Popular Science Monthly/Volume 17/October 1880/The English Precursors of Newton II
ROBERT HOOKE was born at Freshwater, in the Isle of Wight, July 18, 1635. Like Newton, he was a sickly child, and, like Newton too, his early years were distinguished and diverted by his singular mechanical ingenuity. He has left it on record that, having seen an old brass clock taken to pieces, he succeeded in constructing, in imitation of it, a wooden one that would, after a fashion, go; and about the same period he rigged out a miniature ship with ropes, pulleys, and masts, besides a contrivance to make it fire off some small guns while sailing across an adjacent haven; with what childish applause and self-gratulation, we are left to imagine. Nor did his sole gifts lie in this direction. His literary aptitude was beyond the common, and he showed a marked taste for music and painting. His education was as various as his talents. His father, who was minister of the parish, destined him for his own profession; but his infirm health precluded serious study, and it was consequently proposed to bind him apprentice to a watchmaker, or some similarly skilled artisan. After his father's death in 1648, his artistic tendencies so far got the upper hand, that we hear of him in the workshop of Sir Peter Lely, where, however, his occupation seems to have been nothing more æsthetic than color-grinding. Either this preliminary stage of art disgusted him, or (as his biographers prefer to state) the smell of oil-paint aggravated his constitutional headaches, and he was transferred to the care of Dr. Busby, the celebrated master of Westminster School, who kept him gratuitously in his own house for several years. Here his education, properly speaking, may be said to have begun. He not only acquired a competent knowledge of Latin and Greek, with a tincture of Hebrew and other Oriental languages, but is said to have astonished his teachers by mastering the first six books of Euclid in as many days, and by playing, without instruction, twenty lessons on the organ. In 1653 he entered Christ Church, Oxford, as servitor to a Mr. Goodman; and ten years later received, on the nomination of Lord Clarendon, then Chancellor of the University, the degree of Master of Arts, which his poverty had perhaps prevented him from taking in the ordinary course. In 1654 the Hon. Robert Boyle, having finished his travels in Italy and his studies at Leyden, came to reside at Oxford. This amiable and ingenious gentleman has been quaintly panegyrized by an Irish humorist as "the father of chemistry and brother of the Earl of Cork." Although the clauses of this eulogy command different degrees of assent, and claim different kinds of esteem, they may be taken together as roughly summarizing the merits of its subject in the popular apprehension of that time. He was infected to an extraordinary extent with the prevailing experimental fervor, and contributed perhaps more than any of his contemporaries to advance the credit and promote the cultivation of science. The tinge of credulity which occasionally colored his inquiries may be excused (in the words of Bacon's apology for corruption) as vitium temporis non hominis, and we suppress a smile at his solemn testamentary disposition of an infallible recipe for "multiplying gold," when we find Newton and Locke the eager recipients of the secret.
Several members of the "Philosophical or Invisible College" of London finding themselves about this time together at Oxford, their discussions were resumed, and Hooke's singular mechanical skill quickly brought him to their notice. Boyle at once attached him to himself, and, if we are to believe what Anthony Wood tells us, was glad to improve his foreign acquirements by receiving from the young servitor instruction in Euclid, and some much-needed light on the Cartesian philosophy. What is more certain is, that Hooke constructed for him an air-pump vastly superior in design to that recently contrived at Magdeburg by Otto Guericke, and differing in no essential particular from that now in use. He further devised thirty different modes of flying, and emulated Archytas in the production of a "Module" (we quote his own words), "which, by the help of springs and wings, raised and sustained itself in the air; but finding," he adds, "by my own trials, and afterward by calculation, that the muscles of a man's body were not sufficient to do anything considerable of that kind, I applied my mind to contrive a way to make artificial muscles, divers designs whereof I shewed also at the same time to Dr. Wilkins, then Warden of Wadham College, but was in many of my trials frustrated of my expectations."
