tensive improvements in their Nautical Almanac, which distinguish the volume for 1834.

Optics.

The splendid discoveries of Newton in optics seem to have excited very little desire to pursue the subject, for a long time, among his successors. The fact of the unequal dispersion of different media, really observed, as we have remarked, but not understood, by Newton and Lucas, was taken up, and even practically applied to the correction of the colour at the focus of the object-grasses of telescopes, by Mr. Hall, about 1729. This invention, however, seems to have been forgotten until the idea was once more revived, or rather re-discovered, by John Dollond, in 1758, and the achromatic telescope brought to perfection by the labours of that eminent artist and his scientific successors. This principle also enabled the optician to dispense with those inconvenient lengths in telescopes, formerly adopted, as will appear from what we before remarked (p. 245.)

The enquiry into the refractive and dispersive powers of transparent bodies, and the recognition of dispersions not only unequal in amount, but dissimilar in character, was, at the same time, carried on by various eminent observers, and these important physical data ascertained with increasing precision from the determinations of Boscovich, Wollaston, and Brewster. Compound lenses, including liquid media, were proposed and tried, with considerable success, by Dr. Blair; and have still more recently been carried to the highest perfection by Mr. Barlow.

The mathematical theory of optical rays is another extensive branch of enquiry, in the first instance, indeed, of a purely speculative character, but in its results and applications embracing the whole theory of ordinary reflection and refraction. Such a theory, developed in a high degree of generality by Malus, has, at the present day, been almost wholly superseded by investigations of a yet

more commanding abstraction, and prosecuted with the resources of a still more powerful calculus, by Professor Hamilton.

The phenomena of the prismatic spectrum again have been shown to possess characteristics unnoticed before, by the observations, first of Wollaston, and then of Frauenhofer, in the existence of bright and dark lines, crossing the spectrum at all points of length of these, no theory has as yet been attempted.

In those departments of experimental optics which reveal to us the more intimate nature and affections of light, in very recent times astonishing accessions have been made to our knowledge both of the phenomena and of the theory. After slumbering more than a century, during which scarcely a single fact was added to those elicited by Newton, this beautiful and interesting branch of enquiry began to revive. In the hands of Malus, Arago, and Biot, in France, and of Wollaston, Brewster, and Herschel, in Great Britain, the phenomena of polarised light gave a completely new turn to the train of enquiry; and a succession of brilliant experimental facts and laws, served to excite the admiration, and in some degree to exercise the theoretical skill of these and other philosophers. Before this, however, though little noticed or appreciated, the revival of the theory of Huyghens, and the extension of the experiments of Grimaldi, Hooke, and Newton, enabled the masterly genius of Dr. Young to strike out the beautifully simple law of interference, which has since been extended to the most intricate and recondite phenomena, not only of the class to which it was originally applied, but to those of polarised light also, with a precision and success utterly beyond the reach of possible anticipation, in the researches of those two lamented labourers in the field of science, Frauenhofer and Fresnel, and of their living coadjutors, Professor Airy and M. Cauchy.

The curious phenomena on which, as we have before seen, Newton founded his theory of fits, was now brought into connection with the whole range of the

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new results. The intervals, of whatever nature they might be, whose lengths had been so accurately determined, were found to be identified with the intervals of the waves in the Huyghenian theory; and though either explanation might apply, nothing for a long time had been decisively urged to determine which was preferable, until, in the first instance, Fresnel reasoned on the quantity of light reflected (which was one of the points we before alluded to); and, again, those assumptions respecting the action of the two surfaces of the thin plate (which, as we remarked, were the only hypothetical parts of Newton's doctrine,) were examined by Professor Airy; and, by a masterly experimentum crucis, he determined that both surfaces were concerned, and, by consequence, demonstrated the undulatory explanation. To those researches have recently been added the application of Professor Hamilton's system to the theory of undulations; out of which has arisen one of the most singular facts in scientific history, the prediction of an entirely new form which a ray of light ought to take under particular circumstances; totally unlike any thing of which previous observation could supply the smallest idea, but which was completely verified by the experiments of Professor Lloyd. Nor should we omit to mention the ingenious objections which have been raised against the undulatory theory by Mr. Potter; and the suggestion of a case, not, indeed, as yet either an objection or otherwise, but to which the theory has not yet been made applicable, by Mr. Barton, which have both excited some discussion. MM. Arago and Fresnel originated the experiment in which the simple phenomenon of interference is exhibited without any extraneous circumstances. Two diverging beams of light are made (either by reflection or refraction) to cross at a very small angle; and, instead of giving rise to a uniform double illumination at the point where they mix, they exhibit a space striped with alternate bright and absolutely black bands. At the black part, then, we have the paradoxical result that two conspiring rays of light produce absolute darkness: a result totally

