SOURCE: A preface to Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light, by Isaac Newton, Dover Publications, 1952, pp. ix-lxxvii.

[In the following essay, Cohen reviews the content, textual history, and contemporary and later reception of Newton's Opticks.]

Great creations—whether of science or of art—can never be viewed dispassionately. The Opticks, like any other scientific masterpiece, is a difficult book to view objectively; first, because of the unique place of its author, Isaac Newton, in the history of science, and, second, because of the doctrine it contains. One of the most readable of all the great books in the history of physical science, the Opticks remained out of print for a century and a half, until about two decades ago, while the Principia was constantly being reprinted. One of the reasons for this neglect was that the Opticks was out of harmony with the ideas of 19th-century physics. The burden of this book was an exposition of the "wrong" (i.e., corpuscular) theory of light,—even though it also contained many of the basic principles of the "correct" (i.e., wave) theory. Not only had Newton erred in his choice of the corpuscular theory, but also he apparently had found no insuperable difficulty in simultaneously embracing features of two opposing theories. It was not just that Newton couldn't make up his mind between them; that could be easily forgiven. Rather, by adopting a combination of the two theories at once, he had violated one of the major canons of 19th-century physics, which held that whenever there are two conflicting theories, a crucial experiment must always decide uniquely in favor of one or the other.

Every age finds a particular sympathy for certain masterpieces of science and of art. Our present age lends a particularly appreciative eye and ear to the paintings of Lucas Cranach the elder and Jheronimus Bosch, and the poetry of John Donne and William Blake. Since our reading in the past great works of science is conditioned by the science of our own time, our interpretations and evaluations are as different from those of the last century as our tastes in poetry and art. We esteem the Principia as much as the Victorians did, but we know its limitations and cannot help but read it in the light of the theory of relativity. Today our point of view is influenced by the theory of photons and matter waves, or the more general principle of complementarity of Niels Bohr; and we may read with a new interest Newton's ideas on the interaction of light and matter or his explanation of the corpuscular and undulatory aspects of light. It is not surprising, therefore, that the last decades have witnessed a revival of Newton's Opticks, finally reprinted in 1931 by Messrs. G. Bell and Sons of London with a foreword by Albert Einstein and introduction by E. T. (now Sir Edmund) Whittaker, a new printing of which now makes this work again available thanks to the enterprise of Mr. Hayward Cirker of the Dover Publications. Sir Edmund's introduction delineates the effect on the reputation of the Opticks of the vicissitudes of the corpuscular theory of light which Newton espoused, and the general feeling during the latter 19th century that since the wave theory of light was the only true explanation of optical processes, the Opticks was a work of interest chiefly to the historian rather than the scientist, exhibiting, despite its brilliant exposition, an unhappy example of how wrong a great man might be.

So great, however, was Newton's fame among men of science that a number of writers on optics, especially among the British, took care to inform their readers that Newton's corpuscular theory, while clearly incorrect, was nevertheless a very ingenious creation and had been fully able to explain all of the facts about light known in Newton's day. In other words, this theory was not wholly relegated to the realms of the antique and the curious but was rather presented to the reader with an apology and a discussion of the 17th-century situation in physics. The book itself makes plain that Newton's theory was in fact adequate to the phenomena it attempted to explain, and it was in many respects better than the rival theory of Huygens. Yet the sympathetic explanation of Newton's approach to optics, in terms of the physics of his day, is conspicuous in such books because the authors of scientific treatises in the 19th century were not usually quite so generous to their predecessors when it came to "erroneous" theories of the past. Their attitude is all the more remarkable when we see that they usually felt bound to add that Newton's choice of the "wrong" theory of light had been a serious impediment to the search for the "correct" theory.

Thus, in a book on optics oft reprinted during the 19th century, Dionysius Lardner wrote of the corpuscular theory of light: "… probably, from veneration of his [Newton's] authority, English philosophers, until recently, have very generally given the preference to that theory."¹ And as late as 1909 Sir Arthur Schuster, in a general treatise on optics, still felt it incumbent upon him to apologise for the apparent failure of the greatest of British men of science and wrote: "While there is no doubt that Newton's great authority kept back the progress of the undulatory theory for more than a century, this is more than compensated by the fact that the science of Optics owes the scientific foundation of its experimental investigation in great part to him."²

The revival of the wave theory of light, in opposition to the corpuscular theory advocated by Newton, was due largely to the labors of Dr. Thomas Young who, in a series of three papers published in 1802-04 in the Philosophical Transactions of the Royal Society³, added a new fundamental principle, the so-called principle of interference, and established the wave theory on a basis that has from then until now remained unassailable⁴. Despite Young's insistence in all his papers that his own work derived from the experiments and suppositions of Newton, and his introduction of many quotations from Newton to show that he was modifying rather than destroying the Newtonian doctrine, Young was attacked mercilessly. The most important antagonist appears to have been Lord Brougham, in all probability the author of two anonymous discussions of Young's work in the Edinburgh Review. Brougham was particularly incensed at the suggestion of Newton's fallibility and he accused Young of having insinuated "that Sir Isaac Newton was but a sorry philosopher."⁵ Had not Fresnel and Arago in France become interested in the work of Young, it seems probable that the influence of the great name of Newton would effectively have blocked any pursuit of Young's ideas—at least in England—and any further development of the wave theory of light. This situation is not unlike that in mathematics during the 18th century, when a blind adherence to the Newtonian algorithm and a complete rejection of the Leibnizian or Continental methods seem to have deadened the sensibilities of British mathematicians and to have produced an era of almost complete sterility with regard to progress in the calculus.⁶

The comparison of works of science and works of art is especially relevant to our understanding of the history of the reputation of the Opticks. Having known Picasso and Dali, Cranach and Bosch do not seem as strange to us as they would have to a Victorian spectator. Similarly, having known Planck, Einstein, Bohr, de Broglie, Schrödinger and Heisenberg, Newton's suggestions concerning the interaction of particles of light and particles of matter, and his adumbration of a corpuscular theory of light that also embraced undulatory behaviour, are not to us—as they were to the Victorian physicists—a puzzling set of ideas, seemingly devoid of any physical meaning and, therefore, outside the limits of comprehensibility. One of the great classic treatises on light of the late 19th century was written by Thomas Preston, Professor of Natural Philosophy in University College, Dublin. First printed in 1890, it still served as a principal textbook for this subject in the '30's (in the fifth edition published in 1928). It was notable for its magnificent expository style and the clarity of its explanations, and also for the valuable historical discussions and long extracts from Newton's Opticks; indeed, many students of physics, including the writer, learned of Newton's Opticks for the first time through reading the selections printed in Preston's Theory of Light. One group of extracts was intended to present Newton's "theory in his own words" and to "show how much more closely than is generally supposed it resembles the undulatory theory now accepted." Preston wanted his readers to understand that Newton's position was, if not exactly that of the late 19th century, at least very much like it. By "suitably framing his fundamental postulates," Preston wrote, "an ingenious exponent of the emission [or corpuscular] theory … might fairly meet all the objections that have been raised against it." Yet, when these "necessary postulates" are introduced, the corpuscles become endowed "with the periodic characteristics of a wave motion, apd … the corpuscles themselves may be eliminated.…"⁷ Thus Preston's attitude was to "save the theory" by eliminating its fundamental corpuscular character. Writing long before the rise of quantum mechanics, Preston could not find any virtue in a theory of light which simultaneously embraced corpuscular and undulatory aspects and he therefore destroyed the former in order to preserve the latter.

