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Two important investigations of Arago, however, remain to be mentioned. At the date of his chief optical researches the spirit of controversy between the undulatory and the molecular theories was still kept up. But at present, though much objection still remains on the minds of some persons to the undulatory theory, it is not associated with the slightest attempt to revive the credit of the molecular; which, like the system of Ptolemy, has died a natural death, sinking under the oppression of the vast and daily increasing complexity of subsidiary hypotheses which were necessary to adapt it to new phenomena. In Arago's writings, however, we trace everywhere an anxiety to seize opportunities of contrasting the rival theories, and to institute experiments for the purpose of testing their relative capabilities to account for phenomena; and these must still be regarded with great interest.

One such investigation referred to the question, whether a difference of the velocity of the incident light would produce any difference in the amount of refraction. Now on the emission theory, there would be less refraction in proportion as the original velocity might be greater. On the undulatory theory, it is remarkable that neither Fresnel nor Arago, in the first instance, seemed altogether prepared to say what the result ought to be. Though from some expressions of Arago*, it would seem as if he considered that the same result ought to occur. The experiment consisted in observing the refraction of the light of a star situated in that part of the heavens towards which the earth was at the time moving, compared with that of one situated at the opposite part, or from which the earth was receding. The velocity of the light being in the one case increased, in the other diminished, by the whole of the earth's velocity, which (as is well known from the fact of aberration) bears a sensible proportion to that of light.

The apparatus was sufficiently delicate to exhibit differences of the most minute amount; and consisted of a prism, attached to the object end of a telescope furnished with a micrometer. The result of numerous observations was, that no difference whatever could be detected in the refraction, in the two cases. This result was afterwards shown to accord with the wave theory, but as Arago observed, it could be reconciled with the molecular view only by inventing the complicated additional hypothesis, that an infinity of rays of all velocities are sent forth by luminous bodies, but only those of a certain velocity are capable of affecting our eyes. This, however, would only be

See Biographical Notice of Fresnel, tom. i. p. 155.


to add cycle and epicycle,' to save appearances,' and afforded no real solution.

But somewhat connected with this last experiment was another, also proposed by Arago, and the history of which has a sort of melancholy interest attached to it, as one of those conceptions which the infirmities of his later years compelled him to leave to others to put in execution.

We cannot describe this better or more briefly than in the words of Humboldt:


M. Arago had already, as I mentioned above, discovered, in 1818, the remarkable effect produced in the phenomena of interference by a very thin plate, placed in the path of one of the two interfering rays. There is then displacement of the fringes, and retardation of the light, which moves more slowly through a denser substance.

'When Mr. Wheatstone had succeeded in his beautiful researches on the velocity of electrical light (1835), in availing himself of his ingenious rotatory apparatus, M. Arago gained a glimpse at once of the possibility of measuring, by angular deviations, and by applying the same principle of rotation, the difference in the velocity of light in liquid and in the air. He gave an account to the Institute, towards the end of 1838, of the experiment which he proposed to make. Aided by an experienced and clever artist, M. Breguet, Junr., he succeeded, after changes in the apparatus, in realising his project. In the course of these attempts M. Breguet had been able, by disencumbering the axis of the weight of the mirror which it supported, to make the axis turn eight thousand times in a second. All was ready in 1850, and the perfected apparatus could be put in action; but the great and sad alteration which the eyesight of M. Arago had experienced almost suddenly, left him no longer the hope. of being able to take part in the observations. He says, with a noble simplicity, in a note presented to the Institute, the 29th of April, 1850, "My pretensions must confine themselves to having laid "down the problem, and to having indicated (by publishing them), "certain means for solving it. I can do no more, in the present state "of my sight, than accompany with my good wishes those experi"menters who will follow my ideas, and add a new proof in favour of "the system of waves, to the proofs which I deduced from a pheno"menon of interference, too well known by physicists to be necessary "to repeat here." M. Arago was able to see his wishes favourably carried out. Two experimenters, equally distinguished by their talent and by the delicacy of their modes of observation, M. Foucault, to whom we owe the physical demonstration of the rotation of the earth by means of the pendulum, and M. Fizeau, who determined by an ingenious method the velocity of light in the atmosphere, have succeeded in adding some improvements to the means proposed by M. Arago, to resolve the question, and this on the side subversive MM. Foucault and Fizeau presented of the system of emission. the results of their labours to the Academy of Sciences, the first in May, 1850, the second in September, 1851. (Introduction, p. xi.)

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Our remarks thus far have arisen strictly out of the attempt to elucidate the discoveries of Arago. Though we have now arrived at the terminating point of his labours, yet it is by no means here that their influence on the progress of the science will be found to cease;-and it is no more than a strict adherence to what our proposed object demands, to close these remarks (though already, we fear, too extended for the patience of many of our readers), by a few words, as to the general condition and prospects of the theory of Light.

The dynamical views of Fresnel were wanting in systematic completeness: this has been supplied, and new generalisations given to the theory in the researches of M. Cauchy (1830), founded on an extension of the original equations of Euler and D'Alembert, giving the complete dynamical exposition of transverse vibrations, and at length leading to the great desideratum, the explanation of unequal refrangibility, the want of which had hitherto been the reproach of all theories. These researches were extended and simplified by English mathematicians-Airy, Kelland, Lubbock, Lloyd, O'Brien, Green, Hamilton, Tovey, Challis and Stokes; and the precise application to the question of unequal refrangibility, fully evinced by numerical accordance with a limited range of results of high accuracy obtained by Fraunhofer, was carried on through a wider range of refractive indices determined from his own observations, by Professor Powell, and while the accordance was sufficient for all media except two or three, those few doubtless constituted the most important tests: in those instances the theory remains incomplete. An empirical change in the formula satisfies them; but it is not yet shown whether theory can justify such a change.

The formulas of Fresnel for reflected light are essentially dependent on the angle of refraction. Hence when applied to crystals in which there is an extraordinary as well as an ordinary refraction, and still more to those in which both rays. are in fact extraordinary rays,-the Biaxial class, we cannot strictly apply the formulas since there are the two refractions to be taken into account. This difficult and intricate question can hardly be considered as even yet completely investigated, but researches of a very extensive kind have been made towards its solution by the late Professor Maccullagh and by M. Neumann of Königsberg.

We cannot close this rapid enumeration with all its imperfections and omissions, without mentioning the latest and most remarkable optical discovery of the present day. Newton laid it down as a sort of axiom, that' to the same ray ever belongs 'the same refrangibility.' On the molecular theory, some such

property might be conceived inherent; but why so on the undulatory? Professor Stokes, however, has shown it not to be the fact. Certain media, indeed very many, possess a property of changing the nature of a simple primary ray of light passing into them, by a peculiar superficial action. They abstract some ray from a compound beam, which is found wanting when the residue is analysed; they emit it having a less refrangibility, a greater wave length, and a colour lower in the prismatic scale. This property is termed Fluorescence,' being found in fluorspar, canary glass, solution of quinine, &c. fessor Stokes has also pointed out, both theoretically and experimentally, a remarkable change in the direction of polarisation produced by diffraction, which leads directly to the settlement of the long-disputed question whether the plane of the vibrations is parallel or perpendicular to that of polarisation, in favour of the latter view.


The phenomena of light have received explanations from theory in a degree by no means proportioned to their simplicity of character or familiarity of occurrence. In fact some of the most abstruse and complex appearances are precisely those most easily explained, while some of the most common and simple are just the cases where the theory is most at fault. Thus the familiar fact of the varied and apparently capricious absorptions of certain rays and portions of the spectrum exercised by all coloured transparent bodies, and especially by certain gases and vapours, notwithstanding the very elaborate investigations of Lloyd, Van Wrede, and Power, can hardly be said to be explained to the satisfaction of all who have inquired into the subject. And, again, the simple occurrence of innumerable dark lines in the prismatic spectrum has remained without even any attempt at theoretical explanation. No candid inquirer would wish to shut his eyes to the difficulties of the theory; nor can we close a sketch of its triumphs without a passing reference to alleged failures and objections.

We have already adverted to the difficulty relative to dispersion. Some objections have been alleged relative to certain cases of diffraction: another has been recently adduced by Professor Potter, from the results of experiment and calculation, by which he maintains that the explanation of the interior secondary bands of colour seen in very bright rainbows cannot be accounted for by interference, as was originally suggested by Young, and since upheld by others. To these, other instances might perhaps be added. Thus, while it is freely admitted that there are many outstanding classes of facts, to which the undulatory explanation has not yet been applied, and some which even still appear at variance with the deduc

tions from its principles, yet it is eminently worthy of remark that of these points no explanation whatever has been attempted on any other principles. The undulatory theory, defective as it may be, has now no rival. It is not the best but the only theory we have. Not one of its able assailants has maintained, or even imagined, any opposite theory to apply to the same range of subjects. None of the opponents of waves are now advocates of emission. We must now regard as absurd and exploded the party designations of undulationists and emissionists, or of those neutrals, whom Biot designated as 'Rienists.' The undulatory doctrine has successfully maintained at least a very extended and daily enlarging empire. There are still some small rebellious districts, but its past successes warrant the expectation that these will sooner or later be reduced to obedience. There are also large regions explored, but not under its dominion: yet these are, at least, not held by hostile powers: they are simply unoccupied tracts, waiting to be taken possession of, and colonised.

ART. II. —1. Festus: a Poem. By PHILIP JAMES BAILEY. Fifth Edition. 1854.

2. The Angel World, and other Poems. By PHILIP JAMES BAILEY. 1850.



4. The Roman: a Dramatic Poem. By SYDNEY YENDYS. 1850.

5. Balder. Part the First. By the Author of The Roman. Second Edition. 1854.

6. England in Time of War. By SYDNEY DOBELL, Author of Balder,' &c. 1856.

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7. Sonnets on the War.


Author of Balder' and 'The Roman.' 1855.

8. Poems. By ALEXANDER SMITH. Fourth Edition. 1856. 9. The Music Master, a Love Story; and Two Series of Day and Night Songs. By WILLIAM ALLINGHAM. 1855.

10. Poems. By GEORGE MEREDITH. 1851.

11. Clytemnestra, The Earl's Return, The Artist, and other Poems. By OWEN MEREDITH. 1855.

12. Poems. By MATTHEW ARNOLD. Second Edition. 1854 and 1855.

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