inexplicable on any hypothesis of material particles, but a direct consequence of the theory of waves, of which a familiar illustration may be conceived in supposing two waves propagated from different origins, and reaching the same point at that exact interval which shall make the summit of one, as it were, coincide with the bottom of the other; they will then neutralise each other; and that point of the fluid will remain at rest. The agitation of the ethereal medium in waves produces light; when at rest there is darkness. The unequal refrangibility of light is a fact which was not to be reconciled with the particular view of the principles of undulations on which the theory had been framed. has been long, therefore, a topic of enquiry, and several hypotheses have been suggested as to the possibility of so modifying the conception of these first principles as to make the results include this case. This appears at length to have been successfully done by M. Cauchy.

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But, in addition to these, we could, if our limits would allow, increase largely our list of discoveries by reference to a vast number of other interesting facts relative to the absorption of light by different media, the constitution of the prismatic spectrum, the application of polarised light to the detection of the crystalline forms of minerals; the phenomena both of the light reflected from metals as regards its state of polarisation, the colours produced by reflection from grooved surfaces, and numerous others, for which we are almost wholly indebted to the profound experimental skill and indefatigable industry of sir D. Brewster. But into any such enumeration our limits forbid us to enter; and we can only pass, with deep regret, from this interesting portion of our survey to others of which our view must be as rapid and imperfect.

General Physics.

A vast range of science, wholly of modern creation, has arisen in tracing the relations of light heat, mag

netism, electricity, and galvanism. The great principles of electricity originally developed by Gray and Franklin, Du Fay and Æpinus; of galvanism, by Volta and Galvani; and of magnetism, first reduced to determinate laws by Mr. Barlow, were placed in the closest connection by the grand discovery of electro-magnetism, by Oersted, in 1819. This was followed up by the researches of Davy, Faraday, Ampere, Arago, Barlow, Christie, and many other philosophers, following out the numerous new analogies which the subject so abundantly offers. The new facts of thermo-electricity and thermo-magnetism elicited by Dr. Seebeck and Professor Cumming, brought the other subtle agent heat into connection with the former; and the singular influence of the simple rotation of metallic plates on the magnetic needle, investigated by Messrs. Arago, Babbage, Herschel, Barlow, and others, have all jointly contributed to form the new science of electro-dynamics.

The increasing and exact attention paid to observing the constitution of the atmosphere, the influence of moisture and temperature, the effects of wind, and the endless combination arising out of the general causes of pressure, condensation, and rarefaction, have led to the recognition of meteorology as holding a place among the exact sciences. Some, indeed, of its phenomena are intimately connected with the laws of chemical combination, another boundless region of philosophical enquiry, which our subject does not even profess to include, and to which we cannot further refer. Closely connected, however, with all these departments is the science of heat, both in its combined form, or recognised as developed in bodies, the subject of profound research to Black, Wilcke, Dulong, Petit, and Fourier; or in its radiant condition as propagated from hot bodies to sensible distances. In this last department (connected as it also is with the solar heat), the valuable researches of Leslie, Herschel, Prevost, and De la Roche have put us in possession of all the elementary facts and simple laws; while the theoretical inferences from some of