CREATIVE WORK OF THE STATE OPTICAL INSTITUTE

(On the 25th Anniversary of the Founding of the GOI)

S. I. Vavilov

The Optical Institute was conceived and in fact developed as a scientific center of a special character, one that does not fit into the usual classificatory framework. It cannot be called a physical or chemical institution, nor does it correspond to the accepted notion of a technical, branch institute.

Gradually changing, it adapted itself to the real and very complex totality in which optics, in the broad sense of the word, has been realized in our time. In the Institute’s program, the most general questions of the theory of light were joined with the innumerable tasks of various branches of industry and, above all, of the optical-mechanical industry, of military affairs, and of technology in general. An important place was taken by optical glassmaking and the precise optical machining of metals, which entailed broad fundamental problems of the vitreous state of matter and other physicochemical questions.

Contrary to many skeptical and even gloomy predictions concerning the feasibility of such diversity in one place, the GOI has, through the practice of its 25 years, proved the vitality of its sphere of activity and even its necessity.

As early as 1919, at the Institute’s first annual meeting, D. S. Rozhdestvenskii, in his speech, called the GOI an institution of a new type, in which scientific and technical tasks were inseparably linked. “The close cooperation of the technical and purely scientific departments of the Institute,” he said, “opens up, both for technology and for the most abstract scientific experiment itself, possibilities of which we, university workers, had never even dreamed. In the experimental optical workshop, in the computing bureau, in the mechanical workshop, we now have scientific workers who, fully armed with knowledge, are engaged in improving technical instruments and analyzing methods of production. On the other hand, these same technical organs provide us with instruments of high perfection for scientific investigations. The work of all collaborators together—the craftsman and the scientist—constitutes one organic whole; to tear away one or another part, science or technology, means to deaden both.” This scheme, set forth 24 years ago as a program,

we have at the GOI even now, but already as practice verified by twenty-five years of actual production.

Given such a character of the Institute, it is not easy to separate, even if only in exposition, its theoretical activity from its applied or technical activity. The very concept of theory must here be used in a complex and many-tiered form. Speaking of theory at the Optical Institute, one has to set the theory of the atom and of light alongside, say, the theory of a polishing machine or of photographic development. This is inevitable, since in a scientific approach to any question every fact, instrument, or operation can and must have its own “theory,” i.e. analysis or calculation on the basis of simpler and more general concepts or regularities. This is what distinguishes science from a mere collection of facts and accidental findings.

From this point of view, the theoretical work of the GOI over 25 years has been very extensive. The laboratories of the Institute have always sought to approach every task they received from a theoretical standpoint. Thus arose the theory of objectives, of polishing processes, of metal coloration, of the “blooming” of optics, of the lightfastness of dyes, and much else. Sometimes, and unfortunately not always, this extensive theoretical work of the Institute found expression in special monographs. Such are the books by A. I. Tudorovsky, Theory of Optical Instruments; A. A. Gershun, The Light Field, Principles and Methods of Light Masking; G. G. Slyusarev, Methods of Calculating Optical Systems; I. V. Grebenshchikov (ed.), Blooming of Optics; D. D. Maksutov, Shadow Methods for Testing Optical Systems; A. N. Zakharevsky, Short-Baseline Optical Rangefinders; K. V. Nikolsky, Theory of Molecules, etc.

In what follows, several examples are given of the Institute’s theoretical activity, pertaining, so to speak, to different tiers, to very different degrees of breadth. These examples make no claim to portray all the theoretical work of the GOI. Their purpose is to explain, by means of examples, the character of the theoretical directions at the Institute, as well as their specific connection with applied and technical tasks in a number of cases.

1. STRUCTURE OF MATTER AND THE ORIGIN OF LIGHT

The war of 1914–1918 was a barrier that for several years delayed the growth of the new physics, i.e. the doctrine of the structure of matter, the theory of relativity, and quantum theory. After this barrier was broken, an astonishing development of physics began; we were all witnesses to it; it continues to this day, and its consequences for theory and practice are incalculable.

