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CONTENTS.

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1853. On'the Change of Eefrangibility of Light.—II 1

1852. On the Optical Properties of a recently discovered Salt of Quinine 18

1853. On the Change of Eefrangibility of Light and the exhibition thereby of

the Chemical Eays 22

1853. On the Cause of the Occurrence of Abnormal Figures in Photographic

Impressions of Polarized Eings 30

1853. On the Metallic Eeflexion exhibited by certain Non-Metallic Substances 38

1854. Extracts from Letter, to Dr W. Haidinger: on the Direction of the

Vibrations in Polarized Light: on Shadow Patterns and the

Chromatic Aberration of the Eye: on Haidinger's Brushes . . 50

1854. On the Theory of the Electric Telegraph. By Prof. W. Thomson.

(Extract) 61

1855. On the Achromatism of a Double Object-glass . . . . . 63

1856. Eemarks on Professor Challis's paper, entitled "A Theory of the

Composition of Colours, etc." . . . . . . . 65

1856. Supplement to the "Account of Pendulum Experiments undertaken

in the Harton Colliery...". By G. B. Airy, Esq., Astronomer Eoyal 70

1857. On the Polarization of Diffracted Light 74

1857. On the Discontinuity of Arbitrary Constants which appear in Divergent

Developments 77

1857. On the Effect of Wind on the Intensity of Sound .... 110

1859. On the Existence of a Second Crystallizable Fluorescent Substance

(Paviin) in the Bark of the Horse-Chestnut 112

1859. On the bearing of the Phenomena of Diffraction on the Direction of

the Vibrations of Polarized Light, with Eemarks on the Paper of

Professor F. Eisenlohr 117

1860. Note on Paviin 119

1860. On the Colouring Matters of Madder. By Dr E. Schunck. (Extract) 122

1860. Extracts relating to the Early History of Spectrum Analysis . . 127

1861. Note on Internal Eadiation 137

1862. On the Intensity of the Light reflected from or transmitted through

a Pile of Plates . . . .145

1862. Eeport on Double Eefraction 157

1862. On the Long Spectrum of Electric Light 203

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1863. On the Change of Form assumed by Wrought Iron and other Metals

when heated and then cooled by partial Immersion in Water. By

Lieut.-Col. H. Clark, B.A., F.B.S. Note appended by Prof. Stokes. 234

1864. On the supposed Identity of Biliverdin with Chlorophyll, with remarks

on the Constitution of Chlorophyll 236

1864. On the Discrimination of Organic Bodies by their Optical Properties . 238

1864. On the Application of the Optical Properties of Bodies to the Detection

and Discrimination of Organic Substances 249

1864. On the Beduction and Oxidation of the Colouring Matter of the Blood 264

1867. On a Property of Curves which fulfil the condition -=-^ + ;r^ = 0- By

W. J. Macquorn Bankine. (Supplement) 276

1867. On the Internal Distribution of Matter which shall produce a given

Potential at the surface of a Gravitating Mass 277

1868. Supplement to a paper on the Discontinuity of Arbitrary Constants

which appear in Divergent Developments 283

1868. On the Communication of Vibration from a Vibrating Body to

a surrounding Gas 299

1868. Account of Observations of the Total Eclipse of the Sun....By J. Pope

Hennessy. Note added by Prof. Stokes 325

1869. On a certain Beaction of Quinine . . . . . . . 327 .

1872. Explanation of a Dynamical Paradox 334

1872. On the Law of Extraordinary Befraction in Iceland Spar . . 336

1873. Sur l'emploi du prisme dans la verification de la loi de la double

refraction 337

1871. Notice of the Besearches of the late Bev. W. Vernon Harcourt on the

Conditions of Transparency in Glass and the Connexion between the

Chemical Constitution and Optical Properties of Different Glasses . 339

1873. On the Principles of the Chemical Correction of Object-glasses . 344

1874. On the Improvement of the Spectroscope. By Thomas Grubb, F.B.S.

Note appended by Prof. Stokes . 355

1874. On the Construction of a perfectly Achromatic Telescope . . 356

1875. On the Optical Properties of a Titano-Silicic Glass .... 358

1876. On a Phenomenon of Metallic Beflection . . . . . .361

1876. Preliminary Note on the Compound Nature of the Line-Spectra of

Elementary Bodies. By J. N. Lockyer, F.B.S. (Extract) . . 365

Appendix (Correspondence of Prof. G. G. Stokes and Prof. W. Thomson

on the nature and possibilities of Spectrum Analysis) . . . 367

Index 377

MATHEMATICAL AND PHYSICAL PAPERS.

