Reluctancy and Reaction
THE GREAT COURT of Trinity College was mostly complete, with a library and stables, central fountain, and fenced-in plots of grass. An avenue of newly planted linden trees lay to the southwest.1 Newton occupied a chamber upstairs between the Great Gate and the chapel. To the west stood a four-walled court used for the game of tennis. Sometimes he watched fellows play, and he noticed that the ball could curve, and not just downward. He understood intuitively why this should be so: the ball was struck obliquely and acquired spin. “Its parts on that side, where the motions conspire, must press and beat the contiguous Air more violently than on the other, and there excite a reluctancy and reaction of the Air proportionately greater.”2 He noted this in passing because he had wondered whether rays of light could swerve the same way—if they “should possibly be globular bodies” spinning against the ether. But he had decided against that possibility.
He did not go to London to appear before the Royal Society after all—not for three more years—but he did not wait to send Oldenburg his promised account of a philosophical discovery. He composed a long letter in February 1672, to be read aloud at a meeting. Within a fortnight Oldenburg had it set in type and printed in the Philosophical Transactions, along with a description of the East Indian coasts and an essay on music.3
Newton’s letter presented both an experiment and a “theory.”4 Six years before, he wrote, he had aligned his prism in a sunbeam entering a dark room through a hole in the window shutter. He expected to see all the colors of the rainbow fanned against the wall and, indeed, he did—vivid and intense, a very pleasing divertissement, he reported. This phenomenon of colors was ancient. As soon as people had glass—that is, as soon as they had broken glass—they noticed the appearance of colors where two refracting surfaces formed a sharp edge.5 A carefully formed triangular prism manifested colors most perfectly. No one knew where the colors came from, but it had seemed clear enough, almost by definition, that a prism somehow creates colors.
Experimentum Crucis: The sunbeam from the window shutter passes through one prism, separating it into colors; then a beam of colored light passes through a second prism. The second prism has no further separation to perform: the white light is a mixture, but the colored beams are pure.
Newton noted a surprise (or so he claimed): where he would have expected the refracted light to form a circle on the wall—all the sun’s rays being refracted equally—instead he saw an oblong. He tried moving the prism, to see whether the thickness of the glass made a difference. He tried varying the size of the hole in the window shutter. He tried a second prism. He measured the distance from the aperture to the wall (22 feet); the length of the colored oblong (13¼ inches); its width (2⅝ inches); and the angles of incidence and refraction, known to be mathematically linked. He noted that the sun was not a point but a disk, spread across 31 minutes of arc. The sunbeam was always in motion, and he could examine it only for moments at a time, but he did not let go of this small oddity—this peculiar elongation of the image.
It led him (or so he reported) to the Experimentum Crucis—the signpost at a crossroads, the piece of experience that shows which path to trust. Newton took the high-plumed phrase from Hooke, who had adapted it from Bacon.6 The crucial idea was to isolate a beam of colored light and send that through a prism. For this he needed a pair of prisms and a pair of boards pierced with holes. He aligned these and carefully rotated one prism in his hand, directing first blue light and then red light through the second prism. He measured the angles: the blue rays, bent slightly more by the first prism, were again refracted slightly more by the second. Most persuasive, though, was that the second prism never created new colors or altered the colors shining from the first prism. Years before, in his earliest speculation, he had asked himself, “Try if two Prismas the one casting blue upon the other’s red doe not produce a white.”7 They did not. Blue light stayed blue and red stayed red. Unlike white (Newton deduced) those colors were pure.
“And so the true cause of the length of that Image was detected,” Newton declared triumphantly—“that Light consists of Rays differently refrangible.” Some colors are refracted more, and not by any quality of the glass but by their own predisposition. Color is not a modification of light but an original, fundamental property.
Above all: white light is a heterogeneous mixture.8
But the most surprising, and wonderful composition was that of Whiteness. There is no one sort of Rays which alone can exhibit this. ’Tis ever compounded, and to its composition are requisite all the aforesaid primary Colours, mixed in due proportion. I have often with Admiration beheld, that all the Colours of the Prisme being made to converge, and thereby to be again mixed,… reproduced light, intirely and perfectly white.
A prism does not create colors; it separates them. It takes advantage of their different refrangibility to sort them out.
