Sergey Ivanovich
Vavilov

SERGEI IVANOVICH VAVILOV

(1891–1951)

E. V. Shpolsky

I

Soviet science has suffered a grave loss: on January 25, the President of the Academy of Sciences of the USSR, Academician Sergei Ivanovich Vavilov, passed away. A great scientist and organizer of science on a nationwide scale, an ardent patriot, and an outstanding statesman and public figure has departed from life in the full bloom of his creative powers.

Sergei Ivanovich was born in Moscow on March 24, 1891. He received his secondary education at one of the Moscow commercial schools. Commercial schools were at that time a comparatively new and progressive type of secondary educational institution. In their character they were close to Realschulen: the teaching of ancient languages was absent, while great attention was given to natural science, physics, chemistry (not taught at all in gymnasia), and modern languages. Already at school S. I. awakened an interest in physics and another passion, preserved together with his love of physics until the last days of his life—bibliophilia. His active interest in physics found satisfaction not only in reading, but also in attempts at independent experimentation; and his love of books early developed in S. I. a taste for antiquarian rarities, especially for original editions of classical works in the natural sciences. In the second-hand bookshops of Mokhovaya and Nikolskaya, and especially in the book stalls of old Sukharevka, he would with great skill seek out true “pearls” amid heaps of waste paper; and his student library was adorned with such finds as Experimenta Nova de Vacuo Spatio ut Vocantur Magdeburgica by Otto von Guericke, and other rarities.

In 1909, having passed the additional Latin examination required of all non-classical-gymnasium graduates, S. I. entered the Faculty of Physics and Mathematics of Moscow University. At the University S. I. was, of course, attracted by the famous Lebedev laboratory, and already at the end of his first year he took up a topic for scientific work. Since P. N. Lebedev was at that time already suffering from the illness that carried him to the grave in 1912, topics for young students beginning scientific work were assigned by his closest assistant—then still a privat-docent, later Academician P. P. Lazarev.

However, S. I. did not have occasion to bring this work to completion within the walls of Moscow University. At the beginning of 1911, more than 40 progressive professors and a large number of younger teachers resigned from the University in protest against the arbitrary actions of the tsarist Minister of Public Education, Kasso. Among those who left were P. N. Lebedev and P. P. Lazarev. The task arose of creating at least minimal conditions for the continuation of the work of Lebedev himself and of the Lebedev school. After initial setbacks, a peculiar institution, created in Moscow after the revolution of 1905, came forward to help—the Shanyavsky City University and the so-called Ledentsov Society, headed by N. A. Umov. For the Lebedev laboratory, two adjoining apartments were rented in the semi-basement of house No. 20 on Mertvy (now Ostrovsky, on Kropotkin Street) Lane. The apartments were small and, of course, wholly unsuited to laboratories. There were also few instruments; service personnel (laboratory assistants, preparators), an indispensable attribute of modern laboratories, were entirely absent: the workers had to do everything themselves. But despite this modest setting, all the laboratory staff devoted themselves to the work with the greatest enthusiasm, and the work was in full swing.

It was in this laboratory that S. I. also carried out his first scientific work. An important element in the life of the laboratory was the “physics colloquium”—weekly scientific meetings, first organized in Russia by P. N. Lebedev, at which new works published in the literature were reported on and then subjected to lively discussion. Especially festive were those colloquia at which one of the participants reported on his own work. In the colloquia, led by P. N. Lebedev himself, and after him by P. P. Lazarev, everyone took part on equal terms and with equal responsibility, from established scholars down to the young students working in the laboratory. Among them S. I. quickly came to the fore. His brilliant abilities, which allowed him to orient himself easily and quickly in complex scientific works, his excellent knowledge of foreign...

SERGEI IVANOVICH VAVILOV

languages, an ardent interest in science, and overflowing energy—all this made him one of the most active participants in the colloquia. His frequent presentations as a speaker were always interesting and at the same time vividly revealed his rapid growth.

One manifestation of S. I.’s broad erudition, acquired early, was his review article “Photometry of Multicolored Sources,” published in 1913 in the Journal of the Russian Physico-Chemical Society. Physical Section, which chronologically was S. I.’s first scientific publication.