It may be conjectured that the failure of these attempts sufficed to convince the Icarus of Wadham of the impracticability of his projected lunar excursion, as well as to divert their author to less ambitious designs. The improvement of timepieces was then looked upon as the shortest road to the solution of the great practical problem of finding the longitude at sea, and in this direction, accordingly, Hooke next turned his thoughts and his experiments. He was rewarded by the discovery of a contrivance for applying springs to regulate the movement of watches. For this important invention his friends endeavored to procure him a patent, which he, however, refused, being dissatisfied with the terms proposed; and it thus remained undivulged, and by many disbelieved in. But when, in 1675, Huygens published, in the "Journal des Savants," his discovery of spiral watch-springs, Hooke indignantly claimed it as his own, incidentally attacking Oldenburg, then Secretary of the Royal Society, with whom he was never on very civil terms. A sharp paper-conflict ensued, Hooke (quite unjustifiably) accusing Oldenburg of "trafficking in intelligence," and Oldenburg retaliating with the better-founded assertion that Hooke's "pendulum watches" could never be got to go; while Huygens, who might well disdain to wrangle over so small a prize, stood aloof, and let the controversy rage. Hooke's priority, as regards the principle, is unquestionable; but it is equally unquestionable that the modification introduced by Huygens first brought the improved timepieces into general use. That modification was nothing more than the coiling into a spiral of a spring which, in Hooke's design, had remained straight. So fine is the line drawn between failure and success.
The history of this invention is, in brief, the history of Hooke's life. He was a man whose brilliant qualities were neutralized one by the other. His extraordinary ingenuity was marred by his equally extraordinary versatility. His thoughts pursued each other in a rapid succession of vivid and original suggestions; but they found no halting-place on the way. He received them with rapture, but they wearied him if they staid too long. He welcomed all, but made none his friend. He wanted that laborious passion of perfection, apart from which the progeny of invention is but a sterile brood. His mind was like a telescope without clock-work, which shows the moving host of heaven, but can not fix or observe any individual star. Thus, his discoveries and investigations were usually abandoned or postponed when on the point of completion. It was not until some other inquirer, less discursive or more discreet, added the finishing touches still wanting, that he became sensible of the full value of what he had neglected, and, with loud vociferations, stood on the highway of learning, crying "Stop thief!" to the whole scientific world. Nor was his manner of conducting these controversies happier than his choice of occasions for them. His tone in argument was at all times dictatorial, and under excitement it was apt to become shrill. By his arrogance, he exasperated his adversaries; by his irritability, he prejudiced his cause. Thus, when (as not unfrequently happened) he was in the right, he roused animosity; when he was in the wrong, he incurred discredit.
But we anticipate our narrative. The foundation of the Royal Society opened to him the road to fortune and fame. Having raised his reputation by an able paper on "Capillary Attraction," his name was placed on the first list of Fellows, and on November 12, 1662, he was unanimously elected Curator of Experiments, "with the thanks of the Society ordered to Mr. Boyle for dispensing with him for their use." He had at this time entered on his twenty-ninth year, and had within him a spirit of fire, not indeed "grossly," but most inadequately "clad" in the corporeal "dimension" of his species. Pepys, who knew him well and rated him high, notes in his "Diary" that "Mr. Hooke is the most, and promises the least, of any man in the world that ever I saw." His personal appearance, indeed, was to the last degree deplorable. His figure was crooked, his limbs shrunken and emaciated, his aspect meager, his carriage stooping. He wore his hair, which was of a dark brown color, hanging in long, disheveled locks over his face, and it was not until three years before his death that, conforming at last to the fashion of his time, he cut it off, and substituted a periwig. Up to the age of sixteen, he was said to have been straight, and he himself attributed his deformity to his excessive use when young of "incurvating exercises," such as working with a turning-lathe. Waller, his earliest biographer, tells us:
His eyes were gray and full, with a sharp ingenious look while younger; his nose thin, hut of a moderate height and length; his mouth meanly wide, and upper lip thin; his chin sharp and forehead large; his head of a middle size. . . . He went stooping and very fast, having but a light body to carry, and a great deal of spirits and activity, especially in his youth. He was of an active, restless, indefatigable genius even almost to the last, and always slept little to his death, seldom going to sleep till two, three, or four o'clock in the morning, and seldomer to bed, oftener continuing his studies all night, and taking a short nap in the day. His temper was melancholy, mistrustful, and jealous, which more increased upon him with his years. . . . He had a piercing judgment into the dispositions of others, and would sometimes give shrewd guesses and smart characters.