Yet if Newton's Opticks appears to contain ideas that seem like anticipations of our present concepts, we must be very careful not to make too much of it. For, if we were to praise Newton today, simply because his statements about the dual properties of light—corpuscular and undulatory—and the interaction of light and matter are in some ways like those of the 20th century, we are in a sense adopting a procedure of as little worth as did those who disparaged Newton 75 years ago because his optical theory did not quite meet the requirements of late 19th-century physics. However great Newton's insight may have been—and it was almost incredibly penetrating—it was hardly sufficient to give him a prevision of quantum theory; nor to enable him to see two centuries ahead to the science of spectroscopy built upon the foundation of his investigations of the prismatic spectrum of the sun; nor to allow him a guess as to what would be the problems of blackbody radiation and the failure of the theorem of the equipartition of energy, the very data for which were not to be discovered until more than 150 years after his death. Nor could Newton have had even the faintest glimmering of the photo-electric effect and the theory of electrons, since in Newton's day the subject of electricity had not yet attained the status of a separate branch of science. Even the distinction between conductors and non-conductors had not yet been made; nor had the two kinds of electric charge (vitreous and resinous) been yet discovered; and Franklin, who produced in middle age the first unitary theory of electrical action, was not born until two years after the Opticks had been published.

We may find an appeal in the Opticks because we know the theory of photons and quantum mechanics, but we must also keep in mind that the physics of the 20th century was developed from the brute facts of experiment of the late 19th century and the early 20th, and in reaction to the inadequacy of the then-current theories which could not account for the incontrovertible observed data. The physics of the 20th century derives directly from the physics of the 19th century, not from a conscious return to the physics of the 18th century and Newton's Opticks. But the physics of the 20th century does in a definite sense derive from that of the 18th, since the latter produced the physics of the 19th century just as the physics of the 19th century, in turn, produced that of the 20th. In terms of the physics of our own time, then, the importance of the Opticks does not lie in any possible kind of prevision of the theory of photons, but rather in the effect it had on the physicists in its own day and on the generations immediately following its publication. In making an evaluation of the Opticks, therefore, we must choose between (1) the historical or (2) the antiquarian approach to the development of science—between the historian's evaluation of Newton's achievement in terms of the living creation and its influence on the scientists in the century following the publication of his results, or the antiquarian's sifting of the disjecta membra of the Opticks (often out of context) for an occasional "precursorship" of one or another 20th-century physical concept.

My own interest in the Opticks was aroused in the course of extended research into the theories of electricity as developed in the 18th century, as a part of a larger study on the growth in the 18th century of concepts crucial in 19th-century physics, such as charge, field, potential, force, action-at-a distance, atom, energy, etc.⁸ I soon found that of the many references to Newton in 18th-century electrical writings only a very small number were to the Principia, the greater part by far were to the Opticks. This was true not alone of the electrical writings but also in other fields of experimental enquiry.

As an example of the influence of the Opticks, we may look at the Vegetable Staticks of Stephen Hales, a work of the highest rank among experimental treatises, and one that has earned for its author the title of father of plant physiology. This book provides a splendid example of the application of quantitative methods to biology in its account of precise measurements of leaf growth, root pressure, and kindred subjects, and its quantitative studies on air and gases, just as in the companion volume, Haemostaticks, Hales initiated measurements of the blood pressure in animals. The Vegetable Staticks bears on the verso of the title page Newton's imprimatur as President of the Royal Society: "Feb. 16, 1726/7. Imprimatur Isaac Newton. Pr. Reg. Soc." The first mention of Newton occurs in the preface, where Hales notes how "it appears by many chymico-statical Experiments, that there is diffused thro' all natural, mutually attracting bodies, a large proportion of particles, which, as the first Author of this important discovery, Sir Isaac Newton, observes, are capable of being thrown off from dense bodies by heat or fermentation into a vigorously elastick and repelling state …", a reference to the Opticks (see, below, Qu. 30 and Qu. 31). In addition to a discussion of "attraction: that universal principle which is so operative in all the very different works of nature,"⁹ without mentioning Newton's name, there are by actual count 17 places in the book where Newton's name occurs. Of these, 15 are either quotations from the Opticks or references to the Opticks, and neither of the remaining two are concerned with the Principia: one discusses Newton's mode of calibrating thermometers (described in the Phil. Trans. Roy. Soc.¹⁰) and the other is a quotation containing Newton's theory about the dissolution of metals arising from the attractive force exerted by the "particles of acids" (taken from the introduction to vol. 2 of John Harris' Lexicon Technicum¹¹).

In order to understand the extraordinary appeal that the Opticks had in the 18th century, we must compare it to the Principia¹²—in scientific, philosophic, and speculative content; literary style; and the approach of the author to the subject. On such a comparison, an important difference between the two books is immediately apparent. The Opticks invites and holds the attention of the non-specialist reader while the other, the Principia, is as austere and forbidding to the nonspecialist as it can possibly be. Of course, the general reader of the Opticks would be more interested in the final section of "Queries" than in the rest of the work, just as the general reader of the Principia would be drawn to the General Scholium at the end of Book Three; but whereas in the Opticks such a reader could enjoy almost 70 pages, in the Principia there would be but four. The latter would discuss for him the mechanism of universal gravitation and give him a hint of the direction of Newton's thinking about this important problem; but the former would allow the reader to roam, with great Newton as his guide, through the major unresolved problems of science and even the relation of the whole world of nature to Him who had created it.