All three of the directions just enumerated directly concerned cardinal aspects of the doctrine of light; in particular, questions of the structure of matter were at the same time the problem of the origin of light, the problem of spectra. It is therefore not surprising that the young Optical

The Institute from the very beginning had as one of its scientific goals the “investigation of the atom on the basis of spectral analysis”1, and over the course of all 25 years this task did not leave the Institute’s program, assuming only the more general form of investigating the structure of matter by the methods of spectral analysis. A quarter of a century is usually a sufficient period in modern science to determine the value of a scientific result, and now it is possible, with complete objectivity, to judge the significance of at least part of the work of the GOI for the development of the doctrine of the structure of matter.

A proper historical perspective is needed for a correct evaluation of the first major theoretical work by D. S. Rozhdestvenskii, “Spectral Analysis and the Structure of Atoms,” which contained an inspired sketch of a theory of the structure of the simplest atoms on the basis of the first variants of the Bohr and Sommerfeld theory. This investigation was created under conditions of blockade and isolation of Soviet science and at that stage in the development of our theoretical ideas when every physicist was fully convinced of the adequacy of mechanical atomic model conceptions to reality. D. S. Rozhdestvenskii’s ideas about the necessity, in the case of complex atoms, always to keep in mind their comparison with hydrogen terms, about the role of magnetic forces in the atomic model, and many others had great heuristic significance, although they received a different interpretation in the modern theory of the atom. D. S. Rozhdestvenskii clearly foresaw this when in 1919 he wrote: “The method of quantization is a blind method. It cannot satisfy us finally. But two things must be kept in mind. First, it is necessary and advantageous to work with a theory that continually leads to new results. Second, if the theory should change, i.e. if the process of quantization should be clarified, then all the facts encompassed by it, in all their connection, in their totality, will be transferred onto new ground. The work of unification not only will not be wasted, but its results will immediately be illuminated by a new light and will become still clearer and more fruitful than now.” In this sense the entire series of theoretical works by D. S. Rozhdestvenskii devoted to the structure of atoms is a remarkable historical stage.

From the prerevolutionary work of D. S. Rozhdestvenskii the GOI obtained its interference method for the quantitative study of anomalous dispersion in vapors, the so-called “hook” method. With astonishing accuracy this method makes it possible to determine the intensities of the absorption and emission lines of atoms, i.e., in the language of quantum theory, the probabilities of quantum energy transitions in the atom. The original Bohr theory of atoms was powerless to explain this quantitative aspect of spectra. A series of classical experimental works on anomalous dispersion, carried out by D. S. Rozhdestvenskii himself and by his pupils V. K. Prokof’ev and A. N. Filippov

and many others, posed the impeccably and precisely formulated problem of the future theory. The new quantum mechanics solved it, and from that time the systematic study of anomalous dispersion proceeded hand in hand with theory, confirming it, checking it, or requiring its improvement. The coherent line of work on anomalous dispersion constitutes a generally recognized achievement of the GOI, and the Institute with good reason chose dispersion “hooks” as the background of its publishing emblem.

Together with the experimental study of the properties of atoms, the GOI did not cease developing a theory that would make it possible to “calculate” the atom. This line reached its culmination in the approximate method for solving the quantum many-body problem proposed by V. A. Fock.

The exact solution of the many-body problem in wave mechanics, as, incidentally, in ordinary mechanics as well, presents enormous difficulties. In 1928 Hartree proposed solving such problems by a method he called the “self-consistent field” method. He retained, approximately, the old Bohr picture of electronic orbits, describing each of them by a separate Schrödinger wave function. At the same time it is assumed that the potential energy in each corresponding Schrödinger equation arises from the nucleus and from the charge of the other peripheral electrons, distributed continuously.

V. A. Fock at the GOI fundamentally improved Hartree’s method, modifying it so that for the wave functions of the individual electrons equations were obtained that differed from the previous equations by the presence of terms corresponding to the so-called quantum exchange. The new method received general recognition and is usually called in the literature the Hartree–Fock method. At the GOI this method was successfully applied to the calculation of the sodium atom.