The extreme parsimony, which the necessities of his early life had rendered a virtue, degenerated, as years went on, into a weakness if not into a vice. After his death, a large iron chest, which it appeared by evident signs had lain undisturbed for above thirty years, was discovered in his lodgings, and on being opened was found to contain several thousand pounds in gold and silver, accumulated by him in the lucrative employment of surveyor for the rebuilding of the city after the fire of September 3, 1666. Thus he condemned himself to a life of sordid privation, while relegating to dust and cobwebs a treasure which he was too penurious to spend, and too busy even to enjoy the miser's pleasure of counting.
It is difficult to convey an adequate idea of the multifarious and unceasing activity of Hooke's intellectual life during the forty years of his connection with the Royal Society. It reflected the boundless but fortuitous curiosity of an age which had indeed realized the bold vaunt of its herald, by leaving the pillars of Hercules of ancient lore far behind; but now found itself, like Ulysses of old, embarked on a trackless ocean without any sure pilotage to the happy isles of renovated science. Hooke and his contemporaries were inflamed with the unmeasured hopes and vast ambition of the Verulamian prophecies; but they began to be more and more conscious that the Verulamian method was but as the "golden path of rays" leading to the setting sun. They were haunted by the idea that Nature was to be interrogated, not progressively or by installments, but once for all, by a supreme inductive effort, and they could not wholly relinquish the hope that they were destined to witness its consummation. They had been told to expand their souls to the measure of the universe, and they were unwilling to confess their inadequacy to the effort required of them. They were like men groping in the darkness for a door which they had but to throw wide, in order to find themselves in the full blaze of daylight; and they learned with reluctance that only by painful and prolonged exertions could they expect to open a chink here and there for a ray of twilight to enter.
This insensible change of front, as regards scientific method, is very clearly discernible in Hooke's writings. He began life with hopes as large as and more defined than those of Bacon himself. Even before he left Oxford, he had provided himself with what he called a "mechanical algebra," which he regarded as an infallible guide to invention. This he afterward expanded into an elaborate engine of discovery, competent, as he believed, to construct with certainty and swiftness an edifice of knowledge, heretofore unmatched for vastness and durability. The scheme, like all his more ambitious designs, remained incomplete, or, at most, was completed only in the mind of its author; and the tract in which he describes it breaks off just as the momentous secret is about to be disclosed. Whether it was that the difficulties in the way became more clear to him as he advanced, and that he lost faith in his own means of removing them, or whether it was that his jealousy of disclosure overbalanced, at the critical moment, his appetite for fame, we shall never know. We do know, however, enough to show us that the revelation would have been valuable only as a gratification of our curiosity, and as throwing a singular light on the visions which haunted the morning of experimental science.
The following extract from his essay on "The Present State of Natural Philosophy" briefly exposes his ideal of a method. He attempted, as will be seen, to come to closer quarters with the problem than Bacon had done, and succeeded thereby in more clearly defining its insolubility.
"Some other kind of art for inquiry," he writes, "than what hath been hitherto made use of, must be discovered; the intellect is not to be suffered to act without its helps, but is continually to be assisted by some method or engine, which shall be as a guide to regulate its actions, so as that it shall not be able to act amiss. Of this engine, no man except the incomparable Verulam hath had any thoughts, and he indeed hath promoted it to a very good pitch; but there is yet somewhat more to be added, which he seemed to want time to complete. By this, as by that art of Algebra in Geometry, 'twill be very easy to proceed in any natural inquiry, regularly and certainly: and indeed it may not improperly be called a Philosophical Algebra, or an art of directing the mind in the search after philosophical truths."
The first part only of this "Algebra of Discovery," "containing the manner of preparing the mind, and furnishing it with fit materials to work on," was written; the second, which should have set forth "the rules and methods of proceeding and operating with this so collected and qualified Supellex," remained in embryo. Hooke, like Bacon, set out with a classification of the errors incidental to humanity in its actual condition; but his mode of rectifying them was a more patient and practical process than the "expurgation of the intellect," preached by the philosophic Chancellor. The senses are to be helped, he tells us, by skillfully constructed instruments, whereby their sphere of action may be enlarged, and their untutored impressions brought to the test of exact measurement and rigid calculation. The report of one sense must be corrected by comparison with that of the others, until "sensation is reduced to a standard," and the mind is gradually informed with true notions of "things, as they are part of, and actors or patients in the universe, not only as they have this or that peculiar relation or influence on our own senses or selves."
