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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. Collins informs his correspondent, James Gregory, that he had not written to Newton or even seen him for these 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 withthe subject of the moon's libration. In a letter to Oldenburg in 1673, in reference to Huygens's work on Central Forces, he mentions that "he had sometimes thought that the moon's libration might depend upon her 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. Galileo had discovered and explained the diurnal libration, arising from the spectator not viewing the moon from the centre of the earth, but it was reserved for Newton to explain the libration in longitude, which Hevelius, its discoverer, had ascribed to the displacement of the centre of the moon's orbit from the centre of motion. He shewed that it was occasioned by the inequalities of the moon's motion in an elliptic orbit round the earth, combined with the uniformity of her motion round her axis. In the same letter to Mercator he shewed that the libration in latitude arose from her axis of rotation being inclined 88° 17′ to the ecliptic.
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.<129>
"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 theyfind does pretty well, but the cyder will not keep above a year, whereas that made of Red Streaks in other parts will keep three years or more. The ill success of Red Streaks here, I perceive, is generally imputed to the soil; but since the tree thrives, and bears as well here as in other parts, I am apt to think it is in the manner of making the cyder. For upon inquiry of the gardiners, I cannot find that they mixed any other fruit with the Red Streaks, which I have been told they do in the cyder countries, and am apt to believe it necessary; the juice of the finer fruit, oil the one hand, sweetening and ripening the harsh juice of the Red Streaks, as that juice, on the other hand, by its slow ripening, makes the cyder keep long. Sir, if this prejudice we have against Red Streaks could be removed, it would much promote the design of planting, and double the benefit of it to us by bettering the cyder; and therefore I make bold to desire you to inform me, if you know of any practical description of making cyder, printed in any author; and if not, to desire you, if it lye in your way at any time, to inquire, about the manner of making and ordering of it. For which end give me leave to make these queries: — What sort of fruit are best to be used, and in what proportion they are to be mixed, and what degree of ripeness they ought to have? Whether it be material to press them as soon as gathered, or to pare them? Whether there be any circumstances to be observed in pressing them? or what is the best way to do it? If you can direct us to, or procure for us a short narrative of the way of making and ordering cyder in the cyder countries, which takes in a resolution of these, or the most material of these queries, you will oblige your humble servant,
"Sir. If my last letter be not yet sent to Mr. Lucas, I desire you would, for preventing any suspicion of insincerity, insert this parenthesis (as is well known here) between the words [and written a tractate on that subject,] and [wherein I had set down] in the latter part of my letter."
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."
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.
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, heintimated to Oldenburg, "that he had some thoughts of writing a further discourse about colours, to be read at one of your assemblies, but find it yet against the grain to put pen to paper any more on that subject. But, however, I have one discourse by me on that subject, written when I sent my first letter to you about colours, and of which I then gave you notice. This you may command when you think it may be convenient, if the custom of reading weekly discourses still continue." Mr. Oldenburg having been desired by the Society to thank him for this offer, and to desire him to send this discourse as soon as he pleased, Newton again writes to him on the 30th November," that he intended to have sent the papers this week, but that upon reviewing them it came into his mind to write another little scribble to accompany them." This little scribble was his "Hypothesis," to which we shall presently refer.
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 them. But as they are, I hope you will accept of them, though not worth the ample thanks you sent. I remember in some discourse with Mr. Hooke, I happened to say that I thought light was reflected, not by the parts of glass, water, air, or other sensible bodies, but by the same confine or superficies of the ethereal medium which refracts it, the rays finding some difficulty to get through it in passing out of the denser into the rarer medium, and a greater difficulty in passing out of the rarer into the denser; and so being either refracted or reflected by that superficies, as the circumstances they happened to be in at their incidence make them able or unable to get through it. And for confirmation of this, I said further, that I thought the reflexion of light, at its tending out of glass into air, would not be diminished or weakened by drawing away the air in an air-pump, as it ought to be if they were the parts of air that reflected; and added, that I had not tried this experiment, but thought he was not unacquainted with notions of this kind. To which he replied, that the notion was new, and he would the first opportunity try the experiment I propounded. But upon reviewing the papers I sent you, I found it there set down for trial; which makes me recollect that about the time I was writing these papers, I had occasionally observed in an air-pump here at Christ's College, that I could not perceive the reflexion of the inside of the glass diminished in drawing out the air. This I thought fit to mention, lest my former forgetfulness, through my having long laid aside my thoughts on these things, should make me seem to have set down for certain what I never tried.
