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NEWTON ON THE CAUSE OF THE MOON'S LIBRATION — IS OCCUPIED WITH THE SUBJECT OF PLANTING CIDER TREES — SENDS TO OLDENBURG HIS DISCOURSE ON LIGHT AND COLOURS, CONTAINING HIS HYPOTHESIS CONCERNING LIGHT — VIEWS OF DESCARTES AND HOOKE, WHO ADOPT THE HYPOTHESIS OF AN ETHER, THE VIBRATIONS OF WHICH PRODUCE LIGHT — REJECTED BY NEWTON, WHO PROPOSES A MODIFICATION OF IT, BUT SOLELY AS AN ILLUSTRATION OF HIS VIEWS, AND NOT AS A TRUTH — LIGHT IS NEITHER ETHER, NOR ITS VIBRATING MOTION — CORPUSCLES FROM THE SUN ACT UPON THE ETHER — HOOKE CLAIMS NEWTON'S HYPOTHESIS AS CONTAINED IN HIS MICROGRAPHIA — DISCUSSIONS ON THE SUBJECT — HOOKE'S LETTER TO NEWTON PROPOSING A PRIVATE DISCUSSION AS MORE SUITABLE — NEWTON'S REPLY TO THIS LETTER, ACKNOWLEDGING THE VALUE OF HOOKE'S DISCOVERIES — OLDENBURG THE CAUSE OF THE DIFFERENCES BETWEEN HOOKE AND NEWTON — NEWTON'S LETTER TO BOYLE ON THE SUBJECT OF ETHER — HIS CONJECTURE ON THE CAUSE OF GRAVITY — NEWTON SUPPOSED TO HAVE ABANDONED THE EMISSION THEORY — DR. YOUNG'S SUPPOSITION INCORRECT — NEWTON'S MATURE JUDGMENT IN FAVOUR OF THE EMISSION THEORY.
IN the years 1675 and 1676, when Newton was engaged in his fruitless controversy with the Dutch professors, his mind was directed to a great variety of subjects. CollinsMacclesfield Correspondence, vol. ii. P. 280.eleven or twelve months; that he did not wish to trouble him, as he was "intent upon chemical studies and practices," and that Newton and Barrow had "begun to think mathematical speculations at least dry, if not somewhat barren." His attention was at this time occupied with conatus from the sun and earth compared together, till he apprehended a better cause." This better cause he communicated in 1675 to Nicolas Mercator, who published it in the following year in his Astronomical Institutions.Institutiones Astronomicæ; , p. 286.
About this time we find Newton occupied with a subject very different from his usual pursuits — taking an interest, like a country gentleman, in the planting of fruit trees for the manufacture of cider. It does not appear how his attention was directed to this subject. A reference is made to it in a letter to Oldenburg, in November 1676; but we have been fortunate enough to find among his papers a previous letter to the same gentleman, in September, which we need make no apology for inserting here.
"September 2, 1676.
"Sir, — I have now made what inquiry I can into the state we are in for planting, and find there are some gentlemen that of late have begun to plant, and seem to incline more and more to it, but I cannot hear of any professed nurseryman we have. Our gardeners find more profit in cherry trees, and so stock their ground almost wholly with them. The chief of them plant some fruit trees, but it is to find the gentry with plants: to whom I am apt to think your proposition will prove a very reasonable one, considering the new humour of planting that begins to grow among them. But in order to promote the design, I am desired to inquire what sort of trees your friend can furnish us with, at what rates, which way they can most conveniently be conveyed to so great a distance, and what may be the charges of carriage. Also, whether they are to be sent in cions or grafts; the first being more convenient for carriage, and so rather to be wished, unless those trees be found best which are grafted on their native soil. I perceive the gardener I mentioned (Mr. Blackley by name) would gladly embrace the proposal, and provide himself with more ground than he has, for a nursery, to stock his neighbours, if he found he can have good sorts of trees, and the carriage make them not too dear.
"But, upon discoursing with people, I find we lye under one great difficulty; which is an opinion generally taken up here, that Red Streaks (the famous fruit for cyder in other parts) will not succeed in this country. The tree thrives well here, and bears as much fruit, and as good to look as in other countries; but the cyder made of it they find harsh and churlish, and so this fruit begins here to be generally neglected, and other fruit, and which they
"Is. Newton."
"R.Sir.Phil. Trans., No. 128, p. 703.