The next step in the "Preparation" consisted in the compilation of a "Philosophical History," comprising—
A brief and plain account of a great store of choice and significant natural and artificial operations, actions, and effects, ranged in a convenient order, and interwoven here and there with some short hints of accidental remarks or theories, of corresponding or disagreeing received opinions, of doubts and queries, and the like; and, indeed, until this repository be pretty well stored with choice and sound materials, the work of raising new axioms or theories is not to be attempted, lest beginning without materials the whole design be given over in the middle.
The matter of such a history, he says further, is no less than the world; "for there is no body or operation in the universe that is not some way or other to be taken notice of in this great work." And the programme which he proceeds to lay down in no way belies his promise. Fire, air, earth, and water, light and darkness, heat and cold, gravity and levity, all the "prime sensible qualities" of nature, find each its place in this stupendous magazine of knowledge. From ether to anthracite, from a man to a mite or a mushroom, from dreams and influences to arts and sciences, from the starry firmament to the costermonger's cart or the cobbler's stall, no substance, quality, or accident is excluded. No natural process, no commercial product, but has its separate "History." The despised handicraftsman is to yield up his obscure secrets as well as the scientific artisan. A Dollond or a Steinheil is not more stimulating to the catholic curiosity of the natural historian than a Quince, a Bottom, or a Snug. Yet all this encyclopedic mass of information, infinite in its subject, indefinite in its extent, expansive in its nature, Hooke tells us he "has very good reason to believe may be contained in much fewer words than the writings of divers single authors!" This would, indeed, have been to imprison the liberated genius of knowledge within narrower limits than those of Aristotelian tradition. The seal, however, was broken; the vase was already at the bottom of the sea, and it only remained to guide and propitiate a power which it was no longer possible to confine.
The "Philosophical History," of which Hooke traced the gigantic plan, would, in fact, have included what we now understand as the whole body of inductive science, with a considerable margin of heterogeneous material, difficult of classification, and more curious, perhaps, than useful. It would have included not only an enumeration of all possible phenomena, but the knowledge of the laws by which they are governed, and the causes by which they are produced. The natural historian was to be "knowing in hypotheses," that he might set his facts in plausible sequence of cause and effect; he was to be a skilled mechanician, and an able mathematician, that he might investigate their relations by experiment, and deduce the consequences of such relations by calculation. Hooke's "Helps of Discovery" are but another form of Bacon's "Prerogative Instances"; but it is significant that in the later system they appear in the preparatory stage, while in the earlier they form an integral part of the "Organum" itself. The impossible was, in fact, relegated to a distant future, while the possible took possession of the present. The "raising of axioms," and the discovery of "forms," which were supposed to constitute the true business of the philosopher, were postponed in favor of the more modest task of setting facts in order, and connecting them by means of ideas. Thus natural philosophy, in the recondite sense in which it was understood by the theorists of the seventeenth century, came, as time went on, to be more and more fully personated by her handmaiden, "Natural History," until at last the identity of the one was completely merged in that of the other. The intermediary whom they had admitted as a messenger of higher promise, they were compelled to take for better for worse. Like Malvolio, they had wooed the mistress; like Sir Toby, they wedded the maid.
We shall conclude our remarks on this singular essay by transcribing some specimens of the queries directed by Hooke to future investigators. Even after the lapse of above two centuries, they strike us as suggestive and ingenious. Under the heading of "Ether," he asks:
Whether it permeates all bodies, be the medium of light, be the fluid body in which the air is but as a tincture? Whether it cause gravity, in the earth or other celestial bodies?
Of the atmosphere:
Whether it encompasses the sun and planets, and that each of them have a peculiar atmosphere, as well as they have a gravitating power?
Whether the spots in the sun may not be clouds of smoke or vapors, raised up into that atmosphere?Whether meteors have anything of fire in them, or whether the light may not be an effect of their rapid motion?