The research work carried out by S. I. during these student years was connected with the doctoral dissertation of P. P. Lazarev, completed shortly before. In this dissertation Lazarev investigated the laws of the chemical action of light using as an example the fading in light of colloidal films dyed with cyanine dyes. It was known that the same dyed colloidal films are also decolorized under the action of heat. Since the mechanism of dark and photochemical reactions at that time was still completely unclear, it was of considerable interest to investigate the kinetics of the dark fading of dyes and to compare it with the kinetics of the photochemical reaction. This was what S. I. did. Using an ingenious experimental apparatus that he himself had devised, he was able, with great convenience, to follow by an exact optical method the course of the process simultaneously at six different temperatures, without in any way disturbing the course of the reaction. In this way, substantial differences were established in the mechanism of the photochemical and dark reactions, despite the identity of the initial and final products. The work was published in 1914, and in 1915 the Society of Friends of Natural Science, Anthropology, and Ethnography at Moscow University awarded S. I. a gold medal for it.

In the spring of 1914 S. I. graduated from the University. He declined the offer to remain at the University “to prepare for the professorial rank,” although remaining at the University would have exempted him from military service. However, S. I. considered it unacceptable for himself to remain at the University, whose departments, after the rout carried out by the tsarist minister, were being boycotted by the best part of the professors. The declaration of the First World War in August 1914 found him serving as a volunteer in one of the Moscow grenadier regiments, and very soon after the beginning of the war he was sent to the front. S. I. remained in the active army throughout the entire war, serving mainly in technical units. It is remarkable that, while taking part in military operations, he did not forget science: in the field radio laboratory where he worked at the end of the war, he even carried out an experimental and theoretical study, “The Frequency of Oscillations of a Loaded Antenna.” This work was published in 1919.

In 1918, having returned from the army, S. I. began work at the Institute of Physics and Biophysics of the People’s Commissariat of Health, created and headed by P. P. Lazarev. The period from 1918 to 1930, during which S. I. worked at this Institute, was exceptionally fruitful. Making use of the excellent conditions for scientific work created by the Great October Socialist Revolution, S. I. concentrated all his extraordinary energy, scientific giftedness, and truly gigantic capacity for work on scientific research and on teaching in higher education. He chose his field of research independently, moving by a quite natural path from photochemistry to the study—speaking in contemporary language—of the properties of excited states in solutions, more specifically, chiefly of photoluminescence and of physical optics in general. Despite the four years spent at the front, by 1918 he was already a fully formed scholar, in no need of any guidance. At the Institute S. I. found in V. L. Levshin a faithful pupil and collaborator, together with whom he carried out and published a large number of investigations.

His wide-ranging scientific interests and indefatigable activity found expression in constant reports at the Institute colloquium and in numerous abstracts and review articles, which he published in Uspekhi fizicheskikh nauk [Advances in Physical Sciences]. S. I.’s pedagogical activity took place at Moscow University, where from 1919 he held the position of privat-docent, at the Moscow Higher Technical School, and at the Moscow Higher Zootechnical Institute, where in 1920 he was elected professor in the Department of Physics.

In 1929 S. I. was elected professor and head of the Department of General Physics at Moscow State University, and from 1930 he transferred his activity entirely to MSU. Here, with the energy and enthusiasm characteristic of him, he set to work organizing the teaching of physics, developing a special practicum, and developing scientific work. Around him there quickly formed a group of young scientific workers, graduate students, and senior students (I. M. Frank, E. M. Brumberg, A. A. Shishlovsky, and others).

The year 1931 was a turning point in S. I.’s activity. In that year he was elected a corresponding member of the Academy of Sciences of the USSR, and in the following year, 1932, on the initiative of the founder of the State Optical Institute (GOI), Academician D. S. Rozhdestvensky, S. I. was appointed scientific director of this remarkable scientific institution. In connection with this appointment, S. I. had to move to Leningrad. However, he maintained close ties with Moscow University and regularly spent several days each month in Moscow, continuing to supervise the scientific work of graduate students and young scientific collaborators.

SERGEI IVANOVICH VAVILOV

S. I.’s activity at the State Optical Institute—this most powerful of the few similar institutions existing in the world—was extraordinarily broad and fruitful. It embraced all aspects of optics, beginning with the melting of optical glass and the calculation of optical instruments, and ending with the subtle problems of optics as a science. Created by the revolution, the State Optical Institute carried out gigantic work that served as a solid foundation for the large Soviet optical-mechanical industry, which supplied the Soviet Army during the years of the Great Patriotic War with all modern optical instruments. In connection with S. I.’s work on luminescence, we should note the creation at the State Optical Institute of a new luminescence laboratory, where S. I., together with a number of collaborators, continued his scientific work in this field. In the same year, 1932, S. I. was elected a full member of the Academy of Sciences of the USSR and became director of the Physical Institute of the USSR Academy of Sciences. This institute at that time, in 1932, was still a small physical laboratory that formed part of the Physico-Mathematical Institute named after Academician V. A. Steklov. S. I. immediately began energetically to develop the activity of this laboratory and transformed it into an independent institute*).