"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,means to engage me in vain disputes; but I hope a declared resolution to answer nothing that looks like a controversy, unless possibly at my own time upon some by-occasion, may defend me from that fear. And therefore, considering that such an hypothesis would much illustrate the papers I promised to send you, and having a little time this last week to spare, I have not scrupled to describe one, so far as I could on a sudden recollect my thoughts about it; not concerning myself, whether it should be thought probable or improbable, so it do but render the paper I send you, and others sent formerly, more intelligible. You may see by the scratching and interlining it was done in haste; and I have not had time to get it transcribed, which makes me say I reserve a liberty of adding to it, and desire that you would return these and the other papers when you have done with them. I doubt there is too much to be read at one time, but you will soon see how to order that. At the end of the hypothesis you will see a paragraph, to be inserted as is there directed. I should have added another or two, but I had not time, and such as it is I hope you will accept it. —
The Hypothesis, to which this letter is introductory, possesses many points of historical interest. Descartes was the first philosopher who maintained the existence of an ether, a medium more subtle than air, filling the interstices of air, and occupying the pores of glass and all transparent bodies. He considered the ether to be composed of a continued series of molecular globules, along which a motion was propagated constituting light andcolour. Dr. Hooke, who adopted the general view of Descartes, maintained that "the parts of bodies when briskly agitated excite vibrations in the ether which are propagated every way from these bodies in straight lines, and cause a sensation of light by beating and dashing against the bottom of the eye; something after the manner that vibrations in the air cause a sensation of sound by beating against the organs of hearing." In his reply to Hooke, on the 11th of July 1673, Newton distinctly states that this, which he calls the fundamental supposition in Hooke's hypothesis, "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 deepviolet, and the intermediate ones of intermediate colours." Now this modification of Hooke's hypothesis has been very erroneously regarded as an expression of Sir Isaac's own views, whereas he merely gives it as a better form of a hypothesis, the fundamental position of which he pronounces impossible, and contrary both to experiment and demonstration. In judging of Sir Isaac's Hypothesis of 1675, it is necessary to keep this in view, as it appears to be quite clear that this hypothesis is not what he believes, but what he found it necessary to draw up for the information of many of his friends. "Having observed," he says, "the heads of some great virtuosos to run much upon hypotheses, as if my discourses wanted a hypothesis to explain them by, and found that some, when I could not make them take my meaning, when I spoke of the nature of light and colours abstractedly, have readily apprehended it when I 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 condensedby precipitation, and after condensation, wrought into various forms, at first by the immediate hand of the Creator, and ever since by the power of nature; which, by virtue of the command, increase and multiply, became a complete imitator of the copies set her by the protoplast." "Thus," he adds, "perhaps may all things be originated from ether." Newton then proceeds to describe an electrical experiment, which afterwards excited much interest in the Society. He laid upon a table a round piece of glass about two inches broad, set in a brass ring, so as to keep the glass about the sixth of an inch from the table, the air being enclosed on all sides by the ring. Having placed some small pieces of paper within the ring, and rubbed the glass briskly with some rough substance, the pieces of thin paper began to be attracted and fly about even after the friction had ceased. From this result he conceived that some subtle matter lying condensed in the glass was rarefied by friction as water is rarefied into vapour by heat, and by "moving and circulating variously, actuate the pieces of paper till it returns into the glass and be recondensed there." He next supposes that this ether may be imbibed by the earth, and also copiously by the sun, in order to preserve his shining, and keep the planets from receding farther from him; that is, to increase his "gravitating attraction, which may be caused by the continual condensation of some very subtle gummy or unctuous substance diffused through the ether. And as if he were amusing himself with the extravagance of his speculations, he adds, "And 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 at Kepler's firm belief that the earth and other planets are enormous living animals taking their daily and nightly alternations of sleeping and waking, we may be allowed to smile when Newton condescends to feed them with the nectar and ambrosia of the ethereal domains. In the same extravagance of speculation he supposes that the soul may have an immediate power over the whole ether in any part of the body, producing, by processes which he invents, the swelling and shrinking of the muscles, and the animal motions which result from it.
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. Newtononly carried farther in some particulars." When this remark was communicated to Newton, he seems to have been greatly offended, and, on the 21st December, he wrote a letter to Oldenburg, pointing out the difference between his hypothesis and that of Dr. Hooke. Although "he is not much concerned at the liberty of Mr. Hooke's insinuation," yet he wishes to "avoid the savour of having done any thing unjustifiable or unhandsome" to him. He therefore separates the part of the hypothesis that belongs to Descartes and others, and leaves to Hooke the merit of having changed Descartes' progressive motion of the ether into a vibrating one, — "the rotation of the globuli to the obliquation of pulses, and the accelerating their rotation on the one hand, and retarding it on the other, by the quiescent medium to produce colours, to the like action of the medium on the two ends of his pulse for the same end." He gives Hooke the credit also of explaining the phenomena of thin plates, and also the colours of natural bodies, fluid and solid. In the other two paragraphs of the letter, he details more specifically the difference between his explanations and those of his rival.
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 hadoccasion to animadvert on the conduct of Hooke, will peruse this letter with much satisfaction.
Robert Hooke — "These to my much esteemed friend, Mr. Isaack Newton, at his chambers in Trinity College in Cambridge.
"Sir., — The hearing a letter of yours read last week in the meeting of the Royal Society, made me suspect that you might have been some way or other misinformed concerning me; and this suspicion was the more prevalent with me, when I called to mind the experience I have formerly had of the like sinister practices. I have therefore taken the freedom, which I hope I may be allowed in philosophical matters to acquaint you of myself. First, that I doe noe ways approve of contention, or feuding or proving in print, and shall be very unwillingly drawn to such kind of warre. Next, that I have a mind very desirous of, and very ready to embrace any truth that shall be discovered, though it may much thwart or contradict any opinions or notions I have formerly embraced as such. Thirdly, that I do justly value your excellent disquisitions, and am extremely well pleased to see those notions promoted and improved which I long since began, but had not time to compleat. That I judge you have gone farther in that affair much than I did, and that as I judge you cannot meet with any subject more worthy your contemplation, so I believe the subject cannot meet with a fitter and more able person to inquire into it than yourself, who are every way accomplished to compleat, rectify, and reform what were the sentiments of my younger studies, which I designed to have done somewhat at myself, if my other more troublesome employments would have permitted, though I am sufficiently sensible it would have been with abilities much inferior to yours. Your design and mine are, I suppose, both at the same thing, which is the discovery of truth, and I suppose we can both endure to hear objections, so as they come not in a manner of open hostility, and have minds equally inclined to yield to the plainest deductions of reason from experiment. If, therefore, you will please to correspond about such matters by private letters, I shall very gladly embrace it; and when I shall have the happiness to peruse your excellent discourse, (which I can as yet understand nothing more of by hearing it cursorily read,) I shall, if it be not ungrateful to you, send you freely my objections, if I have any, or my concurrences, if I am convinced, which is the more likely. This way of contending, I believe, to be the more philosophical of the two, for though I confess the collision of two hard-to-yield contenders may produce light, [yet] if they be put together by the ears by other's hands and incentives, it will [produce rath]er ill concomitant heat, which serves for no other use but . . . . . . kindle — cole. Sr, 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 correspondence. What's done before many witnesses is seldom without some further concerns than that for truth; but what passes between friends in private, usually deserves the name of consultation rather than contention; and so I hope it will prove between you and me. Your animadversions will therefore be welcome to me; for though I was formerly tyred of this subject by the frequent interruptions it caused to me, and have not yet, nor I believe ever shall recover so much love for it as to delight in spending time about it; yet to have at once in short the strongest objections that may be made, I would really desire, and know no man better able to furnish me with them than yourself. In this you will oblige me, and if there be any thing else in my papers in which you apprehend I have assumed too . . . . . . . If you please to reserve your sentiments of it for a private letter, I hope you [will find that I] am not so much in love with philosophical productions, but that I can make them yield. . . . . . But, in the mean time, you defer 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 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 this respect to you, as you would to me. But not to insist on this, your letter gives me occasion to enquire regarding an observation you was propounding to me to make here of the transit of a star near the zenith. I came out of London some days sooner than I told you of, it falling out so that I was to meet a friend then at Newmarket, and so missed of your intended directions; yet I called at your lodgings a day [or] two before I came away, but missed of you. If, therefore, you continue . . . . . . to have it observed, you may, by sending your directions, command . . . . . . your humble servant,
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 respecting the invention of pendulum clocks. These disputes were running high about the time when Newton's discourse on colours was before the Royal Society, and in both of them Oldenburg took a keen and active part against Hooke. It was, therefore, no improbable supposition, that in communicating to Newton what Hooke had said at the Society, Oldenburg had given it too high a colouring, or even artfully misrepresented it. In a subsequent dispute, in 1686, about the law of gravity, when Newton made some severe animadversions on Hooke's claim, Dr. Halley informs him in reply, that "he feared Mr. Hooke's manner 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 ofour common nature, and influenced by the charity which thinketh no evil, we may find in the physical constitution and social position of Hooke, and to a certain extent in the injustice of his enemies, some apology for that jealousy and quickness of temper which may have been more deeply regretted by himself than it was felt by others.
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 exhalations, which we call the atmosphere, to be nothing else but the particles of all sorts of bodies of which the earth consists, separated from one another, and kept at a distance by the said principle, and he concludes this remarkable speculation with a conjecture about the cause of gravity.
"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."
The Hypothesis of Newton, and his other speculations regarding ether, have led some writers to suppose that hehad abandoned the corpuscular or emission theory, in which light is supposed to be produced by material particles projected from luminous bodies, and that he had adopted views not very different from those of the supporters of the undulatory theory. This opinion has been entertained chiefly on the authority of Dr. Thomas Young, in his theory of light and colours. In introducing this theory, he remarks, that "a more extensive examination of Newton's writings has shown me, that he was in reality the first that suggested such a theory as I shall endeavour to maintain; and that his own opinion varies less from this theory than is now almost universally supposed." "I shall collect," he adds, "from Newton's various writings, such passages as seem to be most favourable to its admission, (Dr. Young's theory,) and although I shall quote some papers which may be thought to have been 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 regarded as maintaining views at all approximating to the undulatory theory. We cannot understand how Dr. Young could overlook the language of caution in which he everywhere guards himself against its being supposed that he believes even in the existence of an ether, — language, too, so precise, that the honest meaning of its author cannot be misinterpreted.
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.
"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 hypotheseshow rays can be alternately in fits of easy reflexion and easy transmission; unless perhaps one might suppose that there are in all space two ethereal vibrating mediums, and that the vibrations of one of them constitute light, and the vibrations of the other are swifter, and as often as they overtake the vibrations of the first, put them into those fits. But how two 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, by one of his friends, that Mr. Newton and he have been more than ever led to believe that light consists of bodies which come actually tous from the sun, and that it is in this way that they explain the different refrangibility of light and colours, as if there were primitive bodies which always kept their colours, and which come materially from the sun to us. The thing is not impossible, but it appears to me difficult to understand how by means of these little arrows which, according to them, the sun darts, we can explain the laws of refraction."
 October 19, 1675. Macclesfield Correspondence, vol. ii. P. 280.
 "Harum . . . librationum causas Hypothesi elegantissima explicavit nobis vir cl. Isaac Newton, cujus humanitati hoc et aliis nominibus plurimum debere me lubens profiteor." — Mercator's Institutiones Astronomicæ; , p. 286.
 Newton's letter had been forwarded to Mr. Lucas, and therefore the sentence does not appear in it. — See Phil. Trans., No. 128, p. 703.
 Eddleston's Correspondence, App. No. xvi. p. 260.
 He wrote a work entitled, Herefordshire Orchards a Pattern for England, 1656. See Birch's Hist. of the Royal Society, vol. iv. p. 235.
 See Appendix, No. II.
 Dr. Whewell states that Descartes regarded light as "consisting of small particles 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.
 Newtoni Opera, tom. iv. pp. 325, 326.
 Phil. Trans., 1672, No. 88, p. 5088.
 Newtoni Opera, tom. iv. pp. 378-381; or Birch, vol. iii. p. 278.
 In a paper entitled "Observations," which accompanied this letter, but which was not printed, Newton says that Hooke, in his 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."
 In his Optics, published many years after this, in 1704, Newton does not give Hooke the credit of having made these observations.
 Letter to Boyle, Newtoni Opera, tom. iv. pp. 385-395.
 Phil. Trans., 1801; or Lectures on Natural Philosophy, vol. ii. p. 614.
 Ibid., vol. i. p. 477.
 Optics, edit. 3d, 1720, pp. 336, 339.
 Fatio D'huillier, the particular friend of Newton.
 Huygenii Exercitationes Mathematicæ, &c., Fascic. i. p. 173.