with chalk and a piece of lime-tree wood, so as to form on it shining zones, about two or three lines in length. The steel, thus prepared, being exposed in a place strongly enlightened by the sun, was found, after some time, to become strongly magnetic in the way that has been described. The time, other things being equal, seemed to depend on the intensity of the solar light; for when I concentrated the light, by means of a lens, on the polished parts, I obtained as strong a magnetism in a few minutes, as was produced by an exposure of several hours to the light in its natural state.

A piece of steel, merely polished at one of its extremities, receives a north pole at that end, and a south pole at the other. If the polished part is in the middle, the two extremities become south poles, and the middle a north pole. If the wire, on the contrary, is polished at the two ends, they become north poles, while the middle is a south pole. Lastly, if there are several polished zones, each of them receives the boreal magnetism, and the dark zones which separate them, the austral. In this manner we may obtain an unlimited number of magnetic poles, provided that the steel wire be of a length proportional to it. I have thus easily procured eight poles, in a wire eight inches long, although of unequal force: for I have constantly found that the poles at the ends were stronger than the rest, and that they preserved their magnetism for a longer time. I have not been able to magnetize by the same means wires, either entirely covered with oxyd, or perfectly polished, nor other wires with longitudinal polished stripes.

The whole of these results were the same, whatever might be the position of the wire with respect to the meridian; and each experiment was repeated several times without any variation in its success. It is unnecessary to add, that before each wire was exposed to the sun's rays, I carefully examined whether or no it was magnetic, and those wires which possessed any magnetic properties were rejected as unfit for the experiment.

iv. Elementary view of the UNDULATORY Theory of LIGHT. By Mr. FRESNEL. From the Supplement to the Translation of Thomson's Chemistry. 8. Par. 1822.

[The Council of the Royal Society having adjudged, on the 8th of February, Count Rumford's PRIZE MEDAL to Mr. FRESNEL, for his application of the undulatory theory of light to the phenomena of polarisation, and this decision having probably tended to give new weight to that theory in the estimation of the British public; it is apprehended, that a translation of Mr. FRESNE's very distinct and wellarranged statement, of the grounds of that theory, will not be judged an improper article fcr insertion in the Nautical and Astronomical Collections.]


The Nature of Light has leng been a subject of dispute among natural philosophers: some of them suppose that it is a material substance, dartel out by luminous bodies; and others, that it consists in the vibrations of an infinitely subtile elastic fluid diffused through all space, in the same way that sound is known to be a vibration of the air. The system of undulations, which was contrived by the genius of DESCARTES, and which was more ably completed in its detail by HuyGENS, has also been adopted by EULER, and in more modern times by Dr. THOMAS YOUNG, to whom the science of optics is indebted for many important discoveries. The system of emission, or that of NEWTON, supported by the great name of its author, and I might arost say, by that reputation for infallibility which his immortal PRINCIPIA had earned for him, has consequently been nore generally adopted. The other hypothesis was apparently altogether abandoned, when Dr. YOUNG recalled the attention of philosophers to it by some curious experiments, whch afford a striking confirmation of it, and which appear a the same time to be scarcely reconcilable with the system ofemission.

A variety of new phenomena, observed since that time, have continued to add to the probability of the truth of the theory of undulations. Though long neglected, and more difficult to be followed, in the detail of its applica

tions, than the hypothesis of emission, it has already enabled us to adapt our calculations much more correctly to the phenomena; and this is one of the least equivocal proofs of the truth of any system. When a hypothesis is true, it must lead us to numerical relations between facts the most different in their appearances: while a false hypothesis, on the contrary, may represent very accurately the class of phenomena from which it has been deduced, in the same way that an empirical formula may represent a limited number of results that lave afforded it; but it can never trace the secret relations which connect these phenomena with others of a totally diferent class.