In 1934, in connection with the relocation of the Academy of Sciences to Moscow, this institute was transferred to Moscow, and here, on a new basis, under S. I.’s leadership, it quickly grew into the largest research institute in physics; on S. I.’s initiative the Institute received a new name: “The P. N. Lebedev Physical Institute of the USSR Academy of Sciences.”

The period from 1933 to 1941 is marked in S. I.’s life by varied and intense scientific, scientific-organizational, and public activity. In addition to directing the two aforementioned largest scientific institutes, he was chairman of the Commission for the Study of the Stratosphere under the Presidium of the Academy of Sciences (1933–1937), chairman of the Commission of the Academy of Sciences for the publication of popular-science literature (from 1933), member of the Presidium of the USSR Academy of Sciences (1935–1938), member of the Leningrad Soviet of Working People’s Deputies (1935), deputy of the Supreme Soviet of the RSFSR from the city of Leningrad (1938), and had many other scientific and public duties.

During the Great Patriotic War S. I. developed intense activity for the defense of the country. Despite the inconvenience connected with the fact that the State Optical Institute and FIAN were evacuated to different cities, S. I. continued to direct both of these institutes.

) See S. I. Vavilov, “The Physics Study—The Physics Laboratory—The Physical Institute of the Academy of Sciences over 220 Years,” UFN*, 28, 1 (1946).

Both Institutes, under the direction of S. I., took a most active part in the development of scientific problems connected with the defense of the country. In 1943 S. I. was appointed commissioner of the State Defense Committee.

In June 1945, in connection with Academician V. L. Komarov’s retirement from the post of President of the Academy of Sciences because of illness, S. I. was elected President of the Academy of Sciences of the USSR. From that moment his activity, in connection with the role of the Academy as the principal center organizing and coordinating scientific work in the country, acquired an especially broad scope and responsibility.

S. I.’s chief attention was directed toward linking science with the needs of life, with the demands of the socialist national economy. He concerned himself with the development of branches of the Academy of Sciences of the USSR in the national republics and regions of the Soviet Union and with coordinating the activity of the Academies of Sciences of the Union republics. He developed extensive activity in rendering assistance to the construction of the grandiose hydroelectric stations on the Volga and the Dnieper and of the Main Turkmen Canal; he personally headed the Committee, created on his initiative, for the Assistance of Scientists in the Construction of the Great Stalinist Projects of Communism.

In the high post of President of the Academy of Sciences, S. I.’s talent as an organizer of science unfolded in full measure. S. I.’s remarkable qualities were his exceptionally broad outlook, his ability quickly to assess new directions in science, and his clear understanding of the most important tasks of Soviet science in the crucial period of the postwar Stalinist five-year plans. His inexhaustible energy, speed, and remarkable precision in work enabled him without difficulty to cope with the most varied responsibilities arising from his many-sided activity.

II

A complete review of S. I.’s scientific works does not enter into our task: the list alone of his published works occupies almost 50 pages in small type and contains more than 300 titles. We shall therefore confine ourselves only to a brief outline of S. I.’s scientific activity*).

As has already been said, at the beginning of his scientific activity S. I. turned to the study of photoluminescence, more specifically to photoluminescence in solutions. This field, throughout all his subsequent activity as well, was the principal subject of his investigations. Summing up what S. I. accomplished, one may say without exaggeration that the most essential results for

*) We do not give references to S. I.’s works. A complete bibliography of his works is published in Vol. XLIV, issue 1 (May, 1951) of our journal.

SERGEI IVANOVICH VAVILOV

For our understanding of the nature of this important phenomenon, acquired over the last 30 years, we are indebted precisely to the work of S. I. and his collaborators.

In one of his early works S. I. measured the absolute energy yields of fluorescence of a number of dyes (fluorescein, eosin, rhodamine B, etc.). On this subject, incorrect notions existed at that time: it was thought that only an insignificant fraction of the absorbed energy is converted into the energy of fluorescence light. S. I. showed that this is completely at variance with the true state of affairs. In reality, the fluorescence yield is by no means small, and in some cases it approaches 100% (for example, for the sodium salt of fluorescein in various solvents the yield is 71%). Although S. I. estimated the accuracy of his measurements very cautiously, indicating that their error lay within 10%, subsequent measurements by other investigators confirmed them excellently. It is not without interest to note that in the 26 years that have elapsed since the publication of this work, only very few investigators have turned to the direct absolute measurement of the fluorescence yield in the way S. I. did. In the overwhelming majority of cases the yield was measured relative to some substance with a known yield, and the figures of S. I. were usually taken as the basis.

