make those vigorous and rapid shoots which soon covered with foliage and with fruit the genial soil to which it had been transferred.

Cambridge was consequently the real birthplace of Newton's genius. Her teachers fostered his earliest studies; her institutions sustained his mightiest efforts; and within her precincts were all his discoveries made and perfected. When he was called to higher official functions, his disciples kept up the pre-eminence of their master's philosophy, and their successors have maintained this seat of learning in the fulness of its glory, and rendered it the most distinguished among the universities of Europe.

It was on the 5th of June, 1660, in the 18th year of his age, that Newton was admitted into Trinity College, Cambridge, during the same year that Dr. Barrow was elected professor of Greek in the university. His attention was first turned to the study of mathematics by a desire to inquire into the truth of judicial astrology; and he is said to have discovered the folly of that study by erecting a figure with the aid of one or two of the problems of Euclid. The propositions contained in this ancient system of geometry he regarded as self-evident truths; and without any preliminary study he made himself master of Descartes's Geometry by his genius and patient application. This neglect of the elementary truths of geometry he afterward regarded as a mistake in his mathematical studies, and he expressed to Dr. Pemberton his regret that "he had applied himself to the works of Descartes, and other algebraic writers, before he had considered the elements of Euclid with that attention which so excellent a writer deserved.* Dr. Wallis's Arithmetic of Infinites, Saunderson's Logic, and the Optics of Kepler were among the books which he had studied with care. On these works he wrote comments during their perusal;

* Pemberton's View of Sir Isaac Newton's Philosophy. Pref.

and so great was his progress, that he is reported to have found himself more deeply versed in some branches of knowledge than the tutor who directed his studies.

Neither history nor tradition has handed down to us any particular account of his progress during the first three years that he spent at Cambridge. It appears from a statement of his expenses, that in 1664 he purchased a prism, for the purpose, as has been said, of examining Descartes's theory of colours; and it is stated by Mr. Conduit, that he soon established his own views on the subject, and detected the errors in those of the French philosopher. This, however, does not seem to have been the case. Had he discovered the composition of light in 1664 or 1665, it is not likely that he would have withheld it, not only from the Royal Society, but from his own friends at Cambridge till the year 1671. His friend and tutor, Dr. Barrow, was made Lucasian Professor of Mathematics in 1663, and the optical lectures which he afterward delivered were published in 1669. In the preface of this work he acknowledges his obligations to his colleague, Mr. Isaac Newton,* for having revised the MSS., and corrected several oversights, and made some important suggestions. In the twelfth lecture there are some observations on the nature and origin of colours, which Newton could not have permitted his friend to publish had he been then in possession of their true theory. According to Dr. Barrow, White is that which discharges a copious light equally clear in every direction; Black is that which does not emit light at all, or which does it very sparingly. Red is that which emits a light more clear than usual, but interrupted by shady interstices. Blue is that which discharges a rarified light, as in bodies which consist of white and black particles arranged

* Peregregiæ vir indolis ac insignis peritiæ.-Epist ad. Lect

Yellow

alternately. Green is nearly allied to blue. is a mixture of much white and a little red; and Purple consists of a great deal of blue mixed with a small portion of red. The blue colour of the sea arises from the whiteness of the salt which it contains, mixed with the blackness of the pure water in which the salt is dissolved; and the blueness of the shadows of bodies, seen at the same time by candle and daylight, arises from the whiteness of the paper mixed with the faint light or blackness of the twilight. These opinions savour so little of genuine philosophy that they must have attracted the observation of Newton, and had he discovered at that time that white was a mixture of all the colours, and black a privation of them all, he could not have permitted the absurd speculations of his master to pass uncorrected.

That Newton had not distinguished himself by any positive discovery so early as 1664 or 1665, may be inferred also from the circumstances which attended the competition for the law fellowship of Trinity College. The candidates for this appointment were himself and Mr. Robert Uvedale; and Dr. Barrow, then Master of Trinity, having found them perfectly equal in their attainments, conferred the fellowship on Mr. Uvedale as the senior candidate.

In the books of the university, Newton is recorded as having been admitted sub-sizer in 1661. He became a scholar in 1664. In 1665 he took his degree of Bachelor of Arts, and in 1666, in consequence of the breaking out of the plague, he retired to Woolsthorpe. In 1667 he was made Junior Fellow. In 1668 he took his degree of Master of Arts, and in the same year he was appointed to a Senior Fellowship. In 1669, when Dr. Barrow had resolved to devote his attention to theology, he resigned the Lucasian Professorship of Mathematics in favour of Newton, who may now be considered

as having entered upon that brilliant career of discovery the history of which will form the subject of some of the following chapters.

CHAPTER III.

Newton occupied in grinding Hypdrbolical Lenses-His first Experv ments with the Prism made in 1666-He discovers the Composition of White Light, and the different Refrangibility of the Rays which compose it-Abandons his Ättempts to improve Refracting Telescopes and resolves to attempt the Construction of Reflecting ones-He quits Cambridge on account of the Plague-Constructs two Reflecting Telescopes in 1668, the first ever executed-One of them examined by the Royal Society, and shown to the King--He constructs a Telescope with Glass Specula-Recent History of the Reflecting Telescope-Mr. Airy's Glass Specula-Hadley's Reflecting Telescopes-Short's-Herschel's -Ramage's-Lord Oxmantown's.

THE appointment of Newton to the Lucasian chair at Cambridge seems to have been coeval with his grandest discoveries. The first of these of which the date is well authenticated is that of the different refrangibility of the rays of light, which he established in 1666. The germ of the doctrine of universal gravitation seems to have presented itself to him in the same year, or at least in 1667 and "in the year 1666 or before"* he was in possession of his method of fluxions, and he had brought it to such a state in the beginning of 1669, that he permitted Dr. Barrow to communicate it to Mr. Collins on the 20th of June in that year.

Although we have already mentioned, on the authority of a written memorandum of Newton him self, that he purchased a prism at Cambridge in 1664, yet he does not appear to have made any use of it, as he informs us that it was in 1666 that he "pro

* See Newton's Letter to the Abbé Conti, dated February 26, 1715-16, in the Additamenta Comm. Epistoirci.

cured a triangular glass prism to try therewith the celebrated phenomena of colours."* During that year he had applied himself to the grinding of "optic glasses, of other figures than spherical," and having, no doubt, experienced the impracticability of executing such lenses, the idea of examining the phenomena of colour was one of those sagacious and fortunate impulses which more than once led him to discovery. Descartes in his Dioptrice, published in 1629, and more recently James Gregory in his Optica Promota published in 1663, had shown that parallel and diverging rays could be reflected or refracted, with mathematical accuracy, to a point or focus, by giving the surface a parabolic, an elliptical, or a hyperbolic form, or some other form not spherical. Descartes had even invented and described machines by which lenses of these shapes could be ground and polished, and the perfection of the refracting telescope was supposed to depend on the degree of accuracy with which they could be executed.

In attempting to grind glasses that were not spherical, Newton seems to have conjectured that the defects of lenses, and consequently of refracting telescopes, might arise from some other cause than the imperfect convergency of rays to a single point, and this conjecture was happily realized in those fine discoveries of which we shall now endeavour to give some account.

When Newton began this inquiry, philosophers of the highest genius were directing all the energies of their mind to the subject of light, and to the inprovement of the refracting telescope. James Gregory of Aberdeen had invented his reflecting telescope. Descartes had explained the theory and exerted himself in perfecting the construction of the common refracting telescope, and Huygens had not

* Newtoni Opera, tom. iv. p. 205, Letter to Oldenburg.