Newton’s letter was itself an experiment, his first communication of scientific results in a form intended for publication.9 It was meant to persuade. He had no template for such communication, so he invented one: an autobiographical narrative, step by step, actions wedded to a sequence of reasoning. He exposed intimate feelings: his pleasure at the display of colors, his uncertainty, and then above all his surprise and wonder.
The account was an artifice, stylizing a process of discovery actually carried out over years, on odd occasions, sometimes below the level of consciousness and computation. A prism in a pencil-thin sunbeam actually makes a smudge of color on a wall, uneven and unstable, its edges shadowy and fading. He idealized what he described; the image made sense only because he already knew what he was looking for. He had already seen, years before, that blue light is bent more than red; he had looked through a prism at red and blue threads and noted their varying refraction. He also knew that refracting lenses smeared colors; that was why he had invented a reflecting telescope.
A prism refracts blue light more than red. (illustration credit 7.2)
When Descartes looked at a prism in sunlight, he had seen a circle of colors, not an oblong. A circle was the shape he expected, and it was tiny, because he directed his prism’s light at nearby paper, not a wall twenty-two feet distant. Newton wanted to see the oblong, the spreading; he wanted to magnify it; he wanted to measure it against his geometrical intuition about the laws of refraction; he believed in precision and in his ability to interpret small discrepancies. Indeed, he believed in mathematics as the road to understanding, and he said so: that he expected even the science of colors to become mathematical. And this meant certain. “For what I shall tell concerning them is not an Hypothesis but most rigid consequence,” he wrote, “not conjectured by barely inferring tis thus because not otherwise … but evinced by the mediation of experiments concluding directly & without suspicion of doubt.”10 Oldenburg omitted this sentence from the version he printed.
What was light, anyway? In this offering of a “theory,” Newton chose not quite to commit himself, but he had a mental picture: a ray of light was a stream of particles, “corpuscles”—material substance in motion. Descartes had thought light was pressure in the ether and color an effect of the rotation of these ether particles; Hooke objected to that and proposed the notion of light as a pulse, a vibration of the ether, or a wave, like sound. Newton found Hooke’s theory galling. “Though Descartes may bee mistaken so is Mr Hook,” he wrote privately, in taking notes on his copy of Micrographia. He had a simple argument against a wave theory: light (unlike sound) does not turn corners. “Why then may not light deflect from straight lines as well as sounds &c?”11 In his notes Newton wrote of light as globules, traveling at finite speed and impinging on the eye. In his letter he stuck abstractly to rays. “To determine more absolutely, what Light is,… and by what modes or actions it produceth in our minds the Phastasms of Colours, is not so easie. And I shall not mingle conjectures with certainties.”12
Certainties or not, Newton’s conclusions represented a radical assault on the prevailing wisdom.13 For the next four years the Philosophical Transactions boiled with controversy, month after month: ten critiques of Newton’s letter and eleven counters from Newton.14 Oldenburg kept assuring him of the society’s applause for his ingenuity and frankness and its concern that the honor of discovery might be snatched from him and assumed by foreigners.15 In his role as a clearinghouse for developments in mathematics, Oldenburg discovered that he could use discoveries by foreigners—for example, Gottfried Wilhelm Leibniz in Germany—to pry secret knowledge from Newton. He grew used to Newton’s tantalizing style, always holding gems just out of reach.