It was in this manner, for example, that Mr. BIOT investigated, with as much sagacity as perseverance, the laws of the beautiful phenomena of colours which Mr. ARAGO had discovered in crystallized plates, and found that the tints exhibited by them followed, with regard to their thicknesses, laws similar to those of the coloured rings already known; that is to say, that the thicknesses of two crystallized plates of the same nature, which exhibited any given tints, were in the same proportion as two plates of air, which afforded similar tints in the production of coloured rings. This relation, indicated by analogy, without any regard to a particular theory, was without doubt very remarkable and very important; but Dr. YOUNG advanced much further, by means of the law of interferences, which is an immediate consequence of the system of undulations. He discovered a much more intimate relation between these two classes of phenomena; that is, that the difference of the lengths of the paths of the ays ordinarily and extraordinarily refracted, in the crystallized plate, is precisely equal to the difference of the paths described by the rays reflected at the first and second surface of the plate of air that exhibits the same tint as the crystal: and the phenomena, instead of a simple analogy, re reduced to the predicament of identity.

I might add that the laws, so complicated in appearance, of the phenomena of diffracion, which had escaped all attempts to detect them with the assistance of experiments, as combined with the theory of emission, are perfectly con

sistent, in all their extent, with the simplest principles of the theory of undulations. Without doubt observation has assisted also in the discovery of this relation; but observation alone could never have developed them; while in this case, as well as in many others, the theory of undulations might easily have gone before the experiments, aud have predicted beforehand the precise results in their minutest details.

The example, which has already been mentioned, sufficiently proves that the choice of a theory is not a matter of indifference in the investigation of physical phenomena. Its utility is not confined to the purpose of facilitating the study of facts, by uniting them into groups more or less numerous, according to the relation which they bear to each other. Another no less important end of a good theory must be to contribute to the essential advancement of science, by the discovery of facts, and of relations between classes of phenomena the most distinct, and in appearance the most independent of each other. Now it is obvious that if we set out with an imaginary hypothesis respecting the cause of light, we shall not reach this end so readily, as if we possessed the true secret of nature. A theory depending upon a true fundamental hypothesis, however ill it may be suited for the application of mathematical analysis to the mechanical operations which it involves, will still lead us to intimate relations between distant facts, which would for ever have remained unknown upon any other suppositions. Hence, to say nothing of our natural curiosity to know the truth in all cases, we see how important it is to the progress of optics, and of the kindred science of chemistry, to know whether luminous molecules are actually projected from the sources of light and enter our eyes, or if light is propagated by the undulations of a continuous medium, to the particles of which, the luminous bodies communicate their vibratory motions. And we have no reason to believe that the decision of this question is impossible, because it has long remained unaccomplished: we may even venture to assert that, as far as probability goes, it is already determined; and that, after comparing attentively the application of the

JAN. MARCH, 1827.


two systems to all the facts which have been hitherto discovered, an impartial judge will not hesitate to admit the superiority of the theory of undulations.

In entering into the detail of the facts in question, it will not be advisable to separate them from the theoretical views which have led so immediately to their discovery; and it appears to be equally conducive to the purposes of instruction, and to the advancement of the science, to explain the essential principles and foundations of a theory which has been too long neglected and misunderstood: it will, however, be proper on this occasion to omit many details of calculation, for the sake of conciseness, and having reduced each case to the condition of a mathematical problem, it will be sufficient to take its solution for granted.

The diffraction of light will be considered in the first place, because it relates to the simplest possible case of a shadow cast by an opaque body illuminated by a single radiant point; and this case will be considered somewhat diffusely, as affording the best test of the comparative value of the two theories.


The term Diffraction is applied to those modifications which light undergoes in passing near the extremities of bodies.

When we admit the rays of light by a very small aperture into a dark room, we observe that the shadows of bodies, instead of being terminated abruptly and distinctly as they ought to be, if the light always passed by them in right lines, are softened in their outlines, and bounded by three very distinct fringes of colours, of unequal breadths, the first being the widest, and the third the narrowest; and when the body intercepting the light is narrow, we see fringes even in its shadow, which then appears to be divided by darker and lighter stripes, placed at equal distances from each other. We may call this latter species internal fringes, and the former external.

Grimaldi is the first philosopher that observed and studied these fringes with care. Newton, who investigated the subject of diffraction, and even devoted to it the last

« VorigeDoorgaan »