The solution of problems of atomic spectroscopy required the use of the most delicate and powerful experimental spectroscopic means. On the way to this, the GOI’s extensive spectroscopic school and methodology were nurtured and grew up, making it possible to approach such problems and to solve them successfully. In this connection one cannot fail to mention the many works on the hyperfine structure of spectral lines, which made it possible, in particular, to approach by optical means the problems of the atomic nucleus, the isotopy and moments of the nucleus—mechanical and magnetic (the works of S. E. Frisch and others).

After the calculation of some more or less complex atoms on the basis of the new quantum mechanics had yielded satisfactory agreement with experiment, interest in the fundamental aspect of the problem weakened; for the given phase of the development of physics it even appears exhausted. Whether this is right or not will be shown by the future (it seems to me that it is wrong), but, in any case, it may be stated that in this temporary exhaustion of the atomic-peripheral problem, important significance was held by the experimental and theoretical work of the spectroscopic school of the GOI.

Of the long, continuous series of works of the GOI devoted to molecular spectroscopy and the structure of molecules, I have the opportunity to mention only very few, although in this field of research the Institute also occupies an important place.

A fundamentally new word about molecules and, at the same time, about the condensed—liquid, glassy, and crystalline—states of matter was said by E. F. Gross, who experimentally discovered a new phenomenon in molecular scattering, theoretically predicted by Brillouin and Mandelstam. The phenomenon consists in a change in the wavelength of scattered light by an amount of the order of the ratio of the velocity of sound in the given medium to the velocity of light in it. Subsequent work by Gross and his collaborators showed that this phenomenon provides a new means of investigating intermolecular bonds in bodies and can become the basis of a distinctive spectroscopic method for studying liquids and crystals. The phenomenon found by Gross is extremely delicate, and at first a number of foreign authors tried to refute it. However, the experimental skill of E. F. Gross, which had grown on the soil of the spectroscopic traditions of the GOI, prevailed.

Another spectroscopic method for studying molecules in the condensed state, likewise new, was developed by P. P. Feofilov in the development of certain phenomena previously found by me in the field of fluorescence of solutions. The degree of polarization of the fluorescence of organic dyes and other substances, generally speaking, can depend very sharply on the wavelength of the exciting light, whereas the fluorescence spectrum depends on it scarcely at all. Thus, distinctive “polarization spectra” are added to the emission and absorption spectra. On very extensive material P. P. Feofilov showed that these spectra are connected with the anisotropy of molecules and therefore, in combination with other experimental techniques, give researchers a new method for investigating molecular structure.

The broad scope of the Institute’s interests naturally leads us to the problem of the structure of matter—from atoms and atomic nuclei through molecules to that complex formation which we call glass. By various paths and methods the GOI approaches the investigation of the structure of this material, fundamental for opticians. The chemists of the GOI analyze glass by their own techniques; the physicists use absorption, light scattering, the luminescence of glass, and other optical characteristics; major results were yielded by the X-ray study of the structure of glass by N. N. Valenkov and E. A. Porai-Koshits under the direction of A. A. Lebedev. The problem is still far from final solution, but much has been achieved along the way, especially on the question of the quasicrystalline nature of glass; much has already passed from the theory of glass into technology.

On the soil of such an Institute as the GOI was and remains, the problem of the structure of matter, immeasurable in its breadth and significance, could not remain in abstract isolation. As was brilliantly shown in the well-known report by D. S. Rozhdestvensky at the March session of the Aca-

... Sciences in 1936, at the State Optical Institute the analysis of spectra naturally developed into practical spectral analysis for the benefit of industry, defense, and mining. The difficulty of the experimental tasks being solved compelled the improvement of instruments, the construction of fluorite interferometers, echelons, especially high-speed spectrographs, and the overcoming of obstacles in the manufacture of diffraction gratings. In this the Institute’s instrument-making skill was tempered, which was later reflected in its technical capabilities.

