FROM THE HISTORY OF PHYSICS

NEWTON’S OPTICAL WORKS¹

L. I. Mandelstam

Among Newton’s immortal creations, his optical investigations occupy an important and honorable place.

His discoveries lie at the foundation of all our knowledge of optics; their subsequent development provided not only physicists, but also astronomers and chemists, with a powerful instrument for the study of nature. It was precisely in his optical works that he revealed himself as a brilliant experimenter.

The principles of physical research laid down in them have remained guiding principles to this day. Newton’s optical works brought him his first fame; to them he owed his election as a member of the Royal Society of London—the English Academy of Sciences. If we add to this that Newton’s theoretical views on optical phenomena exerted an enormous, though perhaps not always beneficial, influence on the whole development of optics for a hundred years after his death, then there is hardly any need for further justification of the choice of the topic of my present lecture—“Newton’s Optical Works.”

Allow me to begin somewhat from afar. In 1661 the nineteen-year-old Newton was admitted to Cambridge University. Newton had received his preparation for university studies at the King’s School in Grantham, a small town in the county of Lincoln, near his native village. He stayed there, in all, for five years. Newton’s biographers emphasize the circumstance that already as a boy Newton was marked by reserve and a love of solitude—traits of character that remained with him throughout his life. The school’s headmaster, Stokes, perceptively appreciated the remarkable abilities of his pupil. When Newton’s mother, after a three-year stay

¹ A lecture delivered at a general meeting of academicians in Borovoe on January 16, 1943.

took her son from school and took him home so that he would help her with the household. Stokes, according to the report of Stekley, a close acquaintance of Newton’s family, told her that “it would be a great loss to the world and, moreover, a futile undertaking to bury such a promising genius by making him work in agriculture, which was so contrary to his temperament,” and he succeeded in having the young Newton return to school.

The scientific atmosphere in Cambridge was not favorable to the study of the natural sciences. The powerful movement of the scientific renaissance had hardly touched university teaching. The influence of Aristotelian scholasticism was still strong. Astrology and other prejudices were still alive. The library was in a deplorable state. Of anything resembling our laboratories there was, of course, no question. But young Newton was fortunate. At the beginning of 1663 a chair was founded at Cambridge University by a certain Lucas, a chair destined to become one of the most famous chairs of physics and mathematics in the world. It still exists today. It is held by one of the most talented young theoretical physicists of our time—Dirac.

Isaac Barrow was invited as the first professor to the Lucasian chair. Barrow was an encyclopedist: a traveler who had experienced more than one adventure, an outstanding theologian, an outstanding mathematician, astronomer, and physicist. If it is at all possible to speak of spiritual paternity in relation to Newton, then that honor must belong to Barrow. Under his direct guidance Newton studied mathematics in depth, and at the same time astronomy and optics. The lectures on optics were delivered by Barrow himself. We know that Newton also studied Kepler’s treatise on light and Descartes’s dioptrics. Besides the physical and mathematical sciences, Newton took from the university a thorough knowledge of Latin; he also studied Greek and, apparently, knew the ancient Hebrew alphabet. Newton had no command of either German or French.

Three and a half years after his admission Newton graduated from the university with the degree of bachelor. By that time, somehow suddenly (at any rate for an outside observer), his remarkable genius had unfolded in all its breadth. Indirectly, one external circumstance contributed to this. In the years 1664–1666 a terrible plague epidemic raged in England, carrying off in 1665 more than 31,000 lives in London alone. Whoever could, fled the cities. For about two years Newton lived almost without interruption in his native village. Here he found that solitude and calm which he always needed in times of creative upsurge. Here, if one believes the biographies, there arose those three great discoveries that immortalized his name. I have in mind: the method of fluxions (the foundations of differential and integral calculus), the law of universal gravitation, and the discovery relating to the nature of light and spectral colors. Of course, these discoveries received their final form later, and still later they became the property of scholars.

To his fundamental optical discovery Newton did not come at once. The first optical problem that attracted his attention was the problem of improving the telescope.

After the telescope, invented at the end of the sixteenth century, in Galileo’s hands led to such remarkable discoveries as the discovery of Jupiter’s satellites, the structure of the Milky Way, the phases of Venus, sunspots, and so on, it was natural that scientists should strive to build ever more powerful telescopes and thereby clear the way to new discoveries.

Almost all the great physicists and astronomers of that time were engaged in improving the telescope: Galileo, Descartes, Huygens, Hooke, and others. In doing so they devoted their efforts not only to developing the physical foundations, but also personally ground objectives, dealt with the mechanical questions that arose here, and so forth.

It soon became clear that great obstacles stood in the way of improving the telescope. As the magnification increased, the image given by the objective became indistinct and blurred, which made observations extremely difficult. It was established purely empirically that this defect could be reduced if, simultaneously with increasing the diameter of the objective, its focal length were increased, which was equivalent to increasing the dimensions of the entire instrument. And indeed, in Newton’s time telescopes reached enormous dimensions. There are, for example, reports that in one of Huygens’s instruments the distance between the objective and the eyepiece reached 210 feet, i.e. about 65 m. Naturally, along this path further improvement of the telescope was impossible.

Newton, apparently, was the first to see quite clearly that, besides the known causes distorting the image, such as spherical aberration, there was another and most essential one, as a result of which, among other things, the blurring of the image is accompanied by its coloration or, more precisely, by the appearance of a colored fringe around it. This is what we now call chromatic aberration. But Newton—this is one of the properties of his remarkable genius—having once noticed, even at first glance, an insignificant and unfamiliar circumstance, did not rest until the question had become clear to the end. Now his work proceeded along two parallel lines. On the one hand, he continued working on improving the telescope; on the other, he undertook the investigation of the physical phenomenon underlying chromatic aberration. And here he soon came to the conviction that chromatic aberration is in principle irremovable. This conviction was erroneous, but it forced Newton to abandon attempts to improve the existing type of telescopes and to set about constructing an instrument based on a new principle.

It was well known that light could be concentrated and images obtained not only by means of lenses, but also by concave mirrors. In this case the image is free from chromatic aberration. And so Newton set about constructing a mirror, reflecting ...

of a telescope (a reflector, as distinct from a refractor, i.e., a telescope with a lens as its objective). The idea of the reflecting telescope was not new. Thus, shortly before this, the Scotsman Gregory had not only proposed building such a telescope, but had even given the corresponding calculations. But Gregory’s proposal was never carried out. In view of the fact that Newton was indeed the first to build a practically usable reflector, in view of the difficulties that he had to overcome in doing so, and in view of the improvements that he introduced, he is rightly considered one of the inventors of the reflecting telescope. Newton, however, always openly acknowledged that Gregory’s work was known to him.

The first trial specimen of the reflector was completed by Newton in 1668.

There is preserved a letter from Newton to an unknown addressee, in which he gives a description of his telescope. Here are some extracts from this letter, giving an idea of what Newton succeeded in achieving and characterizing his view of the whole matter:

“...the instrument which I have made is only six inches long and a little more than one inch across (aperture). Its plano-convex eyepiece has a focal length of \(1/6\) or \(1/7\) of an inch, so that the linear magnification of the instrument is about forty, and this is more than any 6-foot (ordinary) tube can, in my opinion, give while preserving the distinctness of the image. But, owing to the wretched quality of the material and the lack of good polishing means, it nevertheless does not give the distinctness that a 6-foot tube can give; but, in any case, I believe that with its help as much can be found as with the help of any tube 3 or 4 feet long, especially if the object is (self-)luminous. With this telescope I saw Jupiter distinctly round, saw its satellites and the horns of Venus....”

