish sources of objection to the wave hypothesis.

One such difficulty for a long time pressed upon the completeness of theory in regard to the explanation of the thin plates. According to the law of the thickness, it followed that at the point of actual contact at the centre the rays would be in accordance, and the centre ought to be a point of brightness; it is however, in fact, always black when the extreme limit is reached. Hence Dr. Young, and those who followed him for a long time afterwards, supposed that in this, as well as in some other cases which seemed analogous, we must suppose half an undulation to be by some means gained by one ray or lost by the other; and this seemed an arbitrary or empirical assumption, which the theory did not account for. We shall presently see how the difficulty was obviated.

Other cases involving the principle of interference were also investigated by Dr. Young; which are not without a practical bearing. Every one may have remarked the threads of a spider's web occasionally exhibiting brilliant colours in the sunshine. The same thing is seen in fine scratches on the surface of polished metal, and may be artificially produced in several ways. These colours Dr. Young showed were due to interference of the portions of light reflected from the sides of the groove, or narrow transparent thread. His attention was also particularly directed to the more complex tints produced when light was transmitted through a texture of fine threads, such as gauze, or even loose fibres of wool, cotton, silk, &c. Here a similar theoretical explanation was found to apply; and when the fibres were tolerably uniformly spread, a bright point of light seen through them appeared surrounded by a halo of coloured rings. The diameters of these rings varied with the fineness of the fibres; and were theoretically shown to depend on the interference of the portions of light passing through the transparent fibres and the interstices. He was able to show by theory the relation between the diameters of the rings and the thickness of the fibres; but the former could be easily measured by an appropriate apparatus of a very simple construction. Hence the diameters of those minute fibres could be immediately determined, and thus differences among them rendered appreciable which were quite imperceptible to the eye. He proposed this practical consideration as likely to be of importance to those interested in estimating the fineness of wool, cotton, &c. for commercial purposes. It does not appear, so far as we know, whether such a method has ever been found prac