Wholly apart from the general reader, the scientist who was not well trained in mathematics could make little headway in the Principia. Not only was this masterpiece written in an austere mathematical style, consisting largely of definitions, theorems, lemmas, scholia, and demonstrations, but it was in a definite sense written in an archaic mathematical language. Newton did not consistently apply his own discovery of the calculus, but preferred to use the geometrical style of Apollonios and Euclid—whose works he recommended, along with others, to the theologian William Bentley who wished to present the work of Newton in a popular way as proof of the wisdom of the Creator of the universe. Bentley had first written to the Scotch mathematician John Craigie¹³ and appears to have been so alarmed by the number of mathematical authors recommended by Craigie as being necessary to understand the Principia, that he then applied to Newton himself for advice. The latter's list is formidable enough; a non-mathematician would have to be in earnest indeed to undertake the preparation which Newton deemed necessary for reading the Principia even with the limited objective of a "first perusal," for which Newton advised that "it's enough if you understand the Propositions with some of the Demonstrations which are easier than the rest."¹⁴

The famous philosopher John Locke was more sensible than Bentley and freely admitted that his mathematics would never be equal to reading the great book. He was satisfied with an examination of the reasoning behind the propositions and corollaries to be drawn from them. To be sure that all was well, he inquired of the Dutch physicist Christiaan Huygens whether the mathematics were sound, and once assured that this was the case he was content with the physical principles, the doctrine the book expounded, without bothering about the proofs and details of the text itself. We have an account of Locke's procedure written by Newton's disciple and friend, the Rev. J. T. Desaguliers, who informs us that he was told the story "several times by Sir Isaac Newton himself":

But to return to the Newtonian Philosophy: Tho' its Truth is supported by Mathematicks, yet its Physical Discoveries may be communicated without. The great Mr. Locke was the first who became a Newtonian Philosopher without the help of Geometry; for having asked Mr. Huygens, whether all the mathematical Propositions in Sir Isaac's Principia were true, and being told he might depend upon their Certainty; he took them for granted, and carefully examined the Reasonings and Corollaries drawn from them, became Master of all the Physics, and was fully convinc'd of the great Discoveries contained in that Book.'"

Newton himself had given a warrant for Locke's procedure in the opening lines of the third book of the Principia, where he wrote that in the two preceding books he had "laid down the principles of philosophy; principles not philosophical but mathematical," that these "principles are the laws and conditions of certain motions, and powers of forces"; lest they "should have appeared of themselves dry and barren, I have illustrated them here and there with some philosophical scholiums.…"

Desaguliers contrasts Locke's reading of the Principia with the way in which "he read the Opticks with pleasure, acquainting himself with every thing in them that was not merely mathematical."¹⁶ The merely mathematical section consisted of "Two Treatises of the Species and Magnitude of Curvilinear Figures," which Newton omitted after the first edition, since they were in no way connected with the text of the Opticks.

Another distinction between the Opticks and the Principia, apart from the mathematical difficulty of the latter, is that the Opticks was written in English while the Principia was written in Latin. While this did not make as much difference in the 17th century as it would today, there is no question but what the austere Latin of the Principia was as characteristic of its essentially mathematical form as the gentle English of the Opticks was characteristic of the intimate style of that work¹⁷. For in the Opticks Newton did not adopt the motto to be found in the Principia—Hypotheses non fingo; I frame no hypotheses—but, so to speak, let himself go, allowing his imagination full reign and by far exceeding the bounds of experimental evidence.

It should, of course, be borne in mind that Newton's phrase Hypotheses non fingo was applied by him to the nature of the gravitational attraction and was never a guiding principle in his work. It is equally clear, however, that many of the readers of the Principia tended to think of this motto as characteristic of the book. Thus Roger Cotes, who superintended the preparation of the second edition of the Principia under Newton's direction and who wrote a preface to it, begged Newton to revise "the last Sheet of your Book which is not yet printed off," since he felt he could not "undertake to answer any one who should assert that You do Hypothesim finigere, [since] I think You seem tacitly to make this supposition that the Attractive force resides in the Central Body."¹⁸ In a letter written in 1672 by Newton to Henry Oldenberg, Secretary of the Royal Society, in response to an objection that had been raised to his first publication on optics, Newton discussed the function of hypotheses at length:

For the best and safest method of philosophizing seems to be, first diligently to investigate the properties of things and establish them by experiment, and then to seek hypotheses to explain them. For hypotheses ought to be fitted merely to explain the properties of things and not attempt to predetermine them except in so far as they can be an aid to experiments. If any one offers conjectures about the truth of things from the mere possibility of hypotheses, I do not see how anything certain can be determined in any science; for it is always possible to contrive hypotheses, one after another, which are found rich in new tribulations. Wherefore I judged that one should abstain from considering hypotheses as from a fallacious argument, and that the force of their opposition must be removed, that one may arrive at a maturer and more general explanation.¹⁹

The first book of the Opticks deals with the reflection and refraction of light, the formation of images, the production of spectra by prisms, the properties of colored light and the composition of white light and its dispersion. Based on definitions and axioms, and embodying a wealth of experimental data, this first book had, according to Newton, the "Design … not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiments." The second book, devoted largely to the production of colors in what we would call interference phenomena, contains no such declaration, and it is here that Newton introduces the notion of "fits" of easy transmission and easy reflection, and kindred concepts not derived by induction from experiments. And although Newton points out (p. 280) that on the score of fits of easy transmission and easy reflection: "What kind of action or disposition this is; Whether it consists in a circulating or a vibrating motion of the Ray, or of the Medium, or something else, I do not here enquire," he adds:

Those that are averse from assenting to any new Discoveries, but such as they can explain by an Hypothesis, may for the present suppose, that as Stones by falling upon Water put the Water into an undulating Motion, and all Bodies by percussion excite vibrations in the Air; so the Rays of Light, by impinging on any refracting or reflecting Surface, excite vibrations in the refracting or reflecting Medium or Substance, and by exciting them agitate the solid parts of the refracting or reflecting Body, and by agitating them cause the Body to grow warm or hot; that the vibrations thus excited … move faster than the Rays so as to overtake them; … and, by consequence, that every Ray is successively disposed to be easily reflected, or easily transmitted, by every vibration which overtakes it. But whether this Hypothesis be true or false I do not here consider. I content my self with the bare Discovery, that the Rays of Light are by some cause or other alternately disposed to be reflected or refracted for many vicissitudes.

The second book thus admits hypotheses, although without any consideration of their truth or falsity. In the third (and last) book, the opening section deals with Newton's experiments on diffraction, followed by the famous Queries in which, as we shall see, Newton introduced a variety of "hypotheses"—not alone on light, but on a great many subjects of physics and philosophy, as if in his final work he had emptied his mind of the conjectures he had accumulated in a life-time of scientific activity. Clearly, Hypotheses non fingo could not be applied to the Opticks. And it is, in a very real sense, the progressively conjectural character of this book that makes it so interesting to read. As Albert Einstein saw so clearly when he wrote his admirable Foreword, this book "alone can afford us the enjoyment of a look at the personal activity of this unique man."

In its own day, the Opticks aroused interest in a way that was directly related to the Principia. Not only did the reputation of the Principia create a ready market for a more readable book by its author, but in the Principia Newton had raised important philosophical questions which he discussed at greater length in the Opticks, in the Queries at the end of Book Three, and which Newton mentioned—but only in passing—in the famous General Scholium to the third book of the Principia, addressed to the nature of the gravitational attraction between bodies, and in which the phrase Hypotheses non fingo appeared.

Newton had shown that celestial and terrestrial motions were in accordance with a law of universal gravitation in which the attraction between any two bodies in the universe depends only on their masses and (inversely) on the square of the distance between them. This led to an attribution to Newton of ideas that he abhorred. One was that since the gravitational attraction is a function of the masses of bodies irrespective of any other properties save their separation in space, this attraction arises simply from the existence of matter.