And further:

“...and I do not doubt that in time, by this method, a 6-foot tube can be built which will be no worse than any tube 60 or even 100 feet long, made according to the ordinary type. At the same time I am sure that if one makes an ordinary-type telescope with an objective of the purest glass, polished in the best manner, with the most advantageous shape, which any geometer (Descartes, etc.) has already calculated or is able to calculate (and this, after all, is everything toward which people have striven or what they have desired up to this time), then even such an instrument will hardly give much more than an ordinary good instrument of the same length. And this assertion, however paradoxical it may seem, is a necessary consequence of some of my experiments relating to the nature of light.”

We shall now return to these last remarkable words. Already from the description given it is clear how substantial were the results achieved by Newton.

His first telescope’s success became known in fairly wide circles, and at the insistence of those around him Newton set about constructing a second, improved instrument.

It is interesting to note that, in the art of polishing mirrors—the most difficult and responsible part of the whole work—Newton had by that time attained an extraordinarily high degree of perfection, surpassing in this art the professional London craftsmen.

This second specimen was ready in 1671. It was brought to London, apparently made a great impression on the scientists, was demonstrated to the king, and then handed over to the Royal Society of London. At the meeting of January 11, 1672, devoted to discussion of his invention, Newton was elected a Fellow of the Society, in whose museum this telescope is still preserved as one of its most precious treasures.

Before leaving the question of the telescope, which, as we have just seen, played so notable a role in Newton’s life, allow me to say a few words about the subsequent development of this problem.

Despite the substantial result achieved by Newton, reflecting telescopes were slow to enter the practice of astronomers. A decisive shift occurred in the second half of the eighteenth century, after William Herschel had built his famous reflectors—one of them was 40 feet long—with the aid of which he carried out remarkable investigations concerning nebulae and double stars, and discovered the planet Uranus.

But the question of refractors, too, did not stand still.

Newton’s conviction that it was impossible to improve the refractor, so vividly expressed in the letter cited above, was, as has been said, erroneous. But so great was his authority that this conviction was accepted on faith for a long time, and this undoubtedly hindered attempts to improve refractors. In 1757 it became known that Dollond had in practice refuted Newton’s opinion. He actually constructed a compound objective, free or almost free from chromatic aberration. Dollond’s discovery played an enormous role in astronomy. It now became possible to build full-fledged refractors, possessing a number of great advantages over reflectors. From that time onward both types of telescopes have developed in parallel, competing with one another or, more precisely, complementing one another, since each of them has its own major shortcomings as well as major advantages. It may be worth noting that the largest telescopes at present are being built according to the reflecting principle. Such is the famous telescope on Mount Wilson in California, with a mirror 100 inches in diameter. Such, apparently, is the largest of all telescopes ever to have existed, now nearing completion on Mount Palomar, also in California; the diameter of its mirror is 200 inches, i.e., about 5 m.

However important and interesting Newton’s work on the improvement of the telescope may be, it was not this that determined his immortal merits

in the field of optics. The foundations of Newton’s fundamental optical discovery were laid in those experiments concerning the nature of light, about which he wrote at the end of the letter cited above.

About three years passed from that time. A week after his election as a member of the Royal Society, Newton wrote to its secretary, Oldenburg, a letter that became famous. Here is its concluding part:

“I should like you to inform me how much longer the weekly meetings of the Society will continue, since, if they continue for some time yet, I intend to submit, for examination and discussion, a communication about a certain philosophical discovery which prompted the construction of the above-mentioned telescope, but which will, I do not doubt, prove much more substantial than the communication about the instrument, since it concerns, in my opinion, the strangest, if not the most important, discovery that has so far been made with regard to the actions of nature.”

Approximately two weeks later Newton sent to the Society his famous communication—“A New Theory of Light and Colors”—for presentation and publication in the Transactions of the Society.

I should like to dwell on this fundamental discovery in greater detail. But here I feel a particular difficulty. When the subject is such discoveries as Newton’s—discoveries known to all of us from school—it is easy to find oneself, as I know from my own experience, in the position of that lover of literature who, when asked how he liked Woe from Wit, said that in Griboyedov’s comedy he saw, in essence, nothing remarkable, since it consists entirely of long-familiar sayings and proverbs.

In order not to lose perspective, it seems to me best of all to take a historical point of view. We must try to imagine, at least in general outline, the state of the question before Newton; then recall what Newton did; and finally briefly trace the role that his works played in the subsequent development of science.

What, then, can be said about the state of optics at the time when Newton began his investigations concerning the nature of light?

To answer this question, it is expedient to proceed from the classification to which physics now adheres. We assign to geometrical optics those questions and that treatment of them in which the matter concerns the geometrical paths of light rays upon reflection and refraction. This is an important area of optics, since it lies at the basis of the calculation of all optical apparatus and instruments. In essence it is purely computational in character and is very poor in physical content. All other optical phenomena of the external world—first and foremost dispersion phenomena, diffraction, interference, and polarization, in which the most varied play of colors is manifested and which make optics not only one of the most important fields of physics but also the most beautiful—belong to so-called physical optics.

The foundations of geometrical optics were laid by ancient Greek mathematicians. The fundamental law of the reflection of light was known to them. The fundamental law of refraction was discovered and formulated by Snell and Descartes at the beginning of the seventeenth century, though, as Newton’s discovery showed, in a form that did not exhaust the question.

In the Dioptrics Descartes gave interesting and important investigations of the path of rays upon refraction in transparent media and applied them to the explanation of the rainbow, having, among other things, in mind the question of spherical aberration. On the basis of this law he also gave the first theory of the rainbow. Thus, in Newton’s time geometrical optics stood at a rather high level of development. It is possible that this is explained precisely by the poverty of the physical content of this discipline and by the practical importance of the problems found here.

The situation was entirely different with physical optics. It will hardly be an exaggeration if we say that by Newton’s time physical optics as a scientific discipline did not exist.

It goes without saying that such remarkable natural optical phenomena as the azure of the sky—which, incidentally, has received a correct explanation only in very recent times—the blue of the sea, the wealth and variety of colors in the animal and vegetable world, and the colors of the rainbow could not have remained without influence on people at all times. Art responded first of all to immediate impressions; but science, too (in every epoch), could not, of course, pass by these problems (and in ancient science and in the science of the Middle Ages one may find a number of interesting observations and remarks). But the whole (scientific) approach to these questions, the whole spirit of research in the pre-Newtonian period, bore a character entirely different from that which we now regard—chiefly thanks to Newton’s works—as obligatory for every physical scientific investigation.

I cannot dwell on the interesting epistemological question of what scientific research in general was and what it ought to be. But I shall allow myself, as an illustration of what has just been said, to give several examples.

