and diamond, which have their refractive powers two or three times greater in respect of their densities than the other substances in the table, while among themselves their refractive powers are proportional to their densities, without any considerable variation. Hence he concluded that diamond "is an unctuous substance coagulated," -a sagacious prediction, which has been verified in the discoveries of modern chymistry. The connexion between a high degree of inflammability and a great refracting force has been still more strongly established by the high refractive power which I detected in phosphorus, and which was discovered in hydrogen by MM. Biot and Arago.

There is no part of the optical labours of Newton which is less satisfactory than that which relates to the double refraction of light. In 1690, Huygens, published his admirable treatise on light, in which he has given the law of double refraction in calcareous spar, as deduced from his theory of light, and as confirmed by direct experiment. Viewing it probably as a theoretical deduction, Newton seems to have regarded it as incorrect, and though he has given Huygens the credit of describing the phenomena more exactly than Bartholinus, yet, without assigning any reason, he rejected the law of the Dutch philosopher, and substituted another in its place. These observations of our author form the subject of the twenty-fifth and twenty-sixth queries at the end of his Optics, which was published fourteen years after the appearance of Huygens's work. The law adopted by Newton is not accompanied with any of the experiments from which it was deduced; and though he has given it without expressing any doubt of its accuracy, it is, nevertheless, entirely incompatible with observation, and has been rejected by all succeeding philosophers.

In his speculations respecting the successive aisappearance and reappearance of two of the four

images which are formed when a luminous object is viewed through two rhombs of calcareous spar, one of which is made to revolve upon the other, Newton has been more successful. He concluded from these phenomena that every ray of light has two opposite sides originally endued with the property on which the unusual refraction depends, and other two opposite sides not endued with that property; and he suggested it as a subject for future inquiry, whether there are not more properties of light by which the sides of the rays differ, and are distinguished from one another. This is the first occasion on which the idea of a polarity in the rays of light has been suggested.*

From the various optical inquiries in which Newton was engaged, he was strongly impressed with the belief that light consists of small material particles emitted from shining substances, and that these particles could be again recombined into solid matter, so that "gross bodies and light were convertible into one another." He conceived also that the particles of solid bodies and of light exerted a mutual action upon each other, the former being agitated and heated by the latter, and the latter being attracted and repelled by the former, with forces depending on the inertia of the luminous particles. These forces he regarded as insensible at all measurable distances, and he conceived that the distances between the particles of bodies were very small when compared with the extent of their sphere of attraction and repulsion.

With the exception of Hooke, Huygens, and Euler, almost all the contemporaries and successors of Newton maintained the doctrine of the materiality of light. It was first successfully assailed by Dr. Thomas Young, and since that time it has been shaken to its foundation by those great discoveries

*See the twenty-ninth query at the end of his Optics, where the sides of a ray are compared with the poles of a magnet.

which have illustrated the commencement of the present century. The undulatory theory, which has thus triumphed in its turn, is still subject to grave difficulties, and we fear another century must elapse before a final decision can be pronounced on this long-agitated question.

The most important of the optical discoveries of Newton, of which we have given a general history, were communicated to the Royal Society in detached papers; but the disputes in which they had involved their author made him hesitate about the publication of his other discoveries. Although he had drawn up a connected view of his labours under the title of "Opticks, or a Treatise on the Reflexions, Refractions, Inflexions, and Colours of Light," yet he resolved not to publish this work during the life of Hooke, by whose rival jealousy his tranquillity had been so frequently interrupted. Hooke, however, died in 1702, and the Optics of Newton appeared in English in 1704. Dr. Samuel Clark proposed a Latin edition of it, which appeared in 1706, and he was generously presented by Sir Isaac with 500l. (or 1007. for each of his five children), as a token of the approbation and gratitude of the author. Both the English and the Latin editions have been frequently reprinted both in England and on the Continent,* and there perhaps never was a work of profound science so widely circulated

*The English edition was reprinted at London in 1714, 1721, and 1730, and the Latin one at London in 1706, 1719, 1721, 1728, at Lausanne in 1740, and at Padua in 1773.

K

CHAPTER X.

Astronomical Discoveries of Newton-Necessity of combined Exertion to the Completion great Discoveries-Sketch of the History of Astronomy previous to the Time of Newton-Copernicus, 1473-1543Tycho Brahe, 1546-1601-Kepler, 1571-1631-Galileo, 1564-1642.

FROM the optical labours of Newton we now proceed to the history of his astronomical discoveries -those transcendent deductions of human reason by which he has secured to himself an immortal name, and vindicated the intellectual dignity of his species: Pre-eminent as his triumphs have been, it would be unjust to affirm that they were achieved by his single am. The torch of many a preceding age had thrown its light into the strongholds of the material universe, and the grasp of many a powerful hand had pulled down the most impregnable of its defences. An alliance, indeed, of many kindred spirits had been long struggling in this great cause, and Newton was but the leader of their mighty phalanx, the director of their combined genius,the general who won the victory, and therefore wears its laurels.

The history of science presents us with no example of an individual mind throwing itself far in advance of its contemporaries. It is only in the career of crime and ambition that reckless man takes the start of his species, and, uncurbed by moral and religious restraint, erects an unholy dynasty upon the ruins of ancient and venerable institutions. The achievements of intellectual power, though often begun by one mind and completed by another, have ever been the results of combined exertions. Slow in their growth, they gradually approximate to a more perfect condition :-the variety

in the phenomena of nature call forth a variety of intellectual gifts ;-the powers of analysis and com. bination are applied to the humbler labours of obser vation and experiment, and in the ordeal of rival inquiry truth is finally purified from error. How different is it with those systems which the imagination rears, those theories of wild import which are directed against the consciences and hopes of man. The fatal upas-tree distils its poison in the spring as well as the autumn of its growth, but the fruit which sustains life must have its bud prepared before the approach of winter, its blossom expanded in the spring, and its juices elaborated by the light and heat of the summer and the autumnal sun.

In the century which preceded the birth of Newton the science of astronomy advanced with the most rapid steps. Emerging from the darkness of the middle ages, the human mind seemed to rejoice in its new-born strength, and to apply itself with elastic vigour to unfold the mechanism of the heavens. The labours of Hipparchus and Ptolemy had indeed furnished many important epochs and supplied many valuable data; but the cumbrous appendages of cycles and epicycles with which they explained the stations and retrogradations of the planets, and the vulgar prejudices which a false interpretation of Scripture had excited against a belief in the motion of the earth, rendered it difficult even for great minds to escape from the trammels of authority, and appeal to the simplicity of nature.

The sovereign of Castile, the generous and nobleminded Alphonso, had long before proscribed the rude expedients of his predecessors; and when he declared that if the heavens were thus constituted, he could have given the Deity good advice, he must not only have felt the absurdity of the prevailing system, but must have obtained some foresight of a more simple arrangement. But neither he nor the astronomers whom he so liberally protected seem to