Although Hooke's "True Method" was not published until after his death, we may safely attribute it to an early stage of his career. He was a man whose ideas did not change, but were superseded. They retained their original form, but were crowded out of sight by the multitude of new arrivals. Now we have evidence to show that, without wholly abandoning his early faith in the efficacy of his "Philosophical Algebra," his confidence in an approaching renovation of science was replaced, later in life, by a conviction of its infinite complexity and extent. In the preface to a volume of "Lectures," published in 1674, he says:
For as there is scarce one subject of millions that may be pitched upon, but to write an exact and complete history thereof would require the whole time and attention of a man's life, and some thousands of inventions and observations to accomplish it: so on the other side no man is able to say that he will complete this or that inquiry, whatever it be (the greatest part of invention being but a lucky hit of chance, for the most part not in our own power). 'Twill be much better, therefore, to embrace the influences of Providence, and to be diligent in the inquiry of everything we meet with. For we shall quickly find that the number of considerable observations and inventions this way collected will a hundredfold outstrip those that are found by design. No man but hath some lucky hits and useful thoughts on this or that subject he is conversant about, the regarding and communicating of which might be a means to other persons highly to improve them. . . . This way is also more grateful both to the writer and the reader, who proceed with a fresh stomach upon variety, but would be weary and dull'd if necessitated to dwell too long upon one subject.
Thus we see that discovery, which speculation had proclaimed to be the infallible result of system, was by experience declared to be the lucky outcome of chance. Investigators had previously been commanded to march in a compact army along the highway of method toward the metropolis of knowledge; they were now warned to disperse in all possible directions into the wilderness of phenomena, and beat the bushes of nature for what game they might contain. That one view was equally misleading with the other is obvious; that one should form the reaction from the other was inevitable. Hooke's reasons for discursiveness were not so much the guide of his conduct as its apology. His position as Curator of Experiments to a body inordinately greedy of scientific novelty suggested a wide range of subjects for inquiry, which his native versatility induced him to embrace to its fullest extent. The journals and registers of the Royal Society alone convey, by their records, an adequate idea of his prodigious activity of mind, fertility of resource, and experimental skill. Astronomy, optics, acoustics, thermotics, pneumatics, hydrostatics, magnetism, and chemistry; geology, physiology, meteorology, and psychology—all in turn engaged his attention, and all in turn received illustrations from his sagacity, and impulses from his zeal. Of all men who ever lived, he was perhaps the most prolific in mechanical invention. New instruments, or useful modifications of those already in use, flowed from him by the dozen. An arithmetical machine, a triple writing-machine, a deep-sea sounding machine, a wind-gauge, rain-gauge, hygrometer and odometer, a system of telescopic telegraphy, a "water-poise," a "weather-clock," and a species of microphone, were all due to his ingenuity; besides important improvements in astronomical and other instruments—telescopes, quadrants, micrometers, diving-bells, barometers, thermometers, and balances. He speculated curiously on memory, and calculated the number of ideas of which the human mind is susceptible, estimating it at three thousand one hundred and fifty-five million seven hundred and sixty thousand! He constructed a model for the rebuilding of London after the great fire, which was approved, although not adopted; and was the architect of Hoxton Hospital and other buildings. He read before the Royal Society commentaries on Ovid's "Metamorphoses," Plato's "Atlantis," and Hanno's "Periplus," interpolating these critical excursions between geological theories and astronomical observations. To him was due the ingenious idea of measuring the force of gravity at different altitudes by the rate of vibration of a pendulum of a given length; as well as the determination (so far as the actual state of chemical knowledge permitted it to be determined) of the true function of the air in combustion and respiration. His zeal carried him to the length of making, in an exhausted receiver, his own person the subject of his observations—"the only experiment of that kind," his biographer naïvely remarks, "I think ever tried."
At the present time, when weather prophecies have come to form a recognized part of our complex social machinery, it would be ungrateful to omit noticing that Hooke was the first to propose a scientific system of meteorological forecasting. His scheme, as might be expected, had for its basis the close association (remarked by him among the earliest) of changes of weather with barometrical variations; which, he writes to Boyle, October 6, 1664—
If it continue to do as I have hitherto observed it, I hope it will help us one step toward the raising a theorieal pillar or pyramid, from the top of which, when raised and ascended, we may be able to see the mutations of the weather at some distance, before they approach us; and thereby being able to predict and forewarn, many dangers may be prevented, and the good of mankind very much promoted.