The first example I should like to take from ancient times, from the famous poem of Lucretius, De rerum Natura, written, as is known, in the first century before our era. In this poem, in essence, the natural-scientific views of Epicurus are set forth. As far as I know, historians consider that the exposition of the poem stands at the level of the knowledge of its time. The great poetic merits of the poem are generally recognized. In it there are undoubtedly interesting and correct observations and remarks of a general character concerning the phenomena of nature, in particular concerning light and colors, to which rather great attention is devoted. But here is what happens when the matter comes to a scientific explanation in a concrete case (I quote from the translation by F. A. Petrovsky). “Moreover, roosters, accustomed to flap their wings at night and to cry loudly, calling forth the dawn at daybreak, fierce lions are utterly unable to endure and at once, as soon as they see it somewhere, turn ...”

flee. It is clear, of course, to us why this happens: there are certain seeds which, flying off from the bodies of roosters, get into lions’ eyes and bore into their pupils, causing acute pain, and for them, though fierce, it is unbearable. Our vision, however, does not suffer in the least from such seeds.”

But even later, in the Middle Ages, no substantial shift occurred in the question that interests us. The reliability of the facts that were often laid at the basis of scientific reasoning may be judged from an example I have borrowed from P. Tannery. In the work On Equilibrium by the famous Arab scholar-alchemist Geber, who lived in the tenth century, there are about 50 questions. Here is one of them: “Why, as everyone knows, does a cloud not give rain when a woman comes out of the house naked and turns her face toward the cloud?”

But let us return to Newton’s era. What views were held at that time on the fundamental question of the origin of colors? Here is what the famous Kepler writes in his work, which appeared in 1604:

“Color is potential light, enclosed in transparent matter (if, indeed, it can be considered something independent of vision at all), and the various properties in the nature of matter, depending on its rarity or density, transparency or opacity, determine the diversity of colors.”

And further:

“Undoubtedly, they (i.e. colors) owe their existence to the weakening of light and to the addition of watery material.”

In 1611 there was published a treatise on light by the famous—so Newton calls him—Antonius de Dominis, Archbishop of Spalatro. In it, in connection with the explanation of the cause of the rainbow, a theory of colors is set forth:

“If pure light is enclosed in a body, as, for example, in the stars or in fire, and this light for some reason loses its brilliance, then it appears as white light.

If a certain quantity of darkness is mixed with the light, which nevertheless allows the light to pass and does not absorb it completely, then intermediate colors appear. On this basis fire seems reddish, because it is mixed with smoke that darkens it... There are three intermediate colors; the smallest admixture of darkness produces the red color...”

Then come references to quite superficial experiments, to which an obviously incorrect interpretation is given. The view presented here of the origin of colors as the result of mixing light with darkness—a view from which Kepler differs little—was very widespread in Newton’s time. Newton himself apparently has it in mind when, in the Opticks, he contrasts his explanation of one case with the ordinary hypothesis of the philosophers.

The conception underlying these and all similar pre-Newtonian views, insofar as anything definite can generally be put into them, consists in the following: light, by

nature itself has no color; color is a new quality, distinct from light; in Dominicus this quality is darkness, which is mixed with light in refractions and reflections and gives light one color or another. All these views are sustained in the spirit of scholasticism, Aristotle, and medieval reasoning.

I know that the view of the Middle Ages as an epoch of complete stagnation in science is hardly fair. Both in the Middle Ages and later, before Newton, one can point concretely to individual observations remarkable for their precision and to interesting arguments by individual scholars of that time.

But, on the other hand, there is no doubt that theories such as the one just cited—and they dominated (until Newton) in the field of physical optics—led physics into a dead end. Their characteristic feature is a contemptuous and extremely frivolous attitude toward experiment, a striving to penetrate the so-called essence of things by means of reasoning alone, often operating with concepts that are far from obvious, and for which no definition is given. What is pure light? Darkness, probably, cannot be identified with smoke, otherwise the absurdity of the whole theory could not have failed to be clear to its authors. But then what is darkness, which can be mixed with light in various proportions?

Such theories may speak to the imagination; but if the task of physics consists in obtaining, on the basis of the results of definite experiments, the possibility of predicting the results of others without having to carry them out, and thus of learning to reproduce at will particular phenomena and to master them, then it must be admitted that the theories cited above do not solve this task and cannot solve it; they are devoid of scientific significance, they are sterile. This, approximately, was the state in which Newton found the problem of colors.

But one observation must be made. We have just seen how strong, and at times—as, for example, in the question that interests us—dominant the influence of barren scholasticism still was in Newton’s time. But, on the other hand, it is well known that by that time this kind of natural philosophy as such had already lost its dominant significance. New currents, associated chiefly with the names of Galileo, Pascal, Gilbert, Descartes, and others, had already borne remarkable fruit, at the end of the sixteenth and in the first half of the seventeenth century, in a number of concrete physical problems. But physical optics lagged behind. And in the 1660s–1670s (seventeenth century) there came, as if in order to make up for lost time, its remarkable flowering; and these years, as Academician S. I. Vavilov rightly points out, may be regarded as the years of the birth of scientific physical optics, both experimental and theoretical.

At that time, alongside the remarkable experimental investigations of Grimaldi, Hooke, and Huygens, the foundations were laid for wave conceptions, which—though only more than 150 years later—were destined for such a brilliant future.

And especially brightly shining at this time is the genius of Newton. The chief cornerstone of the entire development of optics, which left its imprint on all the further development of physics, is precisely his investigations relating to the central optical problem of colors. And here he had nothing to rely on, least of all wave theory.

In confirmation of this one may cite the words of its founder—the brilliant Huygens. Sending Leibniz his remarkable Treatise on Light (in which the foundations of wave theory were laid), printed, incidentally, in 1690, i.e., more than 20 years after Newton’s works, Huygens wrote:

“In my treatise on light I have said nothing about colors, considering this subject extremely difficult, especially in view of the great variety of the circumstances under which colors arise.”

Thus, Newton could obtain no help from wave theory in his investigations. On the contrary, I think that wave theory yielded such brilliant fruits partly because it could be based on Newton’s discovery. Such, in essence, was the situation with the fundamental question of physical optics, the question of the nature of light and colors.

And one more, final remark. Newton’s works, which will now be discussed, in addition to their enormous factual significance, represent a fundamental turn in the direction of physical science in general. It will hardly be an exaggeration to say that before Newton all investigators, including Galileo, when embarking upon the study of a physical problem, proceeded from certain a priori conceptions. Experiment served to verify them or, at best, to introduce corrections. Newton broke with this tradition. He considered that a priori knowledge of nature is impossible; that the methods of cognition are observation, experiment, and the generalization of the obtained results by induction; that “the best and most reliable method of philosophizing apparently consists in first diligently studying the properties of things and establishing these properties by means of experiment, and then cautiously passing on to hypotheses for their explanation.”

A brilliant result of the application of these principles in practice was his remarkable discovery, set forth in the above-mentioned memoir “Theory of Light and Colors,”—a discovery to the consideration of which I should now like to turn.

In Newton’s homeland there exists a splendid custom. There they try to accompany lectures devoted to the creations of great experimenters by reproducing their original experiments and, where possible, with the very same apparatus with which they were performed.

