miscellaneous optical researches of newton—his experiments on refraction—his conjecture respecting the inflammability of the diamond—his law of double refraction—his observations on the polarization of light—newton’s theory of light—his “optics.”

before concluding our account of newton’s optical discoveries, it is necessary to notice some of his minor researches, which, though of inferior importance in the science of light, have either exercised an influence over the progress of discovery, or been associated with the history of other branches of knowledge.

one of the most curious of these inquiries related to the connexion between the refractive powers and the chymical composition of bodies. having measured the refractive powers and the densities of twenty-two substances, he found that the forces which reflect and refract light are very nearly proportional to the densities of the same bodies. in this law, however, he noticed a remarkable exception in the case of unctuous and sulphureous bodies, such as camphire, olive oil, linseed oil, spirit of turpentine,107 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 disappearance and reappearance of two of the four108 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.32

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 discoveries109 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 100l. 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,33 and there perhaps never was a work of profound science so widely circulated.