The means recommended by him for the furtherance of this noteworthy object were the same in principle as those now in use at all the meteorological observatories of Europe and America. Two hundred years, however, had to elapse before they could be profitably employed. In his "Method for making a History of the Weather," the attention of observers is especially directed to the following "particulars," as "requisite for the raising of axioms whereby the cause or laws of weather may be found out": 1. The strength and quarter of the winds. 2. The degrees of heat and cold. 3. The degrees of dryness and moisture observed with a hygroscope "made with the single beard of a wild oat perfectly ripe, set upright and headed with an index." 4. The degrees of pressure of the air. 5. The constitution and face of the sky.
It is perhaps worth remarking that our present system of meteorological observations corresponds with tolerable accuracy to Bacon's notion of how a "history" of any special branch of physics should be compiled; with this difference in result, that, instead of arriving at "axioms" and "forms," we have as yet obtained only a set of empirical rules which, however practically useful, can scarcely be said to constitute a science.
"Discoursed with Mr. Hooke," Pepys wrote, August 8, 1666, "about the nature of sounds, and he did make me understand the nature of musicall sounds made by strings, mighty prettily; and told me that having come to a certain number of vibrations proper to make any tone, he is able to tell how many strokes a fly makes with her wings (those flies that hum in their flying) by the note that it answers to in musique, during their flying. That, I suppose, is a little too much refined, but his discourse in general of sound was mighty fine."
Notwithstanding Mr. Pepys's skepticism, Hooke was on this occasion not "refining" overmuch. He exhibited in 1681 an instrument (with the principle of which he had doubtless long been acquainted) for counting the pulsations of sound, which seems to have been virtually identical with that now known as "Savart's Wheel." He also anticipated Chladni's celebrated experiment by strewing flour on a vibrating glass bell, thus presenting to the eye, as it were, a picture of the configuration of rest and motion on its surface. It was one of his favorite ideas that, by some future discovery, the sense of hearing would be reënforced as prodigiously as the sense of sight had already been by the telescope—an intuition singularly realized by the recent invention of the telephone. "It has not yet been thoroughly examined," he wrote in 1664, "how far Otocousticons maybe improved, nor what other ways there may be of quickening our hearing, or conveying sound through other bodies than the air." "By very casual trials," he tells us elsewhere, he had made some progress in this direction, and was by no means convinced that they might not be prosecuted so far as to render audible noises made at the distance of the planets! Although acknowledging that to his own prejudices this seemed "a very extravagant conjecture, . . . yet methinks," he adds, "I should have had the same thoughts of a conjecture to find out a help for the eye to see the smaller parts and rocks of the moon," and "would fain persuade myself against concluding or building on the impossibility of such things as I am not able demonstrably to prove not possible."
Of Hooke's private and personal history there is little to be recorded. His life might almost be comprised in two words—experiments and controversies. In 1664 Sir John Cutler instituted, especially for his benefit, a mechanical lecturership of fifty pounds a year; in the following year he was appointed to the professorship of Geometry founded by Sir Thomas Gresham in 1575. His services as curator were remunerated by an annual stipend of thirty pounds, not perhaps very regularly paid, since we hear, on one occasion, that both he and Halley were offered, in lieu of their respective salaries, an equivalent number of copies of that unlucky "History of Fishes," by the publication of which the Royal Society had drained their finances and cumbered their shelves. The famous controversy between Hooke and Hevelius on the subject of plain or telescopic sights, which agitated the learned world of Europe during many years, has long ago sunk into a silence we need not disturb. Hevelius was in the wrong, and obstinate; Hooke was in the right, but offensive. Astronomers in general seemed disposed to prefer some slight uncertainty as to the position of the stars, to being bullied into precision by the magisterial little hunchback of Gresham College. The dispute remained long in the condition of a smoldering flame, with outbreaks of argument at distant intervals, and Halley's mission of conciliation in 1679 helped to soothe the vanity of the irritated philosopher of Dantzic, but did not tend to rectify his method.