This materialist position was castigated by Newton in a letter to Bentley in which he said: "You sometimes speak of gravity as essential and inherent to matter. Pray, do not ascribe that notion to me; for the cause of gravity is what I do not pretend to know.…" And in another letter to Bentley, he amplified his position: "It is inconceivable, that inanimate brute matter should, without the mediation of something else, which is not material, operate upon and affect other matter without mutual contact.…"²⁰

Another point of argument arose in a letter written by Leibniz which had been published in an English translation. Cotes wrote to Newton of "some prejudices which have been industriously laid against" the Principia, "As that it deserts Mechanical causes, is built upon Miracles, & recurrs to Occult qualitys." Newton would find "a very extraordinary Letter of Mr Leibnitz to Mr Hartsoeker which will confirm what I have said," Cotes continued, in "a Weekly Paper called Memoires of Literature & sold by Ann Baldwin in WarwickLane."²¹

In the preface which he wrote to the second edition of the Principia, Cotes replied to Leibniz—although without mentioning his name; "… twere better to neglect him," he had written to Newton.²² Cotes also discussed the general nature of gravitation and forces acting at a distance. For this second edition, Newton wrote the famous General Scholium to Book Three, in which he attacked the vortex theory of Descartes, declared that the "most beautiful system of the sun, planets, and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being," and discussed the nature of God, concluding: "And thus much concerning God; to discourse of whom from the appearance of things, does certainly belong to Natural Philosophy."²³ Newton then addressed himself to the problem of what gravitation is and how it might work, admitting that no assignment had been made of "the cause of this power" whose action explains the phenomena of the heavens and the tides of the seas. This is followed by the famous penultimate paragraph which reads:

But hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy.… And to us it is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies, and of our sea.

It was apparently the purpose of the General Scholium to prevent any misunderstanding of Newton's position such as had been made by Bentley and Leibniz after reading the first edition of the Principia in which this General Scholium did not appear. Yet the cautious wording prevented the reader from gaining any insight into Newton's actual beliefs on this subject, as contained, for example, in a letter to Boyle written on 28 February 1678/9, prior to the publication of the Principia and not published until the mid-18th century. In this letter Newton wrote out his speculations concerning the "cause of gravity" and attempted to explain gravitational attraction by the operation of an all-pervading "aether" consisting of "parts differing from one another in subtility by indefinite degrees."²⁴ Some hint, but not more, of Newton's view was contained in the final paragraph of the above General Scholium, in which Newton wrote:

And now we might add something concerning a certain most subtle spirit which pervades and lies hid in all gross bodies; by the force and action of which spirit the particles of bodies attract one another at near distances, and cohere, if contiguous; and electric bodies operate to greater distances as well repelling as attracting the neighboring corpuscles; and light is emitted, reflected, refracted, inflected, and heats bodies; and all sensation is excited, and the members of animal bodies move at the command of the will, namely, by the vibrations of this spirit, mutually propagated along the solid filaments of the nerves, from the outward organs of sense to the brain, and from the brain into the muscles. But these are things that cannot be explained in few words, nor are we furnished with that sufficiency of experiments which is required to an accurate determination and demonstration of the laws by which this electric and elastic spirit operates.

Thus the 18th-century reader who had become convinced that the system of Newton's Principia accounted for the workings of the universe and then naturally enough wondered what the cause of gravity might be, was tantalized by the final statement that Newton might have elucidated this topic but had decided not to do so. Hungry for a further discussion of the nature of "this electric and elastic spirit," or "aether," so intimately associated with the behaviour of material bodies and the communication of animal sensation, such a reader would devour anything else that Newton wrote on the subject.

Since Newton devoted a considerable portion of the Opticks to this question, neatly avoided in the Principia, we can understand at once why the Opticks must have exerted so strong a fascination on men like John Locke and on all the others who wanted to know the cause of gravity and the fundamental principle of the universe. Indeed, in the 1717 edition of the Opticks Newton inserted an "Advertisement" … explicitly declaring that he did "not take Gravity for an Essential Property of Bodies," and nothing that among the new Queries or Questions added to the new edition was "one Question concerning its Cause," Newton "chusing to propose it by way of a Question" since he was "not yet satisfied about it for want of experiments."

The first edition of the Opticks was published in English in 1704 and contained only the first 16 queries. The Latin version, prepared at Newton's suggestion by Samuel Clarke, was issued two years later in 1706; to it were added Queries 17-23 in which Newton discussed the nature of the aether.²⁵ Here he compared the production of water waves by a stone to the vibrations excited in the refracting or reflecting medium by particles of light (Qu. 17), and proposed the theory that the vibrations so excited "overtake the Rays of Light, and by overtaking them successively, put them into the fits of easy Reflexion and easy Transmission.…" He also suggested that the aether is a "much subtiler Medium than Air" and that its vibrations convey heat, both radiant heat which may pass through a vacuum and the heat communicated by hot bodies "to contiguous cold ones" (Qu. 18); it is also "more elastick and active" than air. This aether (Qu. 19, 20) is responsible for refraction and, because of its unequal density, also produces "Inflexions of the Rays of Light" or diffraction of the sort recorded by Grimaldi. Newton indicated (Qu. 21) his belief that the variations in density of the aether are the cause of gravitation; and he pointed out that this aether must be highly elastic to support the enormous speed of light, but that the aether does not (Qu. 22) interfere with the motions of planets and comets; and he compared the gravitational action of the aether to the action of electrified and magnetic bodies in attracting, respectively, "leaf Copper, or Leaf Gold, at … [a] distance … from the electrick Body.… And … through a Plate of Glass…to turn a magnetick Needle.…" The last of these new Queries (Qu. 23) relates the vibrations of the aether to vision and possibly to hearing.

One of the most interesting aspects of this group of Queries is that it follows in outline the letter written to Boyle some 20 years earlier, and which had not yet been published. While Newton was apparently willing to print his conjectures on the aether and gravitation in the Queries of the Opticks, when he came to revise the Principia for the second edition of 1713, seven years after the appearance of the Latin version of the Opticks, he was much more cautious. He refused, as we saw above, to discuss the cause of gravitation, begging off with the phrase Hypotheses non fingo;²⁶ and he actually declared that all one needs to know is that gravity exists, that it follows the law of the inverse square, and that it serves to describe the motions of the celestial bodies and the tides. Newton concluded the General Scholium with the statement that he might discuss the aether but refrained from doing so. While it can never be proved, it is possible that Newton's procedure in this matter may indicate an appreciation of the fundamentally differing character of his two books—the Principia with its mathematical demonstrations and general avoidance of speculation, and the Opticks with its large speculative content.

