scope, and in 1733 had made several; but his work remained unnoticed till Dollond turned his attention to the question, and in 1758 constructed satisfactory achromatic lenses by the combination of crown and flint glass (Brewster, Life of Newton, i. 99, ed. 1855).
Nor were Hooke, Linus, and Lucas Newton's only opponents. Huyghens himself entered the field, but his objections (Phil. Trans. vii. 6086, 6108) were not very serious. Still these differences of opinion troubled Newton, and he wrote to Oldenburg (Maccl. Corr. ii. 368, 5 Dec. 1674): ‘I have long since determined to concern myself no further about the promotion of philosophy;’ and again (ib. ii. 404, 18 Nov. 1676): ‘I see I have made myself a slave to philosophy; but if I get free of Mr. Linus' business I will resolutely bid adieu to it eternally, excepting what I do for my own satisfaction or leave to come out after me, for I see a man must either resolve to put out nothing new or to become a slave to defend it.’ Collins, writing to J. Gregory (ib. ii. 280, 19 Oct. 1675), sadly asserted that Newton and Barrow were ‘beginning to think mathematical speculations at least dry, if not somewhat barren,’ and that Newton was intent on chemical studies and practices. But wiser counsels prevailed, and Newton did not yet give up philosophy. The ‘Macclesfield Correspondence’ contains some interesting letters from him to Collins, dated between 1672 and 1675, dealing with such topics as reflecting telescopes (Gregory's and Cassegrain's), Barrow's method of tangents, and the motion of a bullet.
On 18 Feb. 1675 ‘Mr. Isaac Newton and James Hoare, jun., esq.,’ were admitted fellows of the Royal Society, to which Newton had been elected nearly three years earlier. On 28 Jan. of the same year he had been excused the weekly payment of 1s. to the society, and he had expressed a wish to resign, alleging as the cause the distance between Cambridge and London. It appears that at the time he was in circumstances of pecuniary difficulty. These, it seems probable, were connected with the expectation that he would have to vacate his fellowship in the autumn, owing to his not being in holy orders. The difficulty was solved by the receipt of a patent from the king permitting Newton as Lucasian professor to hold a fellowship although he was a layman. Thus encouraged, he continued his work, and towards the end of the year he wrote to Oldenburg, offering to send ‘a Discourse about Colours to be read at one of your meetings.’ This was accepted, and on 9 Dec. 1675 ‘there was produced a manuscript of Mr. Newton touching his theory of light and colours, containing partly an hypothesis to explain the properties of light discoursed of by him in his former papers, partly the principal phenomena of the various colours exhibited by thin plates or bubbles, esteemed by him to be of a more difficult consideration, yet to depend also on the said properties of light.’ The experiments recorded the first measurements on the coloured rings of thin plates. The relation between the diameter of the rings and the thickness of the plate was stated, and the phenomena were explained in Newton's clear and masterly way. There was also a reference to the diffraction of light. The reading was continued 20 Jan. 1676, when ‘these observations so well pleased the Society that they ordered Mr. Oldenburg to desire Mr. Newton to permit them to be published’ (Birch, Hist. of Roy. Soc. iii. 278). Newton, in his reply (Maccl. Corr. ii. 388, 25 Jan. 1676), asked Oldenburg ‘to suspend the printing of them for a while, because I have some thought of writing such another set of observations for determining the manner of the production of colours by the prism, which, if done, ought to precede that now in your hands, and will do best to be joined with it.’ Accordingly the paper was not printed in the ‘Philosophical Transactions.’ It is given in Birch (Hist. of Roy. Soc. iii. 247, 262, 272, &c.), while a large part of it appeared in the ‘Optics,’ bk. ii., in 1704, but without the hypothesis. This is printed in Brewster's ‘Life of Newton’ (vol. i. App. ii.) and in the ‘Philosophical Magazine’ (September 1846, pp. 187–213).
After the part of the paper relating to diffraction and a portion of the observations on the colours of thin plates had been read, Hooke said ‘that the main of it was contained in his “Micrographia,” which Mr. Newton had only carried further in some particulars’ (Birch, ib. iii. 269). Newton had moreover referred discourteously to a paper of Hooke's dealing with the inflexion of light which had been read 18 March 1675. Hooke's words were now reported to Newton, possibly with too high a colouring, by Oldenburg, who was then engaged in a dispute with Hooke on other matters, and Newton replied somewhat angrily. On this Hooke wrote privately to Newton (Brewster, Life of Newton, i. 123), expressing a desire to remove the misunderstanding. Newton modestly accepted the friendly advance. ‘You defer (he wrote) too much to my ability in searching into this subject. What Descartes did was a good step. You have added much several ways, and especially in considering the colours of thin plates. If I have seen further it is by standing on the shoulders of giants.’ Shortly after (Maccl. Corr., ii. 394), he asked Olden-