We now come to the relations of Hooke with Newton. The first collision between these two remarkable men occurred on the subject of their respective optical discoveries. Hooke's merits in this direction were very considerable. He was the first to propound that view as to the nature of light now universally accepted under the name of the "undulatory theory." He held that light is a "very short vibrative motion," originating in an agitation of the minute particles of the luminous body, and propagated through a perfectly homogeneous and elastic medium "by direct or straight lines, extended every way, like rays from the center of a sphere, . . . just after the same manner (though indefinitely swifter) as the waves 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 Huygens 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 "colors of thin plates," and with singular sagacity declared it to form the experimentum crucis as regards chromatic light. These "fantastical" tints (which we may recognize 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, the rays of light are broken up into "confused or duplicated pulses," changing in tint with the varying thickness of the reflecting film. Thus, "colors begin to appear, when the pulses of light are blended so well and so near together that the sense takes them for one." 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 color means difference of wave-length, a doubly-reflecting surface, by destroying or reënforcing, according to its varying thickness, undulations of certain lengths, analyzes 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 color. 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 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. He was, 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 principally 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 connection 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, by 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, it was "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 colors of thin plates. Hooke declared that "the main of it was contained in the 'Micrographia,'" a remark extremely offensive to Newton, who, however, with his usual careful justice, immediately extended his somewhat scanty acknowledgment of his rival's labors, 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 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. Toward 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 quwstio 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 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:
1. 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. 2. 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. 3. 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 the 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." 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 his gigantic powers to their equally gigantic task. Having made some progress with the calculations, he, however, again "threw them 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 great 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, "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 present of a book of forty 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 produced this Achilles." For the result of his visit was the "Principia."
The most painful passage in Hooke's life now comes before us. 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 quarreled with the President for overlooking his prior claims; he endeavored 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 unreasonably, incensed, and wrote to Halley concerning it in somewhat acrimonious terms. Halley, who seems to have acted throughout a very creditable part, replied by urging that Hooke's conduct had been represented in worse colors than it deserved; whereupon Newton not only expressed his regret for the angry "postscript to his last," but agreed, with the view of "composing the dispute," to insert into the text of his book the following acknowledgment:
"The inverse law of gravity holds in all the celestial motions, as was discovered also independently by my countrymen, Wren, Hooke, and Halley."
How far Hooke was pacified by this concession does not appear; but there is evidence that be continued, although in a lower key, to claim ownership in the discovery of gravity. It was, indeed, difficult for him to see with equanimity the great scientific prize of the century, which he had set before him as the crowning glory of his own career, carried off before his eyes by a swifter competitor; and he could not be expected to recognize, what to us is evident enough, that his powers were wholly unequal to the unique achievement of his rival. The intuition of a discovery is one thing, its demonstration another; and, while the one excites our interest and curiosity, it is to the other that we justly apportion our unqualified admiration.
Between Hooke and Newton no further intercourse seems at any time to have been set on foot. If Hooke was jealous of Newton, Newton was perhaps somewhat ungenerous toward Hooke. He recognized his merits with reluctance, and acknowledged his inventions only by compulsion. Broils and disquietudes, and the fomentors and originators thereof, were in truth odious to him; and he was at all times disposed to conceal a discovery, rather than risk a controversy. "Philosophy," he wrote to Halley, "is such an impertinently litigious lady, that a man had as good be engaged in lawsuits as have to do with her." Thus the turmoil raised by Hooke on the appearance of the first part of the "Principia" inspired him with so deep a disgust that he seriously contemplated suppressing the remainder; and he could never be induced to publish his work on "Optics" until the death of his unquiet opponent had secured for it a peaceful reception. But the most significant fact as regards the relations of these two men is that Newton, who, during Hooke's lifetime had never sat at the council-table of the Royal Society, was, only a few months after his decease, elected both to that position and the still higher one of President, on the same day, November 30, 1703.