We are deprived of this possibility. The only thing I can do is to adhere, as far as possible, to Newton’s original drawings and formulations. I take them chiefly not from the first memoir, but from Newton’s principal work—his Optics. I shall have occasion to speak further about this remarkable work; here

I shall only note that it, like the Principia, is constructed on the model of Euclid’s Elements. First come definitions, then axioms, further propositions—theorems and problems, and with them explanations and proofs by experiments. The experimental part occupies the main place in the book. Apparently, it is not entirely clear what gave Newton the first impetus toward the study of prismatic colors. But there is no doubt that one of the reasons for his interest in this question was, as we saw above, the desire to clarify for himself the phenomenon of chromatic aberration. And so, in order to clarify the fundamental question, Newton performs the following experiment, which stands in first place in the Opticks.

If one looks at an oblong rectangular piece of paper, half colored red and half blue, through a prism, as is shown in Fig. 1, then the rectangle appears as if broken: the blue half is deflected more strongly than the red.

Fig. 1.

The next experiment: a similar piece of paper, i.e. a rectangle, both halves of which are colored as in the first experiment, is wrapped with several turns of black silk thread, placed vertically (Fig. 2), and illuminated by the flame of a candle. A lens casts the image of the illuminated paper onto a movable screen, and here the following is discovered. Where a sharp image of the red half is obtained—the threads serve for exact setting “at focus”—the image of the blue half is blurred. And conversely, if the screen is moved somewhat closer, a sharp image of the blue is obtained, but the red side is blurred.

Fig. 2.

These experiments illustrate the following fundamental proposition of Newton:

Book I. Proposition I, Theorem I. Rays that differ in color differ also in their degrees of refrangibility.

Newton then passes on to a new series of experiments. The principal one of these experiments is the following.

Sunlight enters through a narrow opening in a shutter into a darkened room. On the opposite wall there appears a small circle (of the same white, or, more precisely, yellowish color as the light of the sun). Now a prism is placed in the path of the rays (Fig. 3). After passing through the prism, the thin beam of light is split up, or, more precisely, after the prism there is observed not a thin cylindrical beam, but a beam diverging in a fan-like way. On the wall there is now seen not a small circle, but a band, the length of which in Newton’s experiment was approximately five times greater than the width, which remained equal to the diameter of the circle. And this band is colored in all the colors of the rainbow, beginning with red at the top, through orange, yellow, green, blue, to violet; moreover—Newton emphasizes this—the color passes continuously through every possible shade. On the wall one obtains what is called the solar spectrum.

Fig. 3.

Comparing this experiment with Proposition I, Newton concludes:

Proposition II, Theorem I. Sunlight consists of rays of different refrangibility.

The indicated propositions are illustrated (checked) by a whole series of experiments, which I shall not set out in detail here. I shall merely note that in these and subsequent experiments Newton for the first time applied methods and techniques of investigation that we still use today in optical laboratories. Such, for example, is the adjustment for the minimum angle of deviation of a prism (Fig. 4), a simple but extremely effective method of crossed prisms, and so on. To obtain a pure spectrum of the greatest possible intensity Newton used an arrangement (a collimating slit, a prism, and focusing apparatus) which even now forms the basis of all prism and other spectroscopic instruments.

Fig. 4.

It is necessary also to mention a series of experiments, simple and elegant, whose purpose was to illustrate the proposition, in a certain sense the converse of Proposition II, that the spectral colors, when mixed in the proper proportions, again produce the sensation of white color.

All this, again, is quite well known, although it must be said that the school exposition and the experiments that are usually demonstrated,

do not provide a sufficient notion of the delicacy of the experiment and of the significance—which we shall see further on—that Newton’s investigations really possess.

I must nevertheless dwell somewhat more fully on one of the subsequent experiments. I have in mind the famous experiment—the sixth in the first book. Newton attached great importance to this experiment: in his first memoir he called it the “experimentum crucis.”

The matter concerns the study of the properties of the individual “colored rays” of the fan emerging from the prism. To this end Newton proceeds as follows. He casts the solar spectrum onto a board (Figs. 5a

Fig. 5a.

and 5b), in which a narrow aperture has been made, so that, depending on the position of this aperture, only a thin bundle of rays corresponding to a definite color of the spectrum emerges outward. Newton then causes this bundle, in turn, to fall upon a second prism. And here the experiment shows the following. This thin “colored” bundle of light is deviated by the prism, and is deviated to different degrees according to its color; but it is no longer decomposed and remains, on leaving the prism, as thin as it was on entering; the “color” of the bundle does not change. Thus this experiment shows the existence of a special kind of light of different color. Light of this kind is characterized, first, by a definite color—this is a physiological property—and, at the same time, by elementarity, in the sense that upon refraction it is not decomposed, in contrast to,

Fig. 5b. Newton’s own drawing, sent by him to Arland.

for example, by sunlight, but is deflected as a whole, and moreover the degree of refraction, i.e. a quantitative physical characteristic, is unambiguously connected with color. This discovery of extraordinary importance Newton formulates in the Opticks in the form of a definition:

“Light, the rays of which are all refracted alike, I call simple, homogeneous, and similar; but light, some of whose rays are more refracted than others, I call compound, heterogeneous, and dissimilar.”

By a series of simple but convincing experiments Newton shows that “simple” light or, as we now say, monochromatic, or spectrally simple, light is not changed either by reflection, or by refraction, or by scattering. Color and the degree of refrangibility are, so to speak, the original properties of color.

But Newton knows that the sensation of one color or another cannot decide the question of its monochromaticity. By mixing, for example, blue and red, one can obtain yellow, which the eye will not distinguish from spectral yellow, but which, on passing through a prism, will split up. The colors of bodies are not monochromatic; therefore, for example, in experiments 1 and 2, discussed above, the matter was essentially—Newton points this out—one of different average deviation of the red and blue halves.

Summarizing briefly what has been set forth above, Newton’s results may be formulated approximately as follows—though, to be sure, somewhat schematically.

There exists a special kind of light—this is monochromatic or simple light of various chromaticities. To every simple color (simple light) there corresponds a quantitative characteristic—a definite degree of refrangibility. The properties of simple light cannot be changed in any way. Every other light, including white light, is a mixture of various “simple colors.”

With the aid of a prism, from a mixture there can be separated the individual simple colors already present in the mixture; but neither prisms nor other devices can impose, as had been thought earlier, one color or another.

It is difficult to overestimate the significance of Newton’s discovery. He was the first to give a genuine doctrine of colors, on the basis of which he himself found an enormous number of new facts quantitatively related to one another, and opened the way to the discovery of new ones. And above all let us note the following. In the light of Newton’s teaching on colors, all pre-Newtonian geometrical optics, insofar as refraction was concerned, was only a first and crude approximation. Indeed, Snellius and Descartes formulated this law thus: the ratio of the sine of the angle of incidence to the sine of the angle of refraction for a given medium is constant. But now it is clear that, in view of the different degree of refrangibility inherent in different colors, one definite angle of refraction does not exist at all. Thus, for Descartes and Snellius, at best, the question can be only of some very indefinite—

... average value of the angle. Hence it is obvious that all questions of colors fell outside the framework of pre-Newtonian geometrical optics. Newton showed the validity of the law of refraction for each color separately. Thus, geometrical optics acquired a truly completed quantitative form only thanks to Newton’s discovery. A whole class of phenomena now became accessible to quantitative consideration. Having determined, for example, experimentally the degree of deviation of various monochromatic rays (say, in a given kind of glass), or, as we say, their refractive index by means of a prism, Newton could already, without further experiments, quantitatively calculate chromatic aberration in lenses. This is what Newton did.