In November 1676, Newton addresses another letter to Oldenburg in the following terms: —
"I am desired to write to you about procuring a recommendation of us to Mr. Austin, the Oxonian planter. We hope your correspondent will be pleased to do us that favour as to recommend us to him, that we may be furnished with the best sort of cider fruit trees. We desire only about 30 or 40 graffs for the first essay, and if these prove for our purposes, they will be desired in greater numbers. We desire graffs rather than sprags, that we may the sooner see what they will prove. They are not for Mr. Blackley, but some other persons about Cambridge."Correspondence, App. No. xvi. p. 260.
The friend mentioned in one of these letters, and the correspondent in the other, was the Rev. Dr. John Beal, Rector of Yeovil, in Somersetshire, who, in imitation of his father and great-grandfather, had distinguished himself by his zeal in the plantation of orchards for the making of cider.Herefordshire Orchards a Pattern for England, 1656. See Birch's Hist. of the Royal Society, vol. iv. p. 235.
But though thus occasionally occupied with other subjects, he was at this time diligent in the prosecution of his optical researches. On the 13th November 1675, he
This discourse was produced in manuscript on the 9th December 1675, with the title of — "A 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 to be of a more difficult consideration, yet to depend also on the said properties of light." This paper was introduced by the following letter to Oldenburg, which possesses much historical interest.
"Sir, — I have sent you the papers I mentioned, by John Stiles. Upon reviewing them I find some things so obscure as might have deserved a further explication by schemes; and some other things I guess will not be new to you, though almost all was new to me when I wrote
"Sir, — I had formerly purposed never to write any hypothesis of light and colours, fearing it might be a Sir, I am your obedient servant,
"Is. Newton."
The Hypothesis,Appendix, No. II.emitted by the luminous body," but Mr. Vernon Harcourt (Letter to Lord Brougham, p. 32) has shewn the incorrectness of this opinion. See Œuvres de Descartes, tom. vii. pp. 193, 240.Opera, tom. iv. pp. 325, 326.seems itself impossible; namely, that the waves or vibrations of any fluid can, like the rays of light, be propagated in straight lines, without a continual and very extravagant spreading and bending every way into the quiescent medium where they are terminated by it. I am mistaken if there be not both experiment and demonstration to the contrary."
In thus summarily rejecting Hooke's hypothesis, Newton suggests a modification of it, or a form in which it will be better fitted to account for the phenomena, or to use his own expression, — "The most free and natural application of this hypothesis I take to be this — that the agitated parts of bodies, according to their several figures, sizes, and motions, do excite vibrations in the ether of various depths or sizes, which being promiscuously propagated through that medium to our eyes, effect in us a sensation of light of a white colour; but if by any means those of unequal sizes be separated from one another, the largest beget a sensation of a red colour, the least or shortest of a deep Phil. Trans., 1672, No. 88, p. 5088.illustrated my discourse with an hypothesis; for this reason I have here thought fit to send you a description of the circumstances of this hypothesis, as much tending to the illustration of the papers I herewith send you."
In order to prevent any misapprehension of his meaning, he goes on to say, "that he shall not assume either this or any other hypothesis;" yet while he is describing this hypothesis "he shall sometimes, to avoid circumlocution, and to represent it more conveniently, speak of it as if he assumed it, and propounded it to be believed."
With this caution, he supposes an ethereal medium rarer than air, subtler, and more elastic, not one uniform matter, but "compounded of various ethereal spirits or vapours, with the phlegmatic body of ether. The whole frame of nature may be nothing but various contextures condensed they that will may also suppose that this spirit affords, or carries with it thither, the solary fuel, and material principle of light, and that the vast ethereal spaces between us and the stars are for a sufficient repository for this food of the sun and planets!" If we laugh
In passing from "the effects and uses of ether," to the "consideration of light," he supposes that light "is neither ether, nor its vibrating motion, but something of a different kind propagated from lucid bodies," such as "multitudes of small and swift corpuscles of various sizes springing from shining bodies, at great distances, one after another, but yet without any sensible interval of time." That it is different from the vibrations of the ether, he infers from the existence of shadows, and the colours of thin plates. His next supposition is, "that light and ether mutually act upon one another, ether in refracting, light, and light in warming ether;" and, after some farther observations on this mutual action, he goes on to explain the manner in which refraction and reflexion are produced upon this hypothesis, and the cause of transparency, opacity, and colour. His discourse concludes with an application of the hypothesis to the colours of thin plates, to the inflexion of light, and to the colours of natural bodies, — subjects to which we shall presently direct the reader's attention.