In subsequent works S. I. investigated the dependence of the fluorescence yield on wavelength. It turned out that in passing from short waves to long ones the energy yield at first increases proportionally to the wavelength, then over a certain spectral interval remains constant, and with a further increase in wavelength (in the anti-Stokes region) falls sharply. It is obvious that the region in which the energy yield is proportional to the wavelength corresponds to the region of applicability of a quantum law of equivalence analogous to the photochemical law of equivalence. Indeed, for a given amount of absorbed energy the number of absorbed photons is proportional to the wavelength, and if the efficiency of each absorbed photon is one and the same, irrespective of its magnitude (in the most favorable case—one fluorescing molecule for each absorbed photon), then the energy yield must also increase proportionally to the wavelength. More complex conditions occur in the anti-Stokes region. Here the magnitude of the emitted photon is greater than the magnitude of the absorbed photon; the deficiency of energy is borrowed from the thermal energy of the surrounding medium. One may imagine that in this case the initial state for absorption will be a state with several quanta of vibrational energy, while upon emission the molecule will pass into a state with a smaller store of vibrational energy. Generally speaking, it is not clear in advance that the quantum yield in this case should

decrease. If, however, it retained the value that it has in the Stokes region, this would mean that the energy yield under certain conditions could become greater than 100%. The results of S. I. show that this in fact does not occur; the quantum yield in this region falls in such a way that the energy yield always remains less than 100%. This conclusion was drawn by S. I. on the basis of his experimental work, subsequently confirmed many times on more extensive empirical material by his students. Quite recently (1947) S. I. showed theoretically that this result is an inevitable consequence of the basic laws of thermodynamics.

It is obvious that the regularity discovered by S. I. is fundamental for the whole energetics of photoluminescence processes in solutions. In all probability it also remains valid for any luminescence processes, although there is as yet no experimental proof of this. It is therefore understandable that this regularity in the literature—both ours and foreign—is quite justly called Vavilov’s law.

Another series of important works by S. I. was devoted to the study of polarized fluorescence. The phenomenon of polarized fluorescence itself was discovered by F. Weigert. However, in the subsequent study of it, a leading role was played by the works of S. I. and his collaborators, especially V. L. Levshin and P. P. Feofilov. The phenomenon of polarization of fluorescence in solutions consists in the following. If a fluorescing solution is illuminated by linearly polarized light with a definite direction of oscillation of the electric vector, then the fluorescence light proves to be partially polarized in the same plane as the incident light. This phenomenon is extremely remarkable. Its study opens the way to deep penetration into the structure of complex molecules capable of luminescence and to the investigation of the interaction of these molecules with one another and with the surrounding medium. Indeed, let us imagine that the molecules of a fluorescing substance are perfectly isotropic. When such molecules are illuminated by linearly polarized light, the fluorescence light could likewise prove to be linearly polarized only if the molecule, during the time of the excited state, were not subjected to any perturbing influences. But in a condensed medium, such as a solution, such conditions are not at all satisfied. If nevertheless the fluorescence light is linearly polarized, then this directly indicates that the molecules are in fact optically anisotropic and that their distribution in an isotropic medium (the solvent) is likewise anisotropic. Under these conditions the light will be absorbed and emitted predominantly by molecules situated in a definite way relative to the direction of oscillation of the electric ...

of the vector of the exciting light—the fluorescence will be polarized. It will, however, be only partially polarized, since there exist factors that upset the polarization. Thus, for example, rotational Brownian motion can cause a rotation of the molecule during the excited state and, since these Brownian rotations are completely chaotic, they are capable of completely erasing the initial anisotropy in the distribution of the excited molecules. In fact, it turns out that only in viscous (for example, glycerol) solutions is the influence of the described rotational depolarization reduced so much that observation of polarized fluorescence becomes possible.

Another important factor that disturbs the polarization of fluorescence is the concentration of the fluorescing substance. As the concentration is increased—beginning with very small concentrations of the order of \(10^{-8}\ \mathrm{g/cm^3}\)—the percentage of polarization at first remains unchanged, but then begins to decrease, gradually tending to zero. This decrease—concentration depolarization—was discovered by V. L. Levshin and simultaneously by a number of other investigators. It begins already at such concentrations when the intermolecular distances considerably exceed the gas-kinetic radii of the molecules, and is therefore a very sensitive indication of intermolecular interactions. Precisely because concentration depolarization begins at concentrations so small that energy exchange by direct collisions of molecules does not yet occur, it represