tion of science, of education, of the public health, and of rational progress in whatever concerns the good of the community.

Dr. Young contributed to the Astronomical and Nautical Collections in Brande's Quarterly Journal several important articles on Refraction. He had observed that the series employed for expressing the refraction in terms of the density failed at the horizon, because the sine of the altitude was a divisor of the co-efficients, and it occurred to him that this inconvenience might be avoided by expressing the density in a series in terms of the refraction. The series which was thus obtained, though not always very convergent in extreme cases, is convenient for obtaining a tolerably accurate result from any proposed theory of the law which governs the relations of the height, temperature, and density of the atmosphere, being capable of determining the refraction by the aid of a small number of its terms. He was accustomed to consider this principle of reversing the ordinary form of exhibiting the series for refraction as one of the happiest ideas that had ever occurred to him in a physical investigation.

These papers gave rise to a very acrimonious controversy between himself and Mr. Ivory, who was engaged about the same time in the same class of researches. Mr. Ivory, whose temper was somewhat morose and jealous, was a worshipper of La Place and the French School of analysts, to whose methods he was the first to give a very general currency in this country; and he was too apt-as in his Memoir on Cohesion of Fluids, to which we have before referred-to make the physical principles on which his investigations were founded in some degree subordinate to the analytical

elegance and completeness of the methods which he employed. As an analist however, and in the direct applications of analysis to physical questions, he had few superiors. With such prepossessions, it was not surprising that he should have failed to recognize, in this as well as in other instances, the importance of Dr. Young's conclusions, in connection with the inelegance and apparent insufficiency of his methods, and should have been disposed to dispute the correctness of the assumptions upon which his series were founded; to doubt their convergency, at least for very low altitudes, and to maintain generally that such series were only resorted to in the infancy of analytical science. Dr. Young replied to these imputations by showing that his method was applicable not only to every hypothesis respecting the distribution of heat in the atmosphere to which the methods of La Place, Bessel and Ivory were applicable, but also to others which they did not venture to investigate, in consequence of considering them beyond the powers of the analysis which those methods required.

Formulæ or series for refraction, though founded upon very different views of the constitution of the atmosphere, may give-when their constants are sufficiently determined by observation-results which are nearly coincident for considerable altitudes and within moderate limits of temperature: it is only when we approach the horizon, or for extreme heat or extreme cold, that the considerable discrepancies begin to appear. Thus, whilst the formula which Dr. Brinkley proposed gives results, for extreme cold, even near the horizon, which have been found, in some remarkable observations, to be nearly correct, those given Works, vol. ii. p. 39.

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by that of Dr. Young, under similar circumstances, are much too small. Other sources of discrepancy have their origin in the different methods which are adopted for ascertaining the temperature. Are we to trust to the indications of the thermometer within or without the observatory, or are we to make use of both? Dr. Young would trust to the first," Dr. Brinkley to the second, and Bessel would take account of both of them; whilst the difficulty in a well constructed or well regulated observatory would be evaded by making the difference between them as much as possible disappear. It is in the searching discussion of questions, like these, apparently so trivial and so unimportant, that those minute quantities are involved, the detection and accurate determination of which will form the basis of the most important results of the astronomy of the future; of the values in fact of the standards by which we shall be able to measure the dimensions of the remotest portions of the universe which are penetrable by our telescopes.

CHAPTER XII.

OPTICAL DISCOVERIES.-SECOND EPOCH.

In the interval of twelve years which elapsed between the publication of his Reply to the Edinburgh Review and the appearance of Fresnel's first Memoir on Diffraction in 1816, the name of Dr. Young was ostensibly connected with no important experimental or theoretical optical investigations. In fact, his previous labours upon the subject seemed to have been absolutely forgotten, and it would be difficult to point out a single allusion made to them in any optical work or memoir published during that period, either at home or abroad. In the intermediate period La Place had published his celebrated Memoir on the double refraction of Iceland spar: Malus had discovered the polarization of light by reflection, and was engaged in a brilliant series of researches connecting his discovery with the optical properties of crystalline bodies, when a premature death brought his labours to a close: Brewster was enriching every department of experimental optics with the most remarkable speculations and discoveries: Arago had found out the colours of crystalline plates produced by polarized light, and though less fertile than some of his contemporaries in the number of his contributions to the science, he was second to none of them in the critical sagacity with which he analysed their labours Biot was combining theoretical and practical

LIFE.

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researches with a success and ingenuity which seemed to promise him the first place amongst optical discoverers, when it was his misfortune to waste his energies and compromise his reputation in the proposition and obstinate maintenance of his theory of moveable polarization: at a later period, the labours of Fresnel,— who-though treading generally in the footsteps of Young, required no foreign aid either to guide or support him, was destined to give unity and system to the vast mass of facts and theories which his predecessors had accumulated and prepared. "Of the splendid constellation of great names just enumerated,” writes Sir John Herschel shortly after the most important of this vast series of investigations had been brought to a conclusion, "we admire the living and revere the dead far too warmly and too deeply to suffer us to sit in judgment on their respective claims to priority in this or that particular discovery; to balance the mathematical skill of one against the experimental dexterity of another, or the philosophical acumen of a third so long as 'one star differeth from another in glory,'-so long as there shall exist varieties or even incompatibilities of excellence,-so long will the admiration of mankind be sufficient for all who merit it."

But in the mean time Young, though he engaged in no continuous optical investigation and preserved strictly the incognito which he considered to be due to his profession, was neither an idle nor an unconcerned observer of what was passing around him. The Memoir of La Place, which took the lead in the series of researches above referred to, was published in

a

Malus and Fresnel were then dead.

Article Light, Encyclopædia Metropolitana, §. 780.

"Sur la loi de la Réfraction extraordinaire dans les Cristaux Diaphanes.”—Journal de Physique for 1809.