To be sure, the speculations of the Opticks were not hypotheses, at least to the extent that they were framed in questions. Yet if we use Newton's own definition, that "whatever is not deduced from the phenomena is to be called an hypothesis," they are hypotheses indeed. The question form may have been adopted in order to allay criticism, but it does not hide the extent of Newton's belief. For every one of the Queries is phrased in the negative! Thus Newton does not ask in a truly interrogatory way (Qu. I): "Do Bodies act upon Light at a distance … ?"—as if he did not know the answer. Rather, he puts it: "Do not Bodies act upon Light at a distance. T. ?"—as if he knew the answer well—"Why, of course they do!" But if the addition of the question mark made it possible for Newton to free himself from the restrictions imposed by that "sufficiency of experiments which is required to an accurate determination and demonstration of the laws," we can only be grateful for the opportunity to see the mind of Newton at work, and to share his profound inspiration. As it was said in the 18th century, any of the fine accounts of the Newtonian system of celestial mechanics—such as those of Pemberton, Maclaurin, and Voltaire—could afford the reader a sufficient understanding of the Principia which, therefore, need not be read in Newton's original; but when it came to the Opticks, no abrege or vulgarization could take the place of Newton's own words.

A glance at the analytical table of contents will show the range of subjects covered by Newton in the Opticks and especially in the Queries. For the latter not only took up questions of light as such, and gravitation, but also chemistry, pneumatics, physiology, the circulation of the blood, metabolism and digestion, animal sensation, the Creation, the flood, the true nature of scientific inquiry, how to make experiments and how to draw the proper conclusions from them by induction, the relation of cause and effect, and natural philosophy in relation to moral philosophy. Here, then, was a rich intellectual feast for philosophers as well as scientists, for poets as well as experimenters, for theologians as well as painters, and for all amateurs of the products of the human imagination at its highest degree of refinement. Not only, therefore, did the Opticks come to enjoy a special place in 18th-century science, but also it had an appeal wholly its own for the British poets of the 18th century. Miss Marjorie Nicolson, in the course of her investigations of science and the literary imagination, arrived independently at the same conclusion concerning the Opticks and the 18th-century literary writers that I had found to be tnie of the Opticks and the 18th-century experimenters. "While reading widely in eighteenth-century poetry for other purposes," she writes, "I found myself constantly teased by dozens of references to Newton which had nothing to do with the Principia, until I became persuaded that, among the poets, the Opticks was even more familiar than the more famous work."²⁷

One reason why the Opticks should enjoy a greater vogue among experimenters than the Principia is plain: the Opticks was, in great part, an account of experiments performed by Newton and the conclusions drawn by him from the experimental results; but in the Principia Newton described only two or three important experiments that he actually had made²⁸ and, for the rest, merely cited data obtained by contemporaries and predecessors. We must remember, furthermore, that throughout the history of science there have been two types of investigator: the theoretician and experimenter, characterized in modern times by such basic figures as Einstein and Rutherford. It is rare that the two are combined within one individual as they were in Newton. Newton the mathematical physicist of the Principia did not in general appeal to those who, by the application of the imagination and the experimental art, were exploring new fields depending on empirical investigation for their future progress: such as plant physiology, chemical reactions, heat, and electricity. The mentor and guide of those who explored these new fields was Newton the heroic experimenter of the Opticks and the author of the Queries; and many of them, such as Franklin, who read and re-read the Opticks, did not even have the mathematical training to attempt to read the Principia, had they wanted to do so.

From the point of view of the 18th century, as indeed from that of the 19th, Newton's Principia with its law of universal gravitation had apparently, but for certain minor revisions and emendations, settled the problem of heavenly motions once and for all. Certain situations were not soluble; for example, the famous three-body problem does not admit of a general analytic solution. Yet the law of universal gravitation was unquestionably true, something to be believed even when all else failed. This faith is mirrored for us in a letter which the naturalist Thomas H. Huxley wrote, in 1860, after the death of his son, "I know what I mean when I say I believe in the law of the inverse squares, and I will not rest my life and my hopes upon weaker convictions."²⁹

But while the Principia seemed the terminal point of an ancient line of inquiry, the Opticks, with its newly discovered phenomena concerning colors and diffraction, clearly marked the beginning of a new direction in physical inquiry. Whereas Newton could end the Principia with a General Scholium and supplement it with The System of the World, he closed the Opticks on a note of uncertainty, on a set of Queries—of which some, but by no means all, might be resolved by the work of future generations.

The point was made earlier that Newton's optical researches are related to our present views, not by a conscious return of 20th-century physicists to the Newtonian ideas, but rather through a chain that extends from one end of the 19th century to the other. The 19th century produced a succession of brilliant research, of which the crowning achievement was Clerk Maxwell's electromagnetic theory of light. In the end, it was the partial failure of this theory that led to the quantum theory of Planck and the photon theory of Einstein. The electromagnetic theory derives in part from the extension by Clerk Maxwell of Faraday's notions concerning the propagation of electric and magnetic effects through space,³⁰ and it brought to a superlative conclusion the wave theory of light, revived by Thomas Young in the opening years of the century.

Young's initial contributions were made as a result of studying a class of optical phenomena which we call today effects of interference and diffraction, but which Newton called the "inflection" of light. The first major account of such phenomena was that published by F. M. Grimaldi in his Physico-Mathesis de Lumine Coloribus et Iride (Bologna 1665). Further studies of diffraction were made by Boyle and Hooke as well as Newton, but the most significant and quantitative results were those obtained by Newton while studying the interference rings produced when the curved surface of a plano-convex lens was pressed against a flat optical surface. Newton's magnificent experiments, described in the text in Book Two, provided conclusive evidence that some kind of periodicity is associated with the several colors into which he divided visible light. Such periodicity can in no way be accounted for by the mechanical action of corpuscles moving in right lines and Newton was, therefore, forced into the position of having to postulate some kind of waves accompanying the corpuscles; these were the famous aether waves. Newton had to account for the successive refraction and reflection that he supposed must occur at the glass-air interfaces between a convex and a flat optical surface when the interference rings are produced. He suggested that the alternate "fits of easy reflection" and "fits of easy refraction" arise from the action of the aether waves which overtake the particles of light and put them into one or the other state.

Newton's measurements of the separation of these rings and his computations of the thickness of the thin film of air between the two glass surfaces were of the highest order of accuracy. After Young had explained the production of "Newton's rings" by the application of his new principle of interference to the wave theory of light, he used Newton's data to compute the wave-length of different colors in the visible spectrum and also the wave-numbers (in "Number of Undulations in an Inch"); Young's computations,³¹ based on Newton's measurements, yielded a wave-length for the "Extreme red" of 0.000, 026, 6 inches, or a wave-number of 37640 "undulations in an inch" and a frequency of 436 x 10⁶ "undulations for a second"; and for the "extreme violet" the same quantities had values 0.000, 016, 7 inches, 59750 per inch, and 735 x 106 per sec, respectively, in close agreement with present-day accepted values.³²

Young was indebted to Newton for more than the data for computing wave-lengths, wave-numbers, and frequencies; the whole wave theory of light was developed by him from the suggestions in Newton's Opticks, with several important additions, chiefly (1) considering the waves in the aether to be transverse, (2) supplementing the wave theory by the principle of interference. It was from Young's work that the 19th-century developments leading to the electromagnetic theory may be said to have begun; and since Young's work was inspired by Newton's, we have an historical chain leading from Newton to Young, and from Young to Fresnel and Arago, and from them to Clerk Maxwell and eventually to Planck and Einstein.