Another example. Descartes, in his famous theory of the rainbow, proceeds from the correct conception that the rainbow is the result of the refraction and reflection of the sun’s rays in drops of water suspended in the air, and on the basis of the law of refraction he derives the magnitude of the angle at which rays of especially great intensity emerge from the drop. It is at this angle, which he determines, that we see the rainbow. This is a great achievement. But Descartes’ theory did not even touch, and could not touch, the most interesting, essentially, side of the problem: why is the rainbow so intensely colored? What is the sequence of the colors? and so on. The answer to this is contained in Newton’s doctrine and consists, of course, in the fact that since the angle at which we see the light emerging from the drops is a consequence of refraction, and the degree of refraction depends on the color, we see different colors in several different directions. Newton gave a quantitative calculation of the phenomenon, determined the sequence of the colors, the width of the rainbow, and so forth. The basic features of his theory remain unchanged even now. A complete theory of the rainbow, taking diffraction into account, was given only in the nineteenth century.

I think that these few examples show the meaning and enormous significance of Newton’s discovery, and above all the effective (concrete) power of his doctrine.

As has already been said, the experimental part of Newton’s optical investigations—by the purposefulness of the experiments, by their simplicity and by their accuracy, within the limits possible at the time—arouses amazement; and it is all the more difficult to understand how Newton fell into an error that had purely practical consequences. In his studies of the different refrangibility of different monochromatic rays, Newton deliberately varied the experiments very greatly, changing the angle of incidence, the size of the aperture, and finally, examining various transparent media, he took different glass prisms, but also a prism “of water,” i.e. a full prismatic vessel with glass walls, filled with water. In doing so he came to the conclusion that the magnitude of the dispersion, i.e. the difference in the degree of deviation of two different rays (near the edges of the spectrum), is proportional to the mean deviation, or (what is the same thing) that the relative dispersion does not depend on the substance.

This result, which led Newton to the conviction that it was impossible to construct an achromatic objective, is, broadly speaking, incorrect. How could Newton have made such an error?

I do not know of a satisfactory answer to this question. It is possible that, as S. I. Vavilov thinks, the circumstance played a role here that, in order to increase the refraction, Newton added lead sugar to the water (he himself says this). A certain theoretical bias is possible; some other hypothetical explanations are also possible; but all these are only suppositions.

One further remark. Newton, as we see, repeatedly emphasizes the stability—the invariability—of the properties of simple light, which he apparently regarded as absolute. This opinion was significant in shaping his theoretical views on the nature of light. The stability (of simple light) is also at present one of the most important propositions in optics, though, to be sure, not in this absolute form. Color—we now say: the wavelength of monochromatic light—is indeed extraordinarily stable, but under certain conditions it nevertheless can change. However, Newton could not have noticed changes of this kind. The means at his disposal were not sufficient for this, since these changes are extremely small and very rare; and each time it proves possible to create conditions that change the color of monochromatic light, this is an event in physics—even up to the most recent times.

One of the sure indicators of the significance of a physical discovery is the influence it has exerted on the further development of science. Approaching Newton’s discovery from this point of view, we must undoubtedly regard it as one of the greatest physical discoveries in general.

It is hard to imagine how optics would have developed without Newton’s investigation and, especially, without the discovery of the simple spectral colors, which are those exceedingly stable elements of which, according to Newton, every light field is composed.

The point is that the most varied, numerous, and many-sided optical phenomena, whose discovery enriched optics subsequently, when observed in ordinary, for example white, light, are so complex that it is often extremely difficult to make sense of them. But the same phenomena, when monochromatic light is used, are considerably simplified, so that here it becomes possible to establish laws. Having done this, and taking into account the principle of superposition—one of the most fruitful principles of optics—we then, without special difficulties, chiefly by computational means, pass to the general case of any light. This was the path by which knowledge of ever new optical phenomena advanced.

The development of Newton’s ideas led to an enormous enrichment of the factual side of science.

In 1814 Fraunhofer found dark lines in the continuous solar spectrum. In the light of Newton’s discovery this meant that in the incom-

that in the light coming to us from the Sun some simple spectral colors are absent or greatly weakened.

On the other hand, it was found that in the spectrum of luminous gases the continuous part of the spectrum is absent, and only individual simple colors are present—a line spectrum, with their combination being specific to atoms of a given kind.

On the basis of these two facts Bunsen and Kirchhoff created spectral analysis, which was destined to play such an enormous role in physics, chemistry, and astronomy.

Indeed, only thanks to it was it possible to solve problems that had seemed absolutely insoluble: to determine the chemical composition of heavenly bodies, to determine the radial velocities of stars (with respect to the Earth), to discover and measure the magnetic field on the Sun, and so on. What would remain of astrophysics if spectral analysis were taken away from it?

And in chemistry? A number of new elements were discovered only thanks to spectral analysis. In very recent times spectral analysis has made it possible to investigate the structure of complex molecules, and these investigations, which have already yielded interesting results, will undoubtedly open a new chapter in this field. Almost all of this is well known, but it is not always remembered that spectral analysis is a close and direct heir to Newton’s discovery.

I should like to add one more remark. When it became clear that the structure of the line spectrum emitted by an atom is specific to the latter, there arose a desire to learn the dynamics of the atom through its spectrum or, in other words, to construct a mechanical—or rather, mechanico-electrical—model of it.

However, all attempts to do this proved futile. Moreover, little by little it became clear—Lord Rayleigh showed this especially convincingly—that this failure was not accidental, that the action of any mechanical model would be in contradiction with the observed regularity in spectra.

A conflict arose, which one of the prominent scientists called the scandal of the physics of that time.

Gordius’ knot was cut, relying on Planck’s famous discovery, by Bohr in 1913.

The point is that people had always proceeded from the tacit premise that the atom, as a mechanical system, obeys the laws of Newtonian mechanics. But this premise is based on nothing. Newtonian mechanics retains all its force for the macrocosm, but for the microcosm, for atoms, a mechanics must be built on new principles. Bohr indicated the fundamental ones. Then not only do the contradictions disappear, but a whole series of spectral regularities, previously quite inexplicable, naturally follows from the new premises.

Thus, one may say that Newton’s optical discoveries provided the weapon which, in the end, helped to destroy the age-old conviction—or rather, prejudice—in the universal applicability of Newtonian mechanics.

But this conflict was not fruitless. It was, as we have just seen, one of the stimuli that brought to life that, in my view, remarkable conception which we call quantum physics.

In tracing the course of Newton’s ideas, we have left aside the external events of his life. It may be useful to fill this gap briefly. We have seen that Newton graduated from the university in 1665 with the degree of bachelor. At the beginning of 1668 he was admitted as a “Major Fellow” at Trinity College, and in the same year became a master. In the following year Barrow left the Lucasian chair in order to devote himself entirely to theology, and handed it over to Newton. At the beginning of 1672, as we have seen, he was elected a member of the Royal Society. Thus, by the age of thirty, Newton had attained that independence and that position which enabled him to devote himself wholly to science. His professorial duties were not burdensome. He had to give one lecture a week—yet even that, apparently, was often canceled for lack of listeners—and twice a week, if necessary, to give consultation to students. Apparently there was no great need for this either.

