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or rings on the surface of the water do swell into bigger and • bigger circles about a point of it, where by the sinking of a stone the motion was begun.''

Further, he hit upon the principle of interference,' which, neglected by Huyghens and ignored by Newton, was destined, in the hands of Young and Fresnel, to afford demonstrative proof of the truth of the hypothesis roughly sketched by Hooke. In his “Micrographia' (justly styled by Pepys a * most excellent piece') he described, besides a series of beautiful observations with the microscope, the phenomenon known in optical treatises as the “colours of thin plates, and with singular sagacity declared it to form the experimentum crucis as regards chromatic light. These fantastical' tints (which we may recognise every summer's day in the iridescent glancing of some insect's wing) Hooke diligently examined in soap-bubbles, in muscovy-glass' (mica), in metallic films, and other similar substances. His explanation of what he observed contains a remarkable, although necessarily imperfect, approximation to a cardinal truth in optics. By a double reflection from two closely adjacent surfaces, he tells us,f the rays of light are broken up into confused or duplicated

pulses,' changing in tint with the varying thickness of the reflecting film. Thus, colours begin to appear, when the * pulses of light are blended so well and so near together that the sense takes them for one.' 1 According to the modern doctrine of . interference,' waves of light, pursuing each other at the distance of half an undulation, mutually destroy each other, and produce darkness. But, because difference of colour means difference of wave-length, a doubly-reflecting surface, by destroying or reinforcing, according to its varying thickness, undulations of certain lengths, analyses white light into the prismatic rays of which it is composed, and thus produces the appearances characteristic of thin plates.'

The flaw in Hooke's theory was his erroneous idea as to the nature of colour. And on this point we are unable to defend him from the charge of culpable ignorance. The true view was proposed to him, and he deliberately rejected it. The keystone of the arch he had attempted to build was offered to him, and he declined to set it in its place. On February 8, 1672, Newton's memorable paper on the composition of white light was read before the Royal Society. Had Hooke frankly accepted the discovery, and applied it as a bulwark to his own

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Micrographia, pp. 56-7.

+ Ibid. p. 66. Posthumous Works, p. 190.

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tottering hypothesis, his name would doubtless have sounded louder in the ears of posterity. But here his moral failings, as well as his intellectual shortcomings, interposed. primarily, an experimentalist. His delight was rather in the things than in the thoughts of Nature. The intimate relations of objects were of less account in his eyes than their external operation on the senses. Add to this the utilitarian tendency impressed upon physical researches by the Baconian

precepts. In the Preface to the • Micrographia' Hooke described as follows the purposes of the Royal Society: “They do not wholly * reject experiments of mere light and theory, but they princi“pally aim at such, whose application will improve and facilitate 'the present way of manual arts.' And similar declarations were made by Boyle and other leading men of the time. Thus, in Hooke's apprehension, the raison d'être of an hypothesis was not so much to suggest a physical connexion of facts as to provide a convenient classification of experiments, and its most essential quality that it should be plausible, not that it should be true.

His judgment was besides warped, even more than that of most men, hy that intellectual egotism which, if it sometimes acts as a spur to progress, more often performs the office of a drag. His self-love blinded him to the real merits of his competitors. His own speculations loomed so large before him as to exclude from his field of view those of every other. Newton acknowledged that if he saw farther than most men, .by standing on the shoulders of giants.' Hooke thought his own mental stature sufficient to entitle him to reject such extraneous aids. He accordingly set aside without hesitation Newton's discovery, offering his criticisms, not indeed discourteously, but with a certain air of superiority which not a little galled his sensitive antagonist. Matters were aggravated three years later when Newton published his beautiful explanation, on the emission hypothesis, of the colours of thin plates. Hooke declared that the main of it was contained in the “ Microis graphia,”' a remark extremely offensive to Newton, who, however, with his usual careful justice, immediately extended his somewhat scanty acknowledgment of his rival's labours, by defining with scrupulous accuracy the measure in which he was indebted to him. That Hooke was not devoid of generous sentiments appears from a letter which he wrote about this time to Newton, proposing a private correspondence on philosophical subjects.* In it he acknowledges the superior

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abilities of the great mathematician, professes a dislike to contention, and hints that their relations had been embittered by the machinations of ill-disposed persons. (Oldenburg is evidently indicated.) Newton's reply was conceived in a corresponding spirit; but the harmony thus established was unhappily not lasting

The problem of gravity was the supreme question of that time. It stood first among the orders of the day of the scientific council. It was instinctively felt that until it should be disposed of, no real progress could be made in physical knowledge. And, slowly but surely, the way was being prepared for a great discovery: Galileo had made Newton possible. Men's ideas were gradually clarifying ; the great cosmical analogies, now so familiar, were step by step emerging out of the dusk of ignorance ; antiquated prepossessions were sinking, in a sediment of cloudy cavil, out of sight. Heaven was assimilated to earth, and earth to heaven; the old gratuitous separation between the starry firmament over our heads and the solid globe under our feet was abolished by acclamation; and it was felt that the coming law, to be valid, must embrace in its operation the whole of the visible universe. Towards this consummation Gilbert contributed something by his theory of universal magnetism; and Galileo, as well as Bacon and Horrocks, foresaw that in this direction lay the coveted secret. In 1645 the Abbé Boulliau (Bullialdus) actually announced * that the force by which the sun holds the planets in their orbits must vary as the inverse square of their distance from him; in 1666, Borelli published at Florence some suggestive speculations on the subject; † in England, Wallis, Wren, and Halley, all eagerly scanned the question, and all arrived at close approximations to the truth. But it was undoubtedly Hooke whose arrow flew nearest to the mark. The first definite proposal of the planetary revolutions as a problem in mechanics is due to him ; and it has been immemorially held that prudens quæstio est dimidium scientiæ. In a paper on Gravity, presented by him to the Royal Society, March 21, 1666, the following noteworthy passage occurs :