Young was extremely explicit about his debt to Newton. Thus, in the first of the three foundational papers in the Philosophical Transactions, Young stated:

The optical observations of Newton are yet unrivalled; and, excepting some casual inaccuracies, they only rise in our estimation as we compare them with later attempts to improve on them. A further consideration of the colours of thin plates, as they are described in the second book of Newton's Optics, has converted the prepossession which I before entertained for the undulatory system of light, into a very strong conviction of its truth and sufficiency; • a conviction which has been since most strikingly confirmed by an analysis of the colours of striated substances.…

A more extensive examination of Newton's various writings has shown me that he was in reality the first that suggested such a theory as I shall endeavour to maintain.…

Young even pointed out that the wave theory of light should be given a hearty welcome because it originated with Newton who was universally venerated.

Those who are attached, as they may be with the greatest justice, to every doctrine which is stamped with the Newtonian approbation, will probably be disposed to bestow on these considerations so much the more of their attention, as they appear to coincide more nearly with Newton's own opinions. For this reason, after having stated each particular position of my theory, I shall collect, from Newton's various writings, such passages as seem to me the most favourable to its admission.…

In conformity with this plan, almost half of the article is made up of quotations from Newton.

In his "Reply to the Edinburgh Reviewers,"³³ Young described the history of his ideas concerning light, once again expressing the degree to which they derived from Newton's writings. Even the principle of interference, he insists, was discovered by him in May 1801 "by reflecting on the beautiful experiments of Newton …," and although "there was nothing that could have led to it in any author with whom I am acquainted," Young noted how Newton had used a similar conception in explaining "the combinations of tides in the Port of Batsha."³⁴ Although there is, therefore, no question of the influence of Newton on Young, one suspects that Young, like the Player Queen in Hamlet, doth protest too much. The Edinburgh Review, in its condemnation of Young, was of course grossly unfair when it used against him the phrase that a person may, "with the greatest justice, be attached to every doctrine which is stamped with the Newtonian approbation," but without quotation marks (as if this were a position contra-Young on the part of a pro-Newton reviewer rather than a statement by Young himself); but Young should not really have been as astonished as he would have us believe he was; after all, in the minds of everyone at that time, Newton stood for the corpuscular theory of light and against the theory of light waves.

This raises for us the very real question of whether we can, with Young and Preston, strip Newton's theory of the corpuscles and leave only the waves as the essential components. And this question rapidly leads us into another, which is, why did Newton build his theory on corpuscles, or, why did he reject the wave theory that others in the 19th century in vain tried to attribute to him?

I shall not attempt to trace here the complete history of Newton's ideas concerning the theory of light, nor even to collate all the evidence relating to this subject that may be found in his various published works and letters, but shall rather limit myself to the information provided in the Opticks itself. Foremost among the reasons why Newton insisted upon the corpuscularity of light was the general atomism of the age; indeed, the very hallmark of the "New Science" in the 17th century, among such men as Bacon, Galileo, Boyle, and others,, was a belief in atomism, in what Boyle called the "corpuscular philosophy." Whereas the scholastic doctrine had placed light and the phenomena of colors in the category of "forms and qualities," men such as Newton opposed to this traditional view an explanation of the phenomena of nature in terms of the mechanical action of atoms, or of matter and motion. Summing up the many reasons for a general belief in atoms in the final Query, Newton wrote:

All these things being consider'd, it seems probable to me, that God in the Beginning form'd Matter in solid, massy, hard, impenetrable, moveable Particles, of such Sizes and Figures, and with such other Properties, and in such Proportion to Space, as most conduced to the End for which he form'd them.…

Now by the help of these Principles, all material Things seem to have been composed of the hard and solid Particles above-mention'd, variously associated in the first Creation by the Counsel of an intelligent Agent. For it became him who created them to set them in order. And if he did so, it's unphilosophical to seek for any other Origin of the World.…

Then, too, it was well known that waves of whatever sort would spread out in all directions in any homogeneous medium, rather than travel in straight lines as light is observed to do when it produces a sharply defined shadow. Thus, says Newton (Qu. 29): "Are not the Rays of Light very small Bodies emitted from shining Substances? For such Bodies will pass through uniform Mediums in right Lines without bending into the Shadow, which is the Nature of the Rays of Light." Furthermore, that material bodies moving in a straight line oblique to a surface will be reflected so as to obey the law of reflection, i.e. that the angle of incidence equals the angle of refraction, had been well known since classical antiquity. Refraction might easily be explained on the basis of the corpuscular theory since the attraction exerted by the particles of glass, say, on the corpuscles of light incident upon the glass from air would produce an increase in the vertical component of the velocity of the particles and, therefore, would result in a bending toward the normal which is always observed to be the case.³⁵

Of course, a rival theory to Newton's, the wave theory of Christiaan Huygens, had offered a geometrical construction for reflection and refraction in wholly different terms. And this theory led to an exactly opposite conclusion to that of Newton's theory in respect to the relative speeds of light in air and in glass or water. Whereas Newton's corpuscular theory demands a speed of light greater in glass or water than in air, the theory of Huygens requires a speed of light in air that must be greater than the speed of light in water or in glass. Unfortunately, the possibility of putting these opposing conclusions to the test of experiment did not occur until well into the 19th century, when the labors of Young and Fresnel had already established the wave theory of light, so that this test, favoring the conclusions of the wave theory rather than the corpuscular, was but an additional argument, rather than the primary one, for the wave theory of light.

Newton's rejection of Huygens' theory was based, in part, on his own cherished belief in atomicity and also on the fact that Huygens' theory was geometrical rather than mechanical, and contradicted physical principles. Although Huygens had provided a brilliant method for constructing the wave front in the case of refraction or reflection, the waves he postulated were without the primary characteristic of a physical wave motion, i.e., periodicity. In fact, Huygens' denial of periodicity in his postulated light waves was an attempt to account for the possibility of a number of waves crossing each other without in any way interfering one with another. But without the property of periodicity, such waves could not account for color, nor any of the other periodic properties of light which Newton had observed in the various types of interference and diffraction phenomena he had studied so carefully in the Opticks. Nor could Huygens, without the principle of destructive interference invented by Young a little more than a hundred years later, adequately account for the simplest of all optical phenomena, rectilinear propagation.