The choice of subject was left to the professor. For the first two years Newton lectured on optics. In these lectures he first set forth his discoveries. The lectures were not published during Newton’s lifetime. Several copies of them were deposited in the university archives and issued to those who wished them.

It may be appropriate here to say a few words about what works on optics Newton left behind. In contrast to his astronomical works, which had not been published beforehand and formed the content of the brilliant Principia, the remarkable report on which by Academician A. N. Krylov we have just heard, his optical works were reported and printed as results were obtained.

The principal memoirs, as well as letters, notes, and polemical writings, were printed in the Transactions of the Royal Society. In 1704 Newton published his chief work on optics: Opticks: or, A Treatise of the Reflections, Refractions, Inflections and Colours of Light. This work of Newton’s, written in English, became widely disseminated. During his lifetime there appeared three English, three Latin, and one French edition.

The Opticks was reprinted many times during the eighteenth century. In it Newton gathered what had been done by him earlier, supplemented it, and in part deepened it. As an appendix Newton placed a series of questions—“Queries.” In them he gave the results of his, undoubtedly many years’, reflections on optics and on the most varied questions of physics. This remarkable work is in itself. I shall say a few more words about the “Queries” below. Finally, in 1729 his Lectiones opticae were published. Their content is approximately, so far as I can judge without having them at hand, the same as that of the Opticks, but in the Opticks there are no mathematical calculations whatever. In the Lectiones the calculations are given in extenso. The Opticks exists—

NEWTON’S OPTICAL WORKS

in the Russian, highly authoritative annotated translation by Acad. S. I. Vavilov. From a letter of Sergei Ivanovich I know that, in connection with the present jubilee, he has translated the Lectiones as well. We shall all impatiently await their appearance.

I should like to point out one more source. This is the remarkable work of Acad. A. N. Krylov, published by him in 1935—“Newton’s Theory of Refraction.” The history of this work, according to Aleksei Nikolaevich himself, with whose permission I cite it, is as follows. In 1832, in the attic of a house in London, a box was discovered containing various manuscripts and old letters. Among these papers were 27 letters from Newton to Flamsteed. All the papers were delivered to Baily, vice-president of the London Astronomical Society, put in order by him, and published in the form of a quarto volume. The book did not go on sale; it was sent out to scientific institutions and to well-known astronomers, so that this book is rare. Aleksei Nikolaevich bought it by chance for two and a half shillings at a junk market in London.

Among Newton’s letters there are some that relate to the theory of refraction.

When determining the position of a luminary from observations, it is necessary to introduce a correction for the curvature of the ray of light in the earth’s atmosphere—the so-called atmospheric refraction. Newton was not satisfied with the existing tables and created his own theory, according to which tables were calculated. Newton did not publish this theory. Only in a letter to Flamsteed does he report the table and formulate the fundamental theorem without proof. The tables were later published once more by Halley.

Aleksei Nikolaevich reconstructed the course of Newton’s thought and gave a complete proof of his propositions, using only those mathematical means that were at Newton’s disposal, and thereby revived this extraordinarily interesting work of Newton. Here is what Aleksei Nikolaevich says at the conclusion of his article about the value of this optical work for astronomy.

“I have gone into all these details in order to show how complete and general was the theory of astronomical refraction which Newton created at the end of 1694 and the beginning of 1695, but which he, unfortunately, did not publish. If Newton’s theory is developed by the elementary methods of analysis that Newton possessed, and compared with modern theories, one can immediately see how simple and natural the exposition becomes, and how little, in essence, has been added to it in 240 years.”

One more remark. Here, in Borovoe, I could not, of course, make sufficient use of sources. I had at hand the detailed new popular biography by Louis Trenchard More, which contains many of Newton’s letters and quotations from certain documents. Of Newton’s works, thanks to the kindness of Aleksei Nikolaevich, I had at my disposal only the Russian translation of the Opticks and Aleksei Nikolaevich’s work, of which I have just spoken. The cited...

the factual data I have taken from these sources. For the illumination and evaluation of them, of course, I myself bear responsibility.

How, then, were Newton’s discoveries received by his contemporaries? Upon receiving his communication on the new theory of light and colors, of which we spoke above, the Royal Society, at its meeting, resolved to express its thanks to the author for his communication, to inform him that it deemed it desirable to print the communication, and also to instruct the secretary to send a copy to Huygens. At the same time a commission was chosen, consisting of the Archbishop of Salisbury, Boyle, and Hooke, for a detailed study of Newton’s communication and a report to the Society.

Already at the next meeting, held a week later, Hooke presented his detailed report. It is difficult, when speaking of Newton and especially of his optical works, to pass over Hooke’s personality entirely in silence. From 1662 he held the post of curator of the Royal Society. His duties included, at every meeting of the Society, i.e. once a week, demonstrating three or four significant experiments (so it was recorded in the minute book of the Society), without expecting remuneration until the corresponding fund should be collected. His duties also included collecting rarities for the Society’s museum. Among the rarities were, for example, a live ostrich, and also grass grown in the stomach of a thrush. Hooke was a very great scholar, but he scattered himself and rarely carried his thought through to the end. His name would now sound more loudly if it were not overshadowed by Newton’s proximity. In Newton’s life Hooke played the role of an evil genius.

There was almost not a single work of Newton’s, beginning with the telescope, that Hooke did not criticize, or on account of which he did not assert his own priority. Newton did not like him and, on his side, suppressed his works. Here is what Newton’s biographer More says of him:

“What sort of man was this, whose personal opposition delayed the publication of the Opticks for 30 years and nearly prevented the completion of the Principia, whose malicious tongue intensified Newton’s tendency toward seclusion and solitude, cooling his youthful enthusiasm toward the Royal Society, and poisoned his taste for science?”

Hooke’s report is remarkable in many respects. It contains individual thoughts that, in my opinion, were far ahead of his time and were understood only at the end of the nineteenth century. Best of all, I shall read to you an excerpt from this report:

“I derived great pleasure from his instructive and subtle observations. But although I wholly agree with him as to the truth of what he asserts, since it is confirmed by hundreds of trials, at the same time I must confess that I cannot regard these observations as irrefutable proof of the truth of his hypothesis for solving the problem of colors...”

Further denying that the sixth experiment had the significance of an experimentum crucis, he points out, among other things, that proposition II does not follow of necessity from this experiment.

For him, the presence of all colors in a white ray is just as incomprehensible as the assertion that all the tones of an organ are already contained in the rush of bellows. And further:

“I should not wish to be understood as objecting by all this to his theory as a hypothesis, since I very readily agree with each of the propositions expressed in it and consider it very subtle and ingenious and capable of explaining all the phenomena of colors. But I cannot regard it either as the only hypothesis, or as so irrefutable as a mathematical proof!”

The Society resolved to send a copy to Newton and to continue, in the event of his consent, to print Newton’s communications; Hooke’s report, however, was to be printed afterward, so that Newton could not consider it disrespectful that a refutation followed so quickly upon a communication greeted by the Society with friendly applause.

Here is what the great Huygens wrote to Oldenburg concerning Newton’s discovery:

“As for his new theory of colors, it seems to me extraordinarily ingenious, but one must see whether it agrees with all the experiments.”

And then, in another letter to Oldenburg:

“What you printed in the last number greatly strengthens his doctrine of colors. But at the same time the cause of light may be quite different, and it seems to me that he ought to have been content to regard his assertion as a probable hypothesis.”