3. THE NATURE OF LIGHT

For physics, and for optics in particular, light is, after matter, the next most important category of phenomena. At the present time, after more than a thousand years of fluctuations in human thought in views on the nature of light, we possess a formally harmonious theory of light, though one difficult to grasp from the model point of view. This theory, as is known, combines features of the wave and corpuscular views, while being, however, neither the one nor the other. The theorists of the State Optical Institute, V. A. Fock, K. V. Nikol’skii, and others, worked a great deal on the difficulties of the theory of light and quantum electrodynamics. V. A. Fock’s ideas have in large measure entered into Dirac’s now generally accepted theory of light. A summary of the most general modern conceptions of light is given in K. V. Nikol’skii’s book Photon.

Among the experimental works of the State Optical Institute directly related to the question of the nature of light, one may point to a large series of investigations carried out by E. M. Brumberg, T. V. Timofeeva, Z. M. Sverdlov, and myself on the question of visual quantum fluctuations. We succeeded in proving the existence of these fluctuations, caused by the quantum structure of light and by the quantum character of absorption. The statistics of these fluctuations were studied, and at the same time, as a by-product, conclusions were obtained concerning the properties of the retina of the human eye, its sensitivity in the ultraviolet spectrum, and so on. The fluctuation method made it possible to probe in the living eye that which previously it had been thought possible to do only on a dead specimen. Our experiments gave proof of the quantum nature of light, obvious in the literal sense of the word. In 1941 these experiments were repeated by American physiologists, with essentially the same result.

In the theory of light, besides the controversy of “waves and corpuscles,” there are also aspects that formally follow directly from classical wave conceptions and the modern theory of the structure of matter, but that until now have escaped the attention of theorist and experimenter. The optician, for example, operates without apprehension with a luminous point radiating equally in all directions. Yet it is easy to prove that such a point is unrealizable and that, on the contrary, strictly speaking, in a careful analysis of optical phenomena it is necessary to distinguish points corresponding to an electric dipole, a magnetic dipole, a quadrupole, and so on. E. M. Brumberg and I showed theoret-

theoretically and experimentally that the nature of the luminous point must manifest itself sharply in interference experiments of the Young–Fresnel type. Later it proved possible to develop a method for establishing the nature of the oscillator, based on certain properties of polarized fluorescence. In development of this, P. P. Feofilov showed that the fluorescence of dyes in solutions, just like that of uranium glass, corresponds to the radiation of dipoles. Thus, into optics there was introduced, not only theoretically but also in concrete work, a differentiated concept of the elementary radiator.

Of course, it is difficult to expect direct technical results from theoretical or experimental works concerned directly with the nature of light. One may only note that from experiments on visual fluctuations there grew our method for measuring extremely small brightnesses, which fully justified itself in the study of the night glow of the sky, in luminescence analysis, and which played a fundamental role in P. A. Cherenkov’s discovery at the Academy of Sciences of a new type of visible glow of electrons moving with superluminal velocity in a medium. At the same time, those same fluctuation experiments gave the physiologist and the physician a new means of deeply studying the eye in its living state.

3. ACTIONS OF LIGHT

The actions of light—photochemistry, the photographic process, and photoluminescence—have always occupied a prominent place in the work of the State Optical Institute. These phenomena long ago became the basis of the corresponding technical fields, while remaining, however, to this day in many respects unclear from the fundamental standpoint.

Great importance in the development of the doctrine of photochemical processes was acquired by the investigations of A. N. Terenin and his collaborators into the photochemical decomposition of vapors of halide salts. A. N. Terenin succeeded in proving the fact of the decomposition of a sodium iodide molecule, upon absorption of an ultraviolet quantum, into an excited sodium atom and an iodine atom. Thus a transition, under the action of light, of a heteropolar bond into a homeopolar one with subsequent decomposition was discovered. On the other hand, for thallium iodide the opposite process was established: the homeopolar molecule decomposed under the action of light into ions, i.e. the atomic bond was transformed into an ionic one. The establishment of these facts was of great significance for the development of modern ideas about the chemical bond. Similarly, under conditions of a vaporous state and illumination by monochromatic light, it proved possible to analyze elementary photochemical processes in triatomic and tetratomic halide compounds. The method of optical excitation applied by A. N. Terenin, with detailed analysis of the luminescence arising in this process, proved to be an exceptionally effective and subtle means for studying elementary photochemical processes.

