Mi. Montague, then chancellor of the exchequer, and afterwards earl ef Halifax, was one of the friends and admiiers of Newton, and sought every opportunity of proving the extent and sincerity of his Iriendship.* An oppo:.tunity having- accordingly presented itself in 1696, by the pinmotion of Mr. Overton, warden of the mint, to be onc of the commissioners of customs, Mr. Mon tagne prevailed upon the king to appoint Mr. New ton warden of the mint ;t an office in w hich hc was of great service during the recoinage which took place soon after his appointment. In 1699, lie was promoted to the situation of master and worker of the mint, an offiee worth from twelve to fifteen hundred pounds a year, which he held during the rest of his life. His whole conduct in this important office gave the highest satisfaction, and he had an opportunity of showing his chemical knowledge in his table of Assays of foreign coins, printed at the cnd or Dr. Arbuthnot's tables of ancient coins, weights, and measures, published in 1727.
In the year 1701, Mr. \Vhiston was appointed his de puty as professor of mathematics at Cambridge, and frem that time he received all the salary, though Mr. Newton did not resign the professorship till 1703. On the 21st February, 1699, he was elected first titular of the seventh place of Foreign Associate of the Academy or Sciences of Paris.
Upon the resignation of John Lord Somers, Mr. New ton was chosen president of the Royal Society on the 30th November, 1703; a situation which he held for 29 years, till the timc of his death. " On the 16th February, 1709, Mr. Newton presented to the Royal Society his book of Optics, " part of which," as he himself says, " was written at the desire of some gentlemen of the Royal Socicty, in thc year 1675, and read at their meetings, and the rest was added about 12 years after, (1687) to complete the theory, except the third book and the last proposition of the second, which were since put together out of scattered papers, * *. If any other papers, writ on this subject, are got out of my hands, they are imperfect, and were perhaps written before I had tried all the experiments here set down, and fully satisfied myself about the laws of refractions and composition of colours. I have here published what I think proper to come abroad, wishing that it may not be translated into another language with out my consent." c• In a letter written to M. Leibnitz, in the year 1679, and published by Dr. Wallis, I mentioned a method by which I had found some general theorems about squar ing curvilineal figures, or comparing them with the conic sections, &c. And some years ago, I lent out a containing such theorems, and having since met with somc things copied out of it, I have on this occasion made it public." This great work underwent other MO editions in English, one in 1717, and the other in 1721. With the approbation of the author, Dr. Samuel Clarke translated his Optics into Latin, and it was done so en tirely to Newton's satisfaction, that lie presented Dr.
Clarke with the sum of 500 upon its publication in 1706.
It is impossible not to observe from the whole c,f our author's conduct respecting his optical discoveries, that Ile valued them even higher than any of his other la bours. The eagerness with which Ile replied to every objection which was started against thetn, and the un ceasing care with which he repeated and perfected them, independent of more direct evidence, mat k the high es timation in which he held them. The labours of Coper nicus, of Kepler, and of Hook, hacl, in some measure, mingled themselves with his astronomical discoveries, and the claim of Leibnitz and the antecedent discoveries of Cavalerius and Fermat had marked the method of fluxions as a sort or common property, of which New ton was the superior,/ but his optical discoveries belong ed to himself alone. No other eye had witnessed the splendid facts which he had observed and studied, and no other judgment hacl attempted to analyze that com pound substance which lie could decompose into its gaudy elements, and which he could with as much fa cility recombine. His theory of the colours of natural bodies too was as new as it was ingenious, and though opposed by those who could not comprehend its beauty, and too profound to gain the applause of those WhO WerC disposed to admire it, it will nevertheless be ranked among the finest speculations in science, and among the highest efforts of the genius of its author.
Disposed, as we arc, to consider the optical discove ries of Newton as the finest specimens of physical re search, we are not blind to the singular oversight which was committed by their author, and which we are per suaded contributed, in no slight degree, to retard thc progress of that branch of knotvledge. Ne‘;ton took it for granted that glass, and water, and all other substan ces, gave a prismatic spectrum of the same length, when the mean refractions of the prisms were equal. He had actually fortned the spectrum with pristns of water and of flint glass, and yet he did not observe that the latter gave much more colour than the former. Nay, though lie had used prisms of numerous substances, all of which differed extremely in their power of producing colour, or dispersing the rays of light, yet lie never sus pected that such a difference existed. The slightest attempt to measure the length of the spectra ; thc slightest accident might have led him to this discovery, and consummated all his wishes by the invention of the achromatic telescope. Trusting to his accuracy, few persons could have ventured to repeat experiments which he had been correcting and revising during the last fifty years of his life, and it was, therefore, only from accidental causes, that future philosophers disco vered the different dispersive powers of different media which led to ihe construction of achromatic telescopes. In the year 1705, Queen Anne conferred upon Mr. New ton the honour of knighthood.