I shall not dwell on the other opponents. I shall point only to one—Pardies, professor of physics in one of the Paris colleges—since, touched by the correct tone of his criticism, Newton answered him with two detailed letters, intended, of course, not only for him, but also for Huygens and Hooke. It must be noted that Newton in general reacted extremely painfully to any criticism of his work.

“I see,” he writes concerning, it is true, the ignorant criticism of a certain Linus, “that I have become a slave of philosophy; when I am free of the business of Linus, I shall decisively take leave of it forever, except for what I shall do for my own satisfaction...”

And further:

“I see that one must either give up offering anything new (in science), or become a slave for its defense...”

What, then, does Newton answer in substance to the criticism of Huygens and Hooke? He believes that their criticism misses the mark. They criticize his hypothesis, whereas the essence of the matter lies in something quite different. Here is what he writes in a letter to Pardies:

“In my work, in my opinion, only certain properties of light are set forth, which, now that they have been discovered, can, I believe, easily be proved. If I did not consider these properties true, I would sooner cast them aside as vain and empty speculation than acknowledge them as a hypothesis.”

Approximately the same thing he writes in his reply to Hooke:

“It is true that from my theory I draw arguments in favor of considering light a corporeal phenomenon. But I do not regard this assertion as obligatory. I know that the properties of light which I have found may, to a certain extent, be explained by other mechanical hypotheses; therefore I prefer to renounce all of them altogether.” And he concludes: “You see how little need there is to argue about hypotheses.”

Thus Newton accuses his opponents of criticizing a wholly secondary part of his work, and that for this reason their criticism is entirely without significance.

As far as I know, most historians of science consider this reply of Newton’s correct. I cannot agree with this. One thing is, of course, indisputable: in his optical works as a whole, and in the passages just cited only, in short, Newton was the first to proclaim the proposition that nature cannot be known a priori, that the highest court of appeal in all physical questions is experiment; and by this he secured for himself the fame of the founder of our modern views on physics. But a completely different question is whether such a full and strict separation of facts from theories, as Newton postulates it, is possible.

I cannot and do not wish to touch upon this question in substance1. It seems to me that in the present case the matter is simpler. If in the Principia the role of hypotheses is reduced to a minimum, this is not so in the Optics. Here the whole structure of the exposition and the individual formulations of facts bear the imprint of the corpuscular hypothesis and often lose their meaning if one adopts the wave point of view. It seems to me, therefore, that both Hooke and Huygens had grounds for their criticism. What they may be reproached for is, in my opinion, that in the heat of criticism they did not sufficiently appreciate the enormous significance of Newton’s discovery as a whole. However, for us, who have before our eyes the fruits which these discoveries have brought, it is easier to do this.

About four years passed after the publication of the first optical memoir. On November 18, 1675, Newton presented to the Royal Society a remarkable memoir devoted to investigations of the colors of thin transparent plates or layers. We all know these investigations under the name “Newton’s rings.” It is also generally known what the essence of the phenomenon consists in.

Transparent uncolored substances display bright colors in very thin layers. The colors of soap bubbles may serve as a good example. In harbors, beautifully colored patches are often visible on calm water. They appear where thin layers of kerosene float on the water. This phenomenon was known before Newton. Boyle, Grimaldi, and, especially in his book Micrographia, Hooke, who gives a detailed and correct description of the relevant phenomena, studied it.

phenomena under the name of “fantastical experiments.” Moreover, Hooke’s interpretation contains the first—though purely qualitative and vague—version of what we now call an interference explanation. But no one, including Hooke, was able to understand this phenomenon. The point is that the observations were made in white light, and here the phenomena are too varied and complex.

Newton approached these experiments, foreseeing with his usual perspicacity that new properties of homogeneous light could be discovered here. He did, indeed, succeed in this. In homogeneous light the phenomena are simplified, and Newton was able to find those quantitative regularities which turned out to be typical for all interference phenomena. Already in the arrangement of the experiments Newton’s exceptional experimental skill was manifested.

Quantitative investigations in this field are difficult, since the thickness of the layers is of the order of

\[ \frac{1}{1000}\ \text{mm}. \]

Such small thicknesses have to be measured. There were then no suitable devices for this. Newton overcame the difficulty of this measurement by a remarkable device. On a plane glass surface he placed, with its convex side down, a plano-convex lens—the objective of a telescope with a very large radius of curvature (Fig. 6). Then between the lower plane and upper convex surfaces there is formed an extremely thin layer of air, revealing variegated bright colors: colored rings in white light and an alternation of single-colored bright and dark rings in homogeneous light.

Fig. 6. \(h=d,\ 2d,\ldots\)

\[ h_1=\frac{d}{2},\ \frac{3d}{2},\ \frac{5d}{2},\ldots \]

The point of the device is that, first, the thickness of the layer is different at different places, i.e. we have here, as it were, a set of layers of different magnitudes; and, most importantly, the geometry here is such that the distance from the center to a given place is considerable—several hundred times greater than the thickness of the layer at that place. By measuring this distance, we determine, by calculation, the thickness which, because of its smallness, cannot be measured directly.

Here is the result—the basic result of Newton. The air layer does not reflect if its thickness is equal to some quantity \(d\) or to a multiple of \(d\). This is a remarkable phenomenon. If the lower surface is left out, then reflection is obtained; when the second surface is added, this reflection, as Newton showed by this experiment, disappears. Conversely, the layer reflects strongly if its thickness is equal to

\[ \frac{1}{2}d,\quad \frac{3}{2}d,\quad \frac{5}{2}d,\quad \text{etc.} \]

Newton experimentally determined this thickness \(d\): for the color at the boundary between red and yellow it proved to be equal to \(\frac{1}{89000}\) of an inch.

Young was the first, in 1801, to give an explanation of this phenomenon from the standpoint of the wave theory and gave it the name interference. We still consider this explanation correct today. It consists in the fact that the light wave is reflected from the first surface, while the transmitted part is reflected from the second. At the point of observation two waves meet, of which the second is somewhat delayed relative to the first. When the total delay is a whole number of waves (depending on the thickness of the layer), the two waves add. When the delay is a half-integer number of waves, they mutually cancel. Newton’s thickness $d$ corresponds, in the wave conception, to $\dfrac{\lambda}{2}$. Thus Newton, in fact, was the first to determine the wavelength of light. His number differs by only a few percent from the modern one.

How did Newton explain this phenomenon to himself? He had to resort to a special hypothesis, attributing to light certain fits—“fits,” as he called them, “impulses”—which made reflection now difficult, now easy. But the main thing is that he understood that the phenomenon definitely indicates that periodicity lies at the basis of the processes determining homogeneous light. This initial property, which in all the subsequent reconstructions of optical views, right up to the present day, has been placed in the forefront, was never disputed in Newton, as Academician S. I. Vavilov notes. Only not everyone knows that Newton was the one who made it; for the most part it passes as anonymous.

In modern physics, interference phenomena play an exceedingly important role. For example, the question of the resolving power of the microscope belongs to the domain of interference. Interference phenomena lie at the basis of the most delicate methods of measuring technique.

Without invoking interference phenomena, Michelson’s famous experiment, which gave the impetus to the creation of the principle of relativity, could not have been carried out. We see that the first fundamental quantitative regularities in this field were established by Newton.

