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MAGNETISATION OF LIGHT.

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The other six needles were washed with dilute nitric acid, and arranged three on a card, in the same manner as the former. After having been exposed to sunshine for some hours, they were examined, and it was found that those needles which had been placed in the direction of the dip had acquired polarity; but no change could be detected in any of the others.

(499.) In 1845, Dr. Faraday communicated to the Royal Society a Memoir on the "Magnetisation of Light, &c.," and on the "Action of Magnets on Light." These subjects are intimately connected with our present consideration, and require some attention. Dr. Faraday's view will be best understood from the following note of explanation.

"Neither accepting nor rejecting the hypothesis of an ether, or the corpuscular, or any other view that may be entertained of the nature of Light; and, as far as I can see, nothing being really known of a ray of Light more than of a line of magnetic or electric force, or even of a line of gravitating force, except as it and they are manifest in and by substances; I believe that, in the experiments I describe in the paper, Light has been magnetically affected, i. e. that that which is magnetic in the forces of matter has been affected, and in turn has affected that which is truly magnetic in the force of Light: by the term magnetic, I include here either of the peculiar exertions of the power of a magnet, whether it be that which is manifest in the magnetic, or the diamagnetic class of bodies. The phrase 'illumination of the lines of magnetic force' has been understood to imply that I had rendered them luminous. This was not within my thought; I intended to express that the line of magnetic force was illuminated as the earth is illuminated by the sun, or, the spider's web is illuminated by the astronomer's lamp. Employing a ray of light, we can tell by the eye, the direction of the magnetic lines through a body; and by the alteration of the ray and its optical effect on the

eye, can see the course of the lines, just as we can see the course of a thread of glass, or any other transparent substance rendered visible by the light: and this was what I meant by illumination."

(500.) The order of the experiments will be understood from the following quotation:-"A ray of light issuing from an Argand lamp was polarised in a horizontal flame by reflection from a surface of glass, and the polarised ray was passed through a Nichol's eye-piece revolving on a horizontal axis, so as to be easily examined by the latter. Between the polarising mirror and the eye-piece, two powerful electro-magnetic poles were arranged, being either the poles of a horse-shoe magnet, or the contrary poles of two cylinder magnets; they were separated from each other about two inches in the direction of the line of the ray, and so placed, that, if on the same side of the polarised ray, it might pass near them, or, if on contrary sides, it might go between them, its direction being always parallel, or nearly so, to the magnetic lines of force. After that, any transparent substance placed between the two poles, would have passing through it, both the polarised ray, and the magnetic lines of force, at the same time, and in the same direction.

(501.) "Sixteen years ago, I published certain experiments made upon optical glass, and described the formation and general characters of one variety of heavy glass, which, from its materials, was called silicated borate of lead. It was this glass which first gave me the discovery of the relation between Light and Magnetism, and it has power to illustrate it in a degree beyond that of any other body; for the sake of perspicuity, I will first describe the phenomena as presented by this substance.

(502.) "A piece of this glass, about 2 inches square, and 0.5 of an inch thick, having flat and polished edges, was placed as a diamagnetic between the poles (not as yet magnetised by the electric current) so that the polarised ray should pass through its length; the glass acted as

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air, water, or any other indifferent substance would do; and if the eye-piece were previously turned into such a position that the polarised ray was extinguished, or rather the image produced by it rendered invisible, then the introduction of this glass made no alteration in that respect. In this state of circumstances the force of the electro-magnet was developed by sending an electric current through its coils, and immediately the image of the lamp-flame became visible, and continued so as long as the arrangement continued magnetic. On stopping the electric current, and so causing the magnetic force to cease, the light instantly disappeared; these phenomena could be renewed at pleasure at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.

(503.) "The character of the force thus impressed upon the diamagnetic is that of rotation; for when the image of the lamp-flame has been thus rendered visible, revolution of the eye-piece to the right or left, more or less, will cause its extinction; and the further motion of the eyepiece to the one side or the other of this position will produce the reappearance of the light-and that with complementary tints, according as this further motion is to the right or left hand.

(504.) "When the pole nearest to the observer was a marked pole, i. e. the same as the north end of a magnetic needle, and the further pole was unmarked, the rotation of the ray was right-handed; for the eye-piece had to be turned to the right hand, or clock fashion, to overtake the ray, and restore the image to its first condition. When the poles were reversed, which was instantly done by changing the direction of the electric current, the rotation was changed also, and became left-handed, the alteration. being to an equal degree in extent as before. The direction was always the same for the same line of magnetic force."

(505.) A great number of substances, solid and fluid,

were employed. The polarised ray being passed through them while they were brought under the influence of powerful electro-magnetic force. Careful examination of all the results which Dr. Faraday obtained, go to prove, that some transparent medium is always necessary, and that, in the greater number of examples, the degree to which the line of light was affected by the magnet was exactly in proportion to the density of the medium. It would, therefore, appear, that the ray of light is not actually magnetised. The molecular arrangement of the medium through which the ray passes is altered, and consequently its refracting powers changed, and thus the effect on light is but secondary. We employ the light, indeed, to enable us to see the amount of change which takes place in the molecules of the transparent body. These results, which are of the most important character, and which serve to guide us in most difficult investigations, do not, therefore, appear to bring the relations of Light and Magnetism much nearer than they were previously to this investigation.

CHAPTER XI.

ANALYSIS OF THE SPECTRUM BY ABSORBENT MEDIA.

(506.) THE considerations to which this chapter is devoted are of the utmost importance, in both a philosophical and physical point of view. The inquiry is still necessarily incomplete, but two reports having been published by the British Association, it is thought advisable to include some notice of the experiments on which these are founded in the present volume.

The object in view is to determine, with all the accuracy possible, the relation which each coloured ray of the prismatic spectrum bears to the chemical action which takes place upon the different agents employed in the production of the sensitive surface. Since different media exhibit very various degrees of absorbent action upon the chromatic rays, as well as on the chemical rays, of the spectrum, by employing them we obtain indications by which we may determine the relation in which these phenomena stand to each other.

(507.) The plan upon which I am proceeding is this. Having obtained a very extensive series of coloured glasses, and, by the solution of chemical compounds, procured a still more varied set of transparent coloured solutions, I analyse the luminous spectrum of a well-formed vertical opening between two knife-edges, by passing the spectrum through a particular absorbent medium. The spectra are obtained, first, by means of an excellent flintglass prism; again, by one of crown-glass of faultless purity, the manufacture of Messrs. Chance Brothers, of Birmingham; and, thirdly, by a hollow prism, in which I

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