Not much now remains to be said. Hooke's growing infirmities of mind and body condemned him to isolation; and isolation is the chosen ally of eccentricity. Repeated disappointments had aggravated the inherent moroseness of his disposition; increasing ill health soured his naturally irritable temper; and the death, in 1687, of his niece, Mrs. Grace Hooke—probably the only person in the world for whom he entertained a sincere attachment—broke the last link uniting him to every-day humanity. Still he pursued his investigations with a feverish energy that age and sickness seemed rather to stimulate than to quell. His jealousy of piratical appropriation increased, with advancing years, almost to a mania; he enveloped his researches in a mysterious reserve; and many of the discoveries which he professed to have made, descended with him into the grave. Among these were a means of finding the longitude at sea, and a secret for perfecting all kinds of optical instruments. It might be conjectured, from the small size of some telescopes used by him, that this latter invention approached that of achromatism (made by Dollond in the middle of the following century); but, on the other hand, we find him laying it down as an axiom that increased power could only be obtained by increased focal length; and he is even said to have entertained as a possibility the construction of an instrument ten thousand feet long, which should bring into view the inhabitants of the moon! We can not, indeed, take his own word for his performances. He was probably not deliberately untruthful; but he was sanguine as well as vain, and apt to discourse largely of results, toward which imagination pointed, but which reason had not yet grasped. The Royal Society, at any rate, so far believed his professions as to make him, in 1696, a grant for the purpose of completing his researches and recording his discoveries. The remaining years of his life and his failing physical powers were dedicated, with almost insane zeal, to the task of raising an adequate monument to his experimental genius. Disease was powerless to divert him from his purpose; fatigue never seemed to approach him. Day after day, and night after night, he meditated, experimented, invented. For several years before his death, he was said never to have undressed or gone to bed. His limbs swelled, his brain reeled, his very eyesight failed; but still he worked, and wrote, and dreamed of immortality. At length a summons came which he was powerless to resist. He died on March 3, 1703, unloved, unlamented, and, at least in his own apprehension, unrecognized. He died, as he had lived, haunted by unfulfilled hopes, and deluded with abortive projects. In the midst of voluntary destitution, he had cherished a magnificent design for the endowment of the Royal Society. But he left no testamentary disposition of his hoarded wealth, which proved as barren after his death as it had been during his life.
Imprisoned in his own egotism, he did not know how to contribute his quota generously to the long day's labor of humanity. He sought to set his trade-mark on every thought. He would have desired a patent of protection for every experiment. His work was in consequence visited with the curse of sterility. A slave to meum and tuum—in his own words, "the great rudder of human affairs"—his peevish reclamations were met with the inexorable Sic vos non vobis of ironical destiny. Of the innumerable inventions which he originated, scarcely one has been associated with his name. His suggestions bore fruit in the hands of others. His ideas were appropriated and perfected by his rivals. His experiments conferred luster on his successors. By tacit consent, his intellectual inheritance was divided, and his claims ignored. Newton took up the theory of light where he abandoned it, and left him far behind in the momentous search for the law of gravitation. Mayow carried forward the investigations which he had set on foot as to the purpose subserved by the air in respiration. His method was used by Picard in 1670, with striking success, in his new measurement of the earth. His observations formed the basis upon which Bradley founded, in 1728, his discovery of the aberration of light. That his repeated disappointments and mischances were in any degree attributable to his own deficiencies, naturally did not occur to him. It was simpler and more consolatory to set them down to the prevalent malignity and injustice of mankind. Hence the deepening shade of misanthropy which enveloped in saturnine reserve the later years of his life.
Nevertheless, Hooke was, in spite of conspicuous defects, by no means a bad man. His morals were irreproachable, his diligence was untiring, and his religious sentiments seem to have been unfeignedly devout. His faults were warpings of the mind, closely dependent, perhaps, on his unfortunate physical constitution. In spirit, as well as in person, Nature had set him somewhat awry. "Certainly," writes Bacon, "there is a consent between the body and the mind; and where Nature erreth in the one, she ventureth in the other. Ubi peccat in uno, periclitatur in altero." It was his misfortune that he could neither win sympathy nor inspire pity. His talents earned for him patronage; but his peculiarities repelled friendship. He lived sixty-eight years without attaching to himself a single human being, and died only to make room for his rival. And yet his intellectual qualities did not demand admiration more than his moral failings claimed tenderness. For surely infirmity has been rarely combined with genius in more painful and pitiable guise than in Robert Hooke.—Edinburgh Review.