Finally, the most brilliant of all the portions of Huygens' Treatise on Light³⁶ provided Newton with an argument against Huygens' theory. For, by extending the geometric construction of wave fronts from isotropic to anisotropic media, Huygens had been able to account for the phenomenon of double refraction in calcite, or Iceland spar, by two different wave forms. Newton (Qu. 28) considered this to be an important weapon against Huygens' "Hypothesis." Newton grasped the salient aspect of Huygens' investigation, which was that "the Rays of Light have different Properties in their different Sides," and he quoted from the original French of Huygens to prove how baffling this phenomenon was to the author of the wave theory himself; plainly, "Pressions … propagated … through an uniform Medium, must be on all sides alike." It never apparently occurred to Huygens, who thought in terms of a geometric scheme, nor to Newton, that the undulations might be perpendicular to the direction of propagation. When, eventually, it was suggested by Young and Fresnel that light waves must be transverse rather than longitudinal, then for the first time was it possible to explain the polarization of light, or the way in which light—to use Newton's phrase—has "sides." The study of the interference of polarized beams of light provided in the 19th century one of the chief arguments for the advocates of the wave theory. But in Newton's day and for a hundred years thereafter, the only way to account for the "sides" of light was to suppose that the corpuscles were not perfectly spherical and would present, therefore, different sides depending on their orientation to the axis of motion.

It is one of the ironies of history that the Opticks, based on Newton's secure belief that light rays consist of streams of corpuscles, should have provided, a century later, as great a contribution to the wave theory as the Traité de la lumière of his rival Huygens. And it is just this feature which makes the Opticks such an attractive work for anyone to read who is interested in that stage of creation when the greatest and clearest of minds is baffled by the observed data and cannot reduce them to a simple, clear, straightforward conceptual scheme. As we watch Newton wrestle with the problems of the nature of light, we get in the following pages some measure of the extraordinary difficulty of scientific research applied to the most fundamental problems. And if Newton's elegant conceptions do not always permit us to follow his argument in every detail, we can at any rate be grateful for the opportunity to observe in action one of the greatest minds that science has ever produced, so beautifully described by Wordsworth's "Newton with his prism and silent face," his "mind forever voyaging through strange seas of thought alone."³⁷

Notes

¹ From the version edited by T. Oliver Harding: Handbook of Natural Philosophy: Optics, sixth thousand, London, James Walton, 1869, p. 164.

² Arthur Schuster: An Introduction to the Theory of Optics, second edition, revised, London, Edward Arnold, 1909, p. 86.

³ These three memoirs, together with other writings on the subject by Young, may be found in George Peacock, editor, Miscellaneous Works of the Late Thomas Young, London, John Murray, 1855. "On the Theory of Light and Colours" was a Bakerian Lecture read at the Royal Society on 12 Nov. 1801 and was published in the Phil. Trans. for 1802, pp. 12 ff. (reprinted in Misc. Works, vol. 1, pp. 140 ff.), "An Account of some Cases of the Production of Colours not hitherto described," was read at the Royal Society on 1 July 1802 and was published in the Phil. Trans. for 1802, pp. 387 ff. (reprinted in Misc. Works, vol. 1, pp. 170 ff.), and "Experiments and Calculations relative to Physical Optics" was a Bakerian Lecture read at the Royal Society on 24 Nov. 1803 and was printed in the Phil. Trans. for 1804, pp. 1 ff. (reprinted in Misc. Works, vol. 1, pp. 179 ff.).

⁴ By "unassailable," I do not of course mean that the wave theory, as an exclusive explanation of optical phenomena, has remained unassailable; but rather that the application of the principle of interference to diffraction phenomena has remained the major basis for our belief in waves, whether these be matter waves, probability waves, electro-magnetic waves, or old-fashioned aether waves. Thus, when Louis de Broglie predicted the existence of matter waves having a wavelength λ = h/mv, evidence for their existence in the case of electrons was provided by the diffraction pattern produced by a crystalline metal on a beam of thermal electrons—in accordance with the application of the principle of interference in much the same manner as it had been applied to visible light in Young's pin-hole experiment and to X-rays in Laue's crystal experiments.

⁵ The attack on Young's three memoirs was published in the Edinburgh Review, nos. II and IX. The authorship of the attack is discussed by Peacock in the note added to the reprint of Young's reply (Misc. Works, vol. 1, pp. 192 ff.).

⁶ Cf. Florian Cajori, A History of the Conceptions of Limits and Fluxions in Great Britain from Newton to Woodhouse, Chicago, Open Court Pub. Co., 1919.

⁷ Thomas Preston, Theory of Light, fifth edition, edited by Alfred W. Porter, London, Macmillan, 1928, p. 21; the preface to the first edition is dated July 1890.

⁸ This research has been sponsored by a generous grant in aid from the American Philosophical Society. A preliminary report has been published in The American Philosophical Society Year Book 1949, Philadelphia, 1950, pp. 240-243. It is hoped that a long memoir, embodying the results of research on the nature of physical theory in the 18th century and its effect on that of the 19th, stressing the role of Newton's Opticks and Franklin's unitary theory of electrical action, will be published by the American Philosophical Society late in 1952.

⁹ "Vegetable Staticks," vol. I of Statical Essays, ed. 2, London, W. Innys et al., 1731. An important study of Hales, clearly indicating the indebtedness of Hales to the Queries in the Opticks, is Henry Guerlac: "The Continental Reputation of Stephen Hales," Archives Internationales d'Histoire des Sciences, 1951, No. 15, pp. 393-404.

¹⁰ Hales refers to Motte's abridgment of the Phil. Trans., vol. 2, p. 1.

¹¹ This may also be found in Qu. 31 of the Opticks; see pp. 380-381 below.

¹² The Principia is available in Florian Cajori's version of Andrew Motte's English trans. of 1729, Berkeley, Univ. of Calif. Press, 1934.

¹³ Craigie's letter to Bentley, dated 24 June 1691, may be found in Sir David Brewster, Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton, Edinburgh, Thomas Constable, 1855, vol. 1, p. 465.

¹⁴Ibid, p. 464.

¹⁵ Cf. preface to Jean-Theophile Desaguliers, Course of Experimental Philosophy, ed. 3, vol. 1, London, A. Millar, 1763, p. viii.

¹⁶Ibid.

¹⁷ A Latin edition of the Opticks was later prepared at Newton's request by Samuel Clarke and published in London in 1706, two years after the English edition.

¹⁸ J. Edleston, ed., Correspondence of Sir Isaac Newton and Professor Cotes …, London, John W. Parker, 1850, p. 153.

¹⁹ This letter, in the original Latin, may be found in Newton's Opera, ed. by Samuel Horsley, vol. 4, London, John Nichols, 1782, pp. 314 ff. The English translation is quoted from Cajori's notes to his edition of the Principia, ed. cit., p. 673. Horsley printed the entire letter; when originally published in the Phil. Trans., this part of the letter had been omitted.

²⁰ The Newton-Bentley correspondence may be found in Newton's Opera, ed. by Horsley, vol. 4, pp. 427 ff.