After the reading of the first part of this discourse on the 9th December, Mr. Hooke said, "that the main of it was contained in his Micrographia , which Mr. Newton Opera, tom. iv. pp. 378-381; or Birch, vol. iii. p. 278.Micrographia, had "delivered many very excellent things concerning the colours of thin plates, and other natural bodies, which he had not scrupled to make use of as far as they were for his purpose."
These controversial discussions seem to have annoyed Hooke as much as they did Newton, and, instead of publicly replying to the two last communications of Newton, he addressed a letter to him, which, with Newton's answer, we had the good fortune to discover among the family papers. These letters are highly interesting; and we are persuaded, that those who, like us, have had
Robert Hooke — "These to my much esteemed friend, Mr. Isaack Newton, at his chambers in Trinity College in Cambridge.
"R.Sir.r, I hope you will pardon this plainness of, your very affectionate humble servt,
"1675-6. Robert Hooke."
To this letter Newton sent the following reply: —
"Cambridge, February 5, 1675-6.
"DR. Sir, — At the reading of your letter I was exceedingly pleased and satisfied with your generous freedom, and think you have done what becomes a true philosophical spirit. There is nothing which I desire to avoyde in matters of philosophy more than contention, nor any kind of contention more than one in print; and, therefore, I most gladly embrace your proposal of a private correIf I have seen farther, it is by standing on the shoulders of giants. But I make no question you have divers very considerable experiments beside those you have published, and some, it's very probable, the same with some of those in my late papers. Two at least there are, which I know you have often observed, — the dilatation of the coloured rings by the obliquation of the eye, and the apparition of a black spot at the contact of two convex glasses, and at the top of a water-bubble; and it's probable there may be more, besides others which I have not made, so that I have reason to defer as much or more in Optics, published many years after this, in 1704, Newton does not give Hooke the credit of having made these observations.
"Is. Newton."
These beautiful letters, emulous of good feeling and lofty principle, throw some light on the character and position of two of the greatest of our English philosophers, and we cannot read their mutual confessions and desires without an anxious hope that two such men may never again be placed in a state of intellectual collision. In alluding to the sinister practices of some intermeddling friend, and to the evil consequences of two hard-to-yield contenders being put together by the ears by other's hands and incentives, Hooke evidently refers to his colleague, Mr. Oldenburg. It was not unlikely that the secretary to the Royal Society, and its Curator and Professor of Mechanics, might have occasional grounds of difference without any imputation upon their social or moral character; but this official jealousy, whatever was its amount, was increased in a high degree during the disputes between Hooke and Hevelius on the subject of plain and telescopic sights, and between Hooke and Huygens remanner of claiming the discovery had been represented in worse colours than it ought." With his usual good feeling, Newton thus expressed his regret: "Now that I understand he was in some respects misrepresented to me, I wish I had spared the postscript in my last. "
When Hooke, in the case more immediately before us, stated "that the main of Newton's discourse was contained in his Micrographia, which he had only carried further in some particulars," he did not do justice to the valuable communication of his rival; but, on the other hand, we have it on the evidence of Newton himself, that he did not, in his discourse, give Hooke the same credit for his discoveries which he afterwards did in the letter that he addressed to him. It has been too much the practice of the admirers of Newton to assail the memory of Hooke with ungenerous animadversions, and unmanly abuse. A distinguished philosopher has even ventured to describe him as "a bad man," as if he added to the intellectual fame of Newton by the moral depreciation of his rival. We cannot give our sanction to so harsh a judgment. Under a due sense of the imperfections of
After the publication of his "Hypothesis, explaining the Properties of Light," Newton seems to have been conversing with Robert Boyle on the subject of its application to chemistry, and on the 28th February 1679, he addressed a letter to him on the subject, in fulfilment of a long deferred promise. The views which he here presents to his friend, he characterizes as indigested and unsatisfactory to himself, and he adds, that "as it is only an explication of qualities that is desired," he "sets down his apprehensions in the form of suppositions." He supposes a subtle and elastic ether to pervade all gross bodies, and to stand rarer in their pores than in free space, and so much the rarer as their pores are less. The ether within solid and fluid bodies diminishes in density towards their surface, while the ether without all such bodies diminishes in density towards their surface. According to this theory there is a certain space within solid and fluid bodies, and a certain space without them, which Newton calls "the space of the ether's graduated rarity." On these suppositions he tries to explain the inflexion of light in passing through this space, the colours of minute particles, and of natural bodies, the repulsion and attraction of bodies coming into contact, the action of menstruums upon bodies, the phenomena of effervescence and ebullition, and the transmutation of gross substances into aerial ones. He conceives the confused mass of vapours, air, and exha