And in fact I know myself how to form a series …
I cannot proceed with the explanation of it now …
I have preferred to conceal it thus …
Once this was known, that other could not long remain hidden from me …
I have another method not yet communicated,… a convenient, rapid and general solution of this problem, To draw a geometrical curve which shall pass through any number of given points.… These things are done at once geometrically with no calculation intervening.… Though at first glance it looks unmanageable, yet the matter turns out otherwise. For it ranks among the most beautiful of all that I could wish to solve.16
His mathematics remained mostly hidden. Regarding light, however, he had exposed himself, and he regretted it. Hooke continued to attack. As Curator of Experiments Hooke assured the society that he had already performed these very experiments, hundreds of times. He was not a little pleased, he said, with the niceness and curiosity of Newton’s observations, but he had to confess that he considered these arguments a mere hypothesis. He said that his own experiments—“nay and even those very expts which he alledged”—proved that light is a pulse in the ether and that color is nothing but a disturbance of that light. He would be glad to see “one Experimentum crucis from Mr Newton” to make him change his mind, but this was not it. A prism adds color to light, he insisted, just as an organ pipe or a violin string adds sound to the air.17 A French Jesuit, Ignace Pardies, wrote from Paris that Newton’s “hypothesis” would overthrow the very basis of optics; that the oblong image could be explained by rays coming from different parts of the sun’s face; and that mixing colored rays of light produces only a dark blur, not white.18
All this angered Newton, especially the word hypothesis. He was not offering a hypothesis, he said again, but “nothing else than certain properties of light which, now discovered, I think are not difficult to prove, and which if I did not know to be true, I should prefer to reject as vain and empty speculation, than acknowledge them as my hypothesis.”19 Oldenburg suggested that he respond without mentioning names—especially Hooke’s—but Newton had a different idea. Months went by, and his rancor festered. When he finally penned a long reply, it named Hooke in its first sentence and on every page. “I was a little troubled to find a person so much concerned for an Hypothesis,” he wrote, “from whome in particular I most expected an unconcerned & indifferent examination.”
Mr Hook thinks himselfe concerned to reprehend me.… But he knows well that it is not for one man to prescribe Rules to the studies of another, especially not without understanding the grounds on which he proceeds. Had he obliged me with a private letter …20
Hooke’s rejection of the experimentum crucis was “a bare denyall without assigning a reason,” he asserted. Newton wrote and rewrote this letter four times. It grew far longer than his original report. He considered colors in bubbles and froth; jabbed slyly at Hooke with suggestions for microscopy; and refined his distinction between pure colors and compounded whiteness. There were many ways to mix colors, he suggested, to produce white or (not so perfect and intense) gray. “The same may be effected by painting a Top (such as Boys play with) of divers colours, for when it is made to circulate by whipping it will appear of such a dirty color.”
Above all, he wished to assert that optics was a mathematical science, rigorous and certain; that it depended on physical principles and mathematical proof; and that since he had learned these principles he had met with constant success.
He implied again and again that Hooke was not really performing the experiments. Hooke had “maimed” his argument. Hooke insisted on “denying some things the truth of which would have appeared by an experimentall examination.” True—Newton conceded—he was arguing for the corporeity of light, but that followed from his theory, not the other way around. It was not a fundamental supposition. In suggesting that light was composed of particles, he had carefully used the word perhaps. “I wonder how Mr Hook could imagin that when I had asserted the Theory with the greatest rigor, I should be so forgetfull as afterwards to assert the fundamentall supposition it selfe with no more than a perhaps.”
Hooke was Newton’s most enthusiastic antagonist now, but not his most able. Christiaan Huygens, the great Dutch mathematician and astronomer, also favored a wave theory of light. His understanding of refraction and reflection was profound—and correct enough, when alloyed with Newton’s, to survive up to the quantum era. But he, too, by way of letters to Oldenburg, raised initial questions about Newton’s “hypothesis” and in return felt the young man’s wrath. He caught subtle errors that Newton would never quite acknowledge; for example, Huygens suggested correctly that white could be created not just by a mixture of all colors but by the blending of pairs such as blue and yellow.21 Fifteen months after his election to the Royal Society, Newton announced that he wished to withdraw—and not just from the society but from all correspondence. “I suppose there hath been done me no unkindness,” he wrote Collins. “But I could wish I had met with no rudeness in some other things. And therefore I hope you will not think it strange if to prevent accidents of that nature for the future I decline that conversation which hath occasioned what is past.”
Oldenburg begged him to reconsider, suggested he no longer feel obliged to pay his dues, and assured him that the Royal Society esteemed and loved him.22 The criticism had been so mild and so ordinary, though perhaps there had been “incongruities.” Newton had still never met any of these men—Oldenburg, Collins, Hooke, or the others. He wrote one more reply. “The incongruities you speak of, I pass by,” he said. “But … I intend to be no further sollicitous about matters of Philosophy. And therefore I hope you will not take it ill if you find me ever refusing doing any thing more in that kind.”23 Oldenburg did not hear from him again for more than two years.24
He had discovered a great truth of nature. He had proved it and been disputed. He had tried to show how science is grounded in concrete practice rather than grand theories. In chasing a shadow, he felt, he had sacrificed his tranquillity.25