²¹Correspondence of Sir Isaac Newton and Professor Cotes, p. 153. [Cotes told Newton that the Leibniz letter appeared in "the 18th Number of the second Volume" of the Memnoirs of Literature. In the second edition of the Memoirs of Literature, "revised and corrected," Leibniz's letter appears in vol. 4 (London, R. Knaplock, 1722), art. LXXV, pp. 452 ff.]

²²Ibid.

²³ All quotations from the Principia are taken from Cajori's edition (note 12).

²⁴ Newton' s letter to Boyle, dated 28 Feb. 1678/9 was first printed in Boyle's Works, edited by Thomas Birch, vol. 1, London, A. Millar, 1744, Life, pp. 70 ff. The aether, in respect of its rarity and density, not only was supposed to produce gravitation but also the diffraction phenomena described by Grimaldi. This letter is reprinted in Sir David Brewster, Memoirs, ed. cit. (note 13).

²⁵ These Queries, 17-23, next appeared in the second English edition, first issued in 1717, together with new Queries 24-31.

²⁶ An important discussion of Newton' s use of hypotheses, and the meaning of. the phrase Hypotheses non fingo, by Cajori may be found in his edition of the Principia, p. 671.

²⁷ Marjorie Hope Nicolson, Newton demands the Muse: Newton's "Opticks" and the Eighteenth Century Poets, Princeton, Princeton Univ. Press, 1946, p. vii.

²⁸ Three major experiments performed by Newton and described at length in the Principia are (1) the production of interference fringes by the diffraction of light produced by the "edges of gold, silver, and brass coins, or of knives, or broken pieces of stone o[r] glass," as "lately discovered by Grimaldi," Scholium following prop. XCVI, th. L, bk. 1 (pp. 229 ff., Cajori's ed.); this Scholium occurs in a group of theorems in which is shown "the analogy there is between the propagation of the rays of light and the motion of bodies," but "not at all considering the nature of the rays of light, or inquiring whether they are bodies or not"; (2) a study of the "resistance of mediums by pendulums oscillating therein," General Scholium following prop. XXXI, th. XXV, bk. II (pp. 316 ff.); (3) an investigation of "the resistances of fluids from experiments," consisting of measurements of the time of descent of spheres made of (or filled with) various substances, when allowed to fall through water and air, Scholium following prop. XL, prob. IX, bk. 2 (pp. 355 ff.).

²⁹ The letter, written to Charles Kingsley, may be found in Life and Letters of Thomas Henry Huxley, edited by Leonard Huxley, vol. 1, London, Macmillan, 1900, p. 218.

³⁰ Faraday used to quote Newton's letter to Bentley as inspiration for his own concern over the role of the medium in the action of one body upon another at a distance from it. Cf. John Tyndall: Faraday as a Discoverer, new ed., London, Longmans, Green, 1870, p. 81.

³¹ "On the Theory of Light and Colours" (see footnote 3 above) in Misc. Works, vol. 1, p. 161.

³² Newton never computed wave-lengths, but on many occasions noted that the vibrations of the "aether" corresponding to the several colors might be likened to the vibrations of air which, according to their several "bignesses, makes several Tones in Sound." One may conclude from the analogy of sound to light (Qu. 28) that Newton's aether waves would be longitudinal pulses; yet Newton also referred to the mode of production of water waves by a stone thrown into a stagnant water (Qu. 17) as an analogy to the "aether" waves arising from the motion of light-particles, and in this case the disturbance would be transverse.

³³ Reprinted in Misc. Works, vol. 1, pp. 192 ff.

³⁴ This example is discussed more fully in I. B. Cohen, "The First Explanation of Interference," Am. J. Physics (1940), vol. 8, pp. 99 ff.

³⁵ In one of his earliest publications in optics, Newton had already indicated that rectilinear propagation was contrary to the suppositions of a wave theory: "For, to me, the Fundamental Supposition it self seems impossible; namely, That the Waves or Vibrations of any Fluid, can, like the Rays of Light, be propagated in Streight lines, without a Continual and very extravagant spreading and bending every way into the Quiescent Medium, where they are terminated by it. I mistake, if there be not both Experiment and Demonstration to the contrary." (Phil. Trans., no. 88, p. 5089). In the Principia, Prop. XLII, th. XXXIII, bk. II, there is a demonstration that "All motion propagated through a fluid diverges from a rectilinear progress into the unmoved spaces." The scholium to Prop. L, Problem XII, bk. II, deals with the determination of the wave-number of a sound-vibration, but begins with a declaration that the rectilinear propagation of light proves that light cannot consist of waves alone.

In Query 28, Newton indicated by examples that "Waves, Pulsations or Vibrations of the Air, wherein Sound consists, bend manifestly, though not so much as the Waves of Water." Does not the evidence of the bending of light in diffraction experiments, as described in the beginning of Book Three, indicate the wave nature of light? Although Newton referred to such experiments (p. 388) as examples of "how the Rays of Light are bent in their passage by Bodies," they were interpreted as illustrating not wave motion or "pression" so much as the way (Qu. 1) "Bodies act upon Light at a distance, and by their action bend its Rays." In Query 28, the difference between the bending of light and of waves is discussed; we find, "The Rays which pass very near to the edges of any Body, are bent a little by the action of the Body, as we shew'd above; but this bending is not towards but from the Shadow, and is perform'd only in the passage of the Ray by the Body, and at a very small distance from it."

³⁶ Huygens' work is available in an English version by S. P. Thompson as Treatise on Light, London, Macmillan, 1912; reprinted by the University of Chicago Press.

³⁷ In addition to the works mentioned in the footnotes, a few others may be cited for those who may wish to pursue some aspect of this subject further. An excellent bibliographical study of the major publications of Newton and important commentaries may be found in A Descriptive Catalogue of the Grace K. Babson Collection of the Works of Isaac Newton and the Material relating to him in the Babson Institute Library, New York, Herbert Reichner, 1950, which supplements but does not entirely supersede George K. Gray: A Bibliography of the Works of Sir Isaac Newton, together with a List of Books illustrating his Works, second ed., Cambridge, Bowes and Bowes, 1907. A number of important and stimulating essays may be found in The Royal Society: Newton Tercentenary Celebrations, 15-19 July 1946, Cambridge, Cambridge Univ. Press, 1947, especially those by E. N. da C. Andrade, Lord Keynes, and S. I. Vavilov. George F. Shirras is writing a new biography which will have, among other merits, the benefit of study of many unpublished manuscript documents, a considerable portion of which were collected by his teacher, the late Lord Keynes. Alexandre Koyré is preparing a series of Newtonian Studies to match his Etudes Galiléennes; he has provided an earnest of this great work in "The Significance of the Newtonian Synthesis," Archives Internationales d'Histoire des Sciences, 1950, vol. 29, pp. 291-311. An admirable exposition of Newton's early work on color and the prismatic spectrum is Michael Roberts and E. R. Thomas, Newton and the Origin of Colours, London, G. Bell & Sons, 1934.