In recent years it has become possible to pass from elementary processes to complex photochemical reactions occurring in organic dyes in the adsorbed state. These phenomena are of great practical interest and are of primary concern to the textile industry.

A special, or from certain points of view more complex, photochemical process may be said to be the phenomenon of the coloration of crystals under the action of X-rays and ultraviolet light. Its essence is that, when light is absorbed in a crystal, under certain conditions a breakdown of the crystal lattice and the precipitation of isolated atoms may occur. For the first time, T. P. Kravets, in his address at the annual meeting of the State Optical Institute in 1929, put forward the proposition that the primary photographic process occurring in the grains—the crystals of an emulsion, i.e., the formation of the latent image—is in reality the coloration of crystallites upon the absorption of light, as a result of the isolation of silver atoms from the crystal lattice. The same proposition was advanced independently and simultaneously by R. Pohl in Göttingen. T. P. Kravets’s idea became the basis of an extensive series of works carried out by his collaborators, especially M. V. Savostyanova, at the Academy of Sciences and at the State Optical Institute. It was observed from the very beginning that, upon illumination of silver bromide crystallites, the atomically distributed metal coagulates and gathers into colloidal particles of silver. The study of this process and of its consequences for the photographic process determined the character of the work on the latent image at the State Optical Institute, which occupies the principal place in the theory of photography.

The photographic process, from the theoretical point of view, even in the simplest cases is considerably more complex than the process of excitation of molecules, which then spontaneously or forcibly return to their initial state. The energy released in such a return, in the simplest case, is emitted, producing the so-called luminescence. At the State Optical Institute, the study of luminescence has not left the program of work, in view of its enormous importance for understanding the structure of matter and the kinetics of molecular processes, and at the same time because of its ever-increasing technical role in illuminating engineering and military affairs.

The beginning of the study of luminescence at the State Optical Institute was laid by A. N. Terenin’s systematic series of works on the resonance radiation of the vapors of mercury, cadmium, thallium, lead, bismuth, zinc, antimony, and arsenic. These investigations, carried out during the period of the most rapid phase in the development of the doctrine of the structure of the atom, were of great importance for establishing the schemes of spectral terms of the listed elements and, at the same time, remain exemplary to this day in their experimental mastery and purity.

The luminescence of vapors, as a powerful means for tracing the links of elementary photochemical processes, figures constantly in other works by A. N. Terenin and his pupils.

At the same time, systematic work is being conducted at the State Optical Institute on luminescence of a considerably more complex type, occurring in complex molecules, in liquid or solid solutions, or in crystals. Thirty years ago these phenomena, known for almost four centuries, remained an accumulation of striking but almost incomprehensible facts. Now much here, though far from everything, has been clarified, systematized, and theoretically explained—at least down to certain empirical constants, for whose theoretical derivation a scarcely attainable exact knowledge of the structure of complex molecules would be required. On the basis of the diffusion theory of molecular collisions in a liquid, the kinetics of quenching of luminescence in solutions by foreign substances has been explained; the dependence of luminescence polarization on viscosity has been explained; and the concept of quantum exchange phenomena has made it possible quantitatively to understand an entire group of complex phenomena involving the influence of the concentration of luminescing molecules on the yield, polarization, and duration of luminescence (S. I. Vavilov, B. Ya. Sveshnikov, A. N. Sevchenko, P. P. Feofilov).

At present luminescence is flowing in a living stream into practice, military and civilian alike, in the form of luminescent lamps, luminous scales, light-masking applications of signaling devices in invisible rays, theatrical technology, innumerable types of luminescent analysis, etc.; and the connection of theoretical work on luminescence with the demands of technology is obvious.