I have dwelt in rather great detail on two of Newton’s principal discoveries. I hope that I have succeeded in conveying the sense that they are fundamental, so to speak, in the literal meaning of the word—the foundation upon which our further knowledge of optics was built and is still being built.

But Newton’s optical investigations are not exhausted by these. In his Opticks there are many interesting remarks, observations, and indications, the interest of which even now may be not only historical. I wish briefly to point out the following. Investigating the double refraction in Iceland spar discovered not long before by Bartholinus, Newton came to the conclusion that each of the two rays emerging from the crystal possesses an interesting property. Whereas an ordinary ray has axial symmetry, both these rays are deprived of it. Ne-

all planes passing through the ray are equivalent among themselves. Such rays Newton calls polarized. You know what an enormous role the doctrine of polarized light plays in modern physics. The indicated important property of plane-polarized light comes from Newton (this remark is not in the text, but in the questions).

Finally, the Opticks ends with an account of certain observations relating to phenomena discovered by Grimaldi and concerning the bending of rays when they pass near the edge of a knife, through slits, etc., i.e. the diffraction of light by obstacles. Newton makes a number of observations in homogeneous light, but arrives at no quantitative laws. The Opticks ends with the following words:

“In view of the fact that I have not completed this part of my plan, I shall conclude by proposing only a few questions for further investigation, which others will carry out.”

These questions, the famous “Queries,” as has been said, are the fruit of many years of persistent reflection, and not only on optical questions, and are of exceptional interest. In them, among other things, Newton renounces his dislike of hypotheses and shows that he can invent hypotheses better than others. These questions are questions only in form; in substance they are assertions.

We are taught that Newton adhered to the corpuscular hypothesis, that Huygens developed the wave hypothesis, which in the end gained complete victory. This is, indeed, almost correct. But this assertion is schematic, pale, and does not, in essence, give any idea of the true state of affairs. And this is especially clearly seen precisely from the Queries. For in such a form this assertion gives the impression that Newton in a certain sense did not rise to the wave theory, and that in this respect Huygens surpasses him. This is profoundly incorrect. Permit me to dwell quite briefly on this question and with it to end my report.

Newton possessed the wave conceptions accessible to that time better than anyone else, including Hooke and Huygens. It was he who pointed out to Hooke that, if the wave hypothesis were correct, then colors would differ by the breadth of the vibration, i.e. by wavelength. And in general he knew excellently all the subtleties of the wave hypothesis, and if he rejected it, then, paradoxical as this may sound, it was because he understood it even better than Huygens, and therefore saw more clearly the insurmountable obstacles that stood in its way at that time.

Newton saw three insurmountable obstacles:

1. If light is a wave motion, then there must accompany it a mechanical medium—a fluid in which the waves propagate, and it must fill all world space. But then the motion of the heavenly bodies must encounter resistance, which, however (according to the laws of mechanics), is in fact absent.

2. The absence of axial symmetry in a polarized ray contradicts wave motion; other waves, except longitudinal ones, by analogy with sound, were then not considered by anyone.

1. If light is a wave motion, then it must bend around an obstacle, like sound, and this, in Newton’s opinion, had never been observed. A planet, passing by stars, obscures them.

Huygens, apparently, did not see these contradictions as clearly—contradictions which at that time were quite real. In any case, he, too, was unable to resolve them; but either he did not take them much to heart, or he did not feel their hopelessness.

Thus Huygens, too, assessed the situation as in no way better than Newton did. Only here the difference in temperaments made itself felt. For Newton, contradictions from which he saw no way out were intolerable. Huygens did not experience them so acutely—to the good fortune of science, because he did not allow these contradictions to distract him from developing those aspects of the wave hypothesis that survived in its evolution. And perhaps here there was a repetition of what has happened more than once in the history of science. Some instinct prompts a genius to turn away from contradictions and go further. As has been said, this is a matter of temperament. Newton could not do this. Newton’s genius shone with other facets.

Was Newton’s corpuscular theory contradictory in his own judgment?

To this question one may give an affirmative answer, but only in the sense that, although not all known phenomena found a concrete explanation, Newton indicated—or thought that one could indicate—the path along which, in his opinion, one could arrive at an explanation; and there was then no reason to think that contradictions would arise in doing so. He experienced particular difficulties in explaining the fits of easy reflection. Here he stopped at the following conception. When light corpuscles fall upon bodies, they excite waves in them. Newton shows that, under a certain supposition about the interaction of light corpuscles with these waves, the observed phenomena can, possibly, be explained.

Thus Newton’s hypothesis synthesizes corpuscular and wave conceptions. The best proof of how serious the contradictions were that forced Newton to abandon the wave hypothesis is the fact that nearly 200 years had to pass before wave theory, at the cost of a complete reconstruction, freed itself from them.

Only at the beginning of the nineteenth century did Young and Fresnel create the hypothesis—by then ripe—of the transverse character of the oscillations of light. Thanks to this, the contradiction with polarization disappeared. But the position of Newton’s supporters was still strong; among them were Poisson, Laplace, and Biot, who at that time asserted that the corpuscular theory could be considered definitively proven. In order to put an end to the wave theory, they proposed, as a topic for a major competition of the Paris Academy of Sciences, the problem of diffraction, expecting that its treatment would be the definitive triumph of the corpuscular theory. Despite all obstacles, however, the prize was awarded,

Fresnel’s work, which proceeded from wave conceptions of the nature of light. The remarkable, unexpected qualitative and quantitative predictions, whose correctness was confirmed by experiment, could not fail to triumph.

However, Newton’s first difficulty, connected with the necessity of a material medium filling space, was not only not eliminated thereby but, on the contrary, seemed still more insuperable. The most desperate efforts were made to remove it. But only in 1864 did Maxwell, having proclaimed his electromagnetic theory, make this difficulty, if one may put it so, immaterial (since the need for a mechanical medium had disappeared). The golden age of optics began. True, certain difficulties arose in time, but they were not regarded as especially dangerous for the whole edifice (of optics) as a whole. In this sense Michelson’s experiment, too, did not disturb the harmony.

Einstein’s genius, through the principle of relativity, brought still greater generality and elegance into the electromagnetic picture. But at the beginning of this century discoveries began to multiply that compelled a reconsideration of all the foundations. Today we have quantum optics; this is, undoubtedly, an immense achievement. But it has internal contradictions, the way out of which is not yet visible. In it, too, the question is one of a synthesis of corpuscles and waves.

In view of this it is sometimes said that we have come closer to Newton’s point of view, and this is used to explain the present heightened interest in his investigations. I do not think that this is so. The words are indeed similar, but the physical content that we now put into the concepts of particle and wave is entirely different from that which Newton put into them. No, this is not where the greatness and significance of Newton’s optical discoveries lie.

The modern edifice of optics is incomparably more extensive than it was in Newton’s time. It is not yet completed, but it is already clear that its architectural style is entirely different from Newton’s. We do not know exactly what it will look like. But if we believe in its future strength, it is because it rests on a solid foundation, the cornerstone of which is formed by the immortal creations of the great Newton.

1. I cannot, however, deny myself the pleasure of quoting the statement of one of the French philosophers of the end of the eighteenth century: “Sans théorie on ne sait ce qu’on dit quand on parle et ce qu’on fait quand on agit.” ↩