"I shall set down," he says, "one conjecture more, which came into my mind even as I was writing this letter; it is about the cause of gravity. For this end I will suppose ether to consist of parts differing from one another in subtlety by indefinite degrees; that in the pores of bodies there is less of the grosser ether in proportion to the finer, than in open spaces; and consequently, that in the great body of the earth there is much less of the grosser ether in proportion to the purer, than in the regions of the air; and that yet the grosser ether in the air affects the upper regions of the earth, and the finer ether in the earth the lower regions of the air, in such a manner, that from the top of the air to the surface of the earth, and again from the surface of the earth to the centre thereof, the ether is insensibly finer and finer. Imagine now any body suspended in the air or lying on the earth; and the ether being by the hypothesis grosser in the pores which are in the upper parts of the body, than in those which are in its lowest parts, and that grosser ether being less apt to be lodged in these pores than the finer ether below, it will endeavour to get out and give way to the purer ether below, which cannot be without the bodies descending to make room above for it to go out into."Opera, tom. iv. pp. 385-395.
The Hypothesis of Newton, and his other speculations regarding ether, have led some writers to suppose that he Phil. Trans., 1801; or Lectures on Natural Philosophy, vol. ii. p. 614.Ibid., vol. i. p. 477.partly retracted at the publication of the 'Optics,' yet I shall borrow nothing from them that can be supposed to militate against his maturer judgment." In another place he states in language still more explicit, "that Newton considered the operation of an ethereal medium as absolutely necessary to the production of the most remarkable effects of light."
In direct contradiction to these statements, we have already found Newton distinctly maintaining "that light is neither ether nor its vibrating motion, but something of a different kind propagated from lucid bodies," such as "multitudes of small and swift corpuscles of various sizes springing from shining bodies;" and when in order to please his friends and illustrate his views, he invents a speculation "not propounded to be believed," he cannot be re
The matured judgment of Newton, of which Dr. Young speaks, and against which his quotations directly militate, is given in the following explicit passage, published in 1717, in the second edition of his Optics, revised by himself.Optics, edit. 3d, 1720, pp. 336, 339.
"Are not all hypotheses erroneous in which light is supposed to consist in pression or motion propagated through a fluid medium? For in all these hypotheses the phenomena of light have been hitherto explained by supposing that they arise from new modifications of the rays, which is an erroneous supposition.
"If light consisted only in pression propagated without actual motion, it would not be able to agitate and heat the bodies which refract and reflect it. If it consisted in motion propagated to all distances in an instant, it would require an infinite force every moment in every shining particle to generate that motion. And if it consisted in pression or motion propagated either in an instant or in time, it would bend into the shadow. For pression or motion cannot be propagated in a fluid in right lines, beyond an obstacle which stops part of the motion, but will bend and spread every way into the quiescent medium which lies beyond the obstacle. . . . . .
"And it is as difficult to explain by such hypotheses ethers can be different through all space, one of which acts upon the other, and by consequence is reacted upon, without retarding, shattering, dispersing, and compounding one another's motions, is inconceivable. And against filling the heavens with fluid mediums, unless they be exceeding rare, a great objection arises from the regular and very lasting motions of the planets and comets in all manner of courses through the heavens. For thence it is manifest that the heavens are void of all sensible resistance, and by consequence of all sensible matter."
That this passage contains the mature and the latest judgment of Newton on the subject of light cannot be doubted. All the quotations from Newton referred to by Dr. Young bear the date of 1672 and 1675, and the letter to Boyle the date of 1679; but the preceding passage was published in 1704, 1717, and 1721, in the lifetime of Newton, when it was in his power to alter or retract it. But in addition to this argument, we have the evidence of Leibnitz in a letter to Huygens, dated 26th April 1694, that Newton at that time was more convinced than ever of the truth of the emission theory. "I have learned," says Leibnitz, "from Mr. Fatio,Exercitationes Mathematicæ, &c., Fascic. i. p. 173.