4. COMPUTATIONAL OPTICS

The transition from broad themes concerning the structure of matter and the nature of light to considerably narrower and more special questions of the action of light means a descent to a lower and narrower theoretical tier. Let us descend still lower, into an even more specialized field, which nevertheless constitutes one of the most important sectors of the optical front: let us dwell briefly on the theoretical work of the calculators.

The Institute had to create computational work almost from nothing—without skills, without traditions—and yet the pioneers of this work, A. I. Tudorovskii, E. G. Yakhontov, and G. G. Slyusarev, did not confine themselves to mechanically copying known computational templates; they sought their own theoretical path and independently developed a methodology for calculating optical systems on the basis of theories of third-order aberrations. It later turned out that Western optical thought had followed the same paths. Higher-order aberrations were subsequently investigated by G. G. Slyusarev for a cemented two-lens objective. General results were obtained that made it possible to calculate an objective without the customary laborious trigonometric trials.

For many years, up to the present time, the Institute’s calculators and opticians have been seeking ways to solve the fundamental problem of the so-called “quality of the image.” Simple calculations on the basis of geo-

metrical optics provide only what may be called the “skeleton” of the image. The distribution of energy over the area of the aberrational image remains unexplained. For cases in which it is permissible not to take into account the specific influence of diffraction, the solution was found by G. G. Slyusarev. Allowance for diffraction in certain problems was carried out by L. P. Moroz, G. D. Rabinovich, and G. G. Slyusarev. The question of the limiting resolution of photographic optical instruments has been advanced (taking into account not only the influence of the objective but also the photographic emulsion); the influence of image contrast on resolving power has been studied (L. P. Moroz). One of D. S. Rozhdestvensky’s recent works was a profound theoretical investigation of certain cases of imaging in the microscope, taking into account interference and diffraction phenomena. This work contains promising practical conclusions of fundamental importance for the further improvement of the microscope.

The general theoretical basis for concrete computational work is formed by a series of investigations on the theory of expanding aberrations in series, on methods for calculating the coefficients of the expansion, and on finding procedures for passing from geometrical aberrations to wave aberrations. The influence of changes in the position of the object and of the entrance pupil on the coefficients of third-order aberrations was studied by A. I. Tudorovsky. He also developed a vector method for calculating the passage of rays through systems with plane surfaces, i.e., through prisms.

Among individual theoretical computational works I shall also note the theory of the distribution of image illumination in wide-angle objectives (G. G. Slyusarev) and the theory of systems with variable focal length (D. S. Volosov).

The deepening of theoretical ideas about the possibilities of optical systems, together with the enormous empirical material recorded in the archive of the calculators, makes it possible to seek and find new paths in this seemingly exhausted field. Proof of this is the great success in the realization of modern high-aperture and wide-angle objectives, series of original catadioptric systems with unexpected prospects (including the meniscus systems of D. D. Maksutov), and the successful application of aspherical surfaces. Optical calculators are clearly in a phase of great creative upsurge, which is the result of a broadened theoretical horizon.

5. THEORETICAL ILLUMINATION ENGINEERING

Illumination engineering, the science of rational lighting, belongs, of course, to purely technical branches of knowledge. However, it too must have its own theory, and the GOI has produced a number of very instructive investigations of this kind. A. A. Gershun showed that an illumination-engineering calculation can be represented as a problem of general physical—

