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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 have the means of employing fluids of very different refracting powers. For obtaining the chemical impression of the spectrum, I procure a flame-like chromatic image of great intensity, 1 inch in length, from a vertical opening in my steel plate. I have adopted this as my measure throughout, dividing it into 100 equal parts: thus, all the numbers employed are intended to express inches, or the one-hundredth part of an inch.

(508.) The photographic agents employed were the collodio-iodide of silver on glass plates, and the iodide of silver excited by gallic acid. All the first results were on the former preparation.

The collodion was made with gun-cotton which had been prepared with nitrate of potash and sulphuric acid. This being well washed, was dissolved in ether. Iodide of potassium was dissolved in spirits of wine iodide of silver added so long as it would take up any, and two drachms of this solution mixed with one fluidounce of the collodion. The solution of silver employed was thirty grains to the fluidounce of distilled water. The image was always developed by pyrogallic acid.

(509.) The NORMAL SPECTRUM was formed by a very pure flint-glass prism. – Light admitted between two knife-edges, separated fth of an inch, and generally passed through a hole of the same diameter in an inner screen. The chromatic image was received on a white tablet in a perfectly black box ; its length, when most accurately adjusted, was 1 inch and ths, but for convenience this has been reduced to 1 inch and divided into 100 parts, and relatively to this all the chemical spectra have been corrected (1:2).

Without any interposed medium. - Chemical action commences •40 above the lower end of red, and from this point extends to the length of 11 inch. Over the space covered by the red and orange rays are indications of a well-defined circle of protective action; immediately above this a dusky brown commences, forming a kind

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