On The Change Of Refrangibility Of Light.—No. IL

[From the Philosophical Transactions for 1853, pp. 385—396.
Received June 16, read June 26, 1853.]

The chief object of the present communication is to describe a mode of observation, which occurred to me after the publication of my former paper, which is so convenient, and at the same time so delicate, as to supersede for many purposes methods requiring the use of sun-light. On account of the easiness of the new method, the cheapness of the small quantity of apparatus required, and above all, on account of its rendering the observer independent of the state of the weather, it might be immediately employed by chemists in discriminating between different substances.

I have taken the present opportunity of mentioning some other matters connected with the subject of these researches. The articles are numbered in continuation of those of the former paper *.

Method of observing by the use of Absorbing Media.

241. Conceive that we had the power of producing at will media which should be perfectly opaque with regard to rays belonging to any desired regions of the spectrum, from the extreme red to the most refrangible invisible rays, and perfectly transparent with regard to the remainder. Imagine two such media prepared, of which the second was opaque with regard to those rays of the visible spectrum with regard to which the first was transparent, and vice versa. It is clear that if both media were held in front of the eye no light would be perceived. The same would still be the case if the first medium were removed from the eye, and placed so as to intercept all the rays which fell on certain objects, which were then viewed through the second, provided the objects did nothing more than reflect, refract, scatter, or absorb the incident rays. But if any of the objects had the property of emitting rays of one refrangibility under the influence of rays of another, it might happen that some of the rays so emitted were capable of passing through the second medium, in which case the object would appear luminous in a dark field.

* [Ante, Vol. in. p. 267.] S. IV. 1

242. Let us consider now how the media must be arranged so as to bring out to the utmost the sensibility of a given substance. To take a particular instance, suppose the substance to be glass coloured by uranium. In this case the sensibility of the medium begins, with almost absolute abruptness, near the fixed line b of Fraunhofer, and continues from thence onwards. The dispersed light has the same, or at least almost rigorously the same, composition throughout, and consists exclusively of rays less refrangible than b. Consequently, we should have to prepare a first medium which was opaque with regard to the visible rays less refrangible than b, and transparent with respect to the rays, whether visible or invisible, more refrangible, and a second medium complementary to the former in the manner described in the preceding article. If the pair of media were still strictly complementary in this manner, but the point of the spectrum at which the transparency of the first medium began and that of the second ended were situated at some distance from b, the sensibility of the glass would be exhibited as before, only the maximum effect would not be produced, on account of the absorption of a portion either of the active or of the dispersed rays, according as the point in question was situated above or below b.

Now, although the commencement of the sensibility of canary glass is unusually abrupt, it generally happens that the sensibility of a medium, or at least the main part of it, comes on with great rapidity, and lasts throughout the rest of the spectrum, though frequently it is most considerable in a region extending not very greatly beyond the point where it commenced. In those cases in which the dispersion of different tints commenced at two or three different places in the spectrum, I have almost always had evidence of the independent presence of different sensitive principles, to which the observed effects were respectively due.

Hence, if we could prepare absorbing media at pleasure, we should get ready for general use in these observations a few pairs of media complementary in the particular manner already described, but having the points of the spectrum at which the transparency of the first medium commenced and that of the second ended different in different pairs, situated say in the yellow for one pair, in the blue for another, in the extreme violet for a third.

243. It is not of course possible to prepare media in this manner at pleasure, and all we can do is to select from among those which occur in nature. Nevertheless it is useful, as a guide in the selection, to consider what constitutes the ideal perfection of absorbing media for this particular purpose. But before proceeding to mention the media which I have found convenient, I will describe the arrangement which I have adopted for admitting the light.

A hole was cut in the window-shutter of a darkened room, and through this the light1 of the clouds and external objects entered in all directions. The diameter of the hole was four inches, and it might perhaps have been still larger with advantage. A small shelf, blackened on the top, which could be screwed on to the shutter immediately underneath the hole, served to support the objects to be examined, as well as the first absorbing medium. This, with a few coloured glasses, forms all the apparatus which it is absolutely necessary to employ, though for the sake of some experiments it is well to be provided also with a small tablet of white porcelain, and an ordinary prism, and likewise with one or two vessels for holding fluids.

244. In the observation, the first medium is placed resting on the shelf so as to cover the hole; the object is placed on the shelf immediately in front of the hole; the second medium is held anywhere between the eyes and the object. As it is not possible to obtain media which are strictly complementary, it will happen that a certain quantity of light is capable of passing through both media. This might no doubt be greatly reduced by increasing the absorbing power of the media, but it is by no means advisable to do so to any great extent, because it is important that the second medium should transmit as many as possible of the rays which are of such refrangibilities as to be stopped by the first.

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