light-field theory, by which is understood a space studied from the point of view of the distribution within it of fluxes of radiant energy. Problems of illumination engineering can be solved on the basis of the concept of the light field, using vector analysis; moreover, as the principal characteristic there is introduced the volume density of light energy and the so-called “light vector,” which determines, in magnitude and direction, the light pressure (it corresponds to the mean value of the Poynting vector). The theory of the light field was successfully applied in the works of Soviet illumination engineers and at the same time attracted the attention of European and American science. In the polemic that arose with the late French Academician Blondel concerning the principles of the doctrine of the light field, A. A. Gershun proved to be the victor. In 1938 the Optical Society of America stated that the development of the application of vector concepts in illumination engineering was, in the main, the achievement of Soviet scientists, collaborators of the GOI V. A. Fock, A. A. Gershun, M. M. Gurevich, and N. V. Boldyrev; A. A. Gershun’s book The Light Field was published in the United States, and the editor of the translation, P. Moon, writes: “Theoretical photometry represents a case of ‘retarded development,’ and essentially it remained unchanged from 1760, while the other chapters of physics were developing triumphantly. However, in recent years the growing demands of modern illumination engineering have rendered the absurdly antiquated concepts of traditional photometric theory helpless. At the present time there is a strong movement to bring the theory of the light field into accord with the spirit of physics. Professor Gershun, at the State Optical Institute in Leningrad, is one of the pioneers of this movement.”

The theory of the light field found practical application in the development of the illumination engineering of natural lighting. Much work was carried out on the question of the distribution of light in media that absorb and scatter light. The results were applied in the study of the properties of milk glasses, of the atmosphere, and of marine optics. The GOI’s work in hydrophotometry, carried out under the direction of A. A. Gershun, contributed much to the physics of the sea and served as the basis for solving important military problems.

6. THEORY OF ANNEALING OPTICAL GLASS

Optical glass is the principal, most complex, and most delicate material in an optical instrument; it is therefore not surprising that optical glass has been the leitmotif of the GOI since the time of its founding. The Institute did not confine itself to introducing into industry the optical-glass technology first borrowed from the English firm Chance. In 25 years it may be said to have revised and relearned this technology from top to bottom, transferring a great deal directly to production. On this basis there grew up new theoretical ideas about opti-

optical glass as a whole and with respect to individual links in the technology of its production and processing. It may, however, be put another way: the new theoretical results obtained at the State Optical Institute concerning optical glass at times exerted a profound influence on technology.

I do not intend here to set forth the entire complex body of new theoretical conclusions obtained at the State Optical Institute concerning glass by I. V. Grebenshchikov, N. N. Kachalov, A. A. Lebedev, V. V. Vargin, A. I. Stozharov, and their collaborators. That would require a very substantial book, which, one hopes, will be written in the near future by the persons named. I shall confine myself to just one very illustrative example.

The annealing of optical glass is one of the most important production operations, although before the Institute’s work it had no acceptable explanation. Formerly, the beneficial effects of annealing were chiefly ascribed to the removal of stresses in glass blocks. A. A. Lebedev radically changed this point of view. He showed that the cause of the change in the basic optical properties of glass during annealing lies in polymorphic transformations associated, in silicate glasses, with the familiar transformation of quartz from the $\alpha$- to the $\beta$-modification. At the same time, A. A. Lebedev’s experiments made the quasi-crystalline nature of glass probable. In contrast to the idea of glass as a supercooled liquid, one may think that glass is an aggregate of pseudocrystals of infinitesimal size. This conclusion was confirmed in the X-ray spectroscopic investigations mentioned earlier.

The physicochemical theory of annealing proposed by A. A. Lebedev became the basis for many subsequent works in this field, both in our country and abroad, and at the same time provided the basis for the rational technology used in production.

The limits of a short article compel me to confine myself to the examples listed of the Institute’s theoretical activity; very much, and very many people, have not even been mentioned here. I hope, however, that even from the little that has been said, the special character of theory at the Optical Institute is clear. From the very first stages of its development, the State Optical Institute, even in the broadest questions, never broke away from the tasks and needs of Soviet practice, never departed into the cloud-high abstract realms of so-called pure knowledge; but at the same time it always fought against undisguised empiricism. Theory, as a necessary condition for solving practical questions, is and must remain at the Institute an indispensable condition of its work.

1. D. S. Rozhdestvenskii, Spectral Analysis and the Structure of Atoms. Transactions of the GOI, issue 1920. ↩