* If such a principle (central attraction) be supposed, all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions; and possibly the prosecuting this speculation may give us a true hypothesis of their motion, and from

* Astronomia Philolaica. Paris, 1645.
† Theoricæ Mediceorum Planetarum. Florence, 1666.

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some few observations, their motions may be so far brought to a certainty, that we may be able to calculate them to the greatest exactness and certainty that can be desired.' *

On this matter, at least, Hooke's ideas were persistent and progressive. In 1674 he announced a forthcoming system of ' the world, answering in all things to the common rules of mechanical motions, and founded on the three following suppositions:

* First, that all celestial bodies whatsoever have an attraction or gravitating power towards their own centres, whereby they attract not only their own parts . . . but also all the other celestial bodies that are within the sphere of their activity. Second, that all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a straight line till they are, by some other effectual powers, deflected and sent into a motion describing a circle, ellipsis, or some other more compounded curve line. Third, that these attractive powers are so much the more powerful in operating by how much the nearer the body wrought upon is to their own centres. Now, what these several degrees are, I have not yet experimentally verified, but it is a notion which, if fully prosecuted, as it ought to be, will mightily assist the astronomer to reduce all the celestial motions to a certain rule, which I doubt will never be done without it. But this I durst promise the undertaker, that he will find all the great motions of the world to be influenced by this principle, and that the true understanding thereof will be the true perfection of astronomy.' †

Our readers will perceive that he was at this time still at fault as to the rate of decrease of the central force; but, some years later, this too was divined by him—divined, not demonstrated. In 1679 he wrote to Newton, suggesting the law of inverse squares, or reciprocal duplicate proportion, and it was this letter which led the Cambridge philosopher to resume ' his former thoughts concerning the moon.' I He first, as is well known, attempted the problem of assimilating the force of gravity at the earth's surface to the deflecting power exerted on the moon's orbital motion, in 1665, when he gathered'the duplicate proportion from Kepler's third law; but the defective data then at his command obliged him to suspend his speculations. Now, with the results of Picard's improved degree measurement in his hands, he once more set bis gigantic powers to their equally gigantic task. Having made some progress with the calculations, he, however, again threw them

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* Birch, The History of the Royal Society,' vol. ii. p. 91.

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† An Attempt to prove the Motion of the Earth, p. 28.

Brewster, Life of Newton,' vol. i. p. 291.

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* by, being upon other studies; ? * and it required a further fillip to induce him to complete

them. It was given thus. One January day in 1684, Edmund Halley, a young and rising astronomer, having independently worked out the problem so far as to perceive the necessity for the ratio of inverse squares, came to town from Islington, and, falling into discourse with Wren and Hooke on the subject, the latter affirmed that upon that principle all the laws of the celestial motions were to be demonstrated, and that he himself had * done it. I declared, continues Halley,f 'the ill-success of 'my attempts, and Sir Christopher, to encourage the enquiry,

said that he would give Mr. Hooke some two months' time to 'bring him a convincing demonstration thereof, and besides ‘the honour, he of us that did it should have from him a pre'sent of a book of 40 shillings. Mr. Hooke then said he had it, but should conceal it for some time, that others trying and failing might know how to value it when he should make it * public. However, I remember that Sir Christopher was little

satisfied that he could do it, and though Mr. Hooke then ' promised to show it him, I do not find that in that particular • he has been so good as his word.'

The two months' interval allowed by Wren for the production of the desired solution elapsed four times over, and Hooke made no sign. Then, at last, Halley started for Cambridge, and laid the difficulty before Newton. In after life he was accustomed to boast that he had been the Ulysses who pro• duced this Achilles.'For the result of his visit was the • Principia.' The most painful passage in Hooke's life now comes before

When the first book of his rival's immortal work was, on April 28, 1686, received by the Royal Society with the applause which it deserved, he was unable to restrain his jealous disappointment within the bounds of moderation or decency. He quarrelled with the President for overlooking his prior claims; he endeavoured to persuade the members that Newton was indebted to him for the first hint of a discovery which he pretended was but a small part of what he himself had conceived, and was engaged in perfecting; he did not attempt to conceal that he regarded Newton's triumph in the light of a personal injury. When this strange carriage' was reported (probably with some exaggeration) to Newton, he was, not

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* Letter to Halley, quoted by Brewster, vol. i. p. 292.
+ Letter to Newton, quoted by Brewster, vol. i. p. 293, note.
I Brewster, Life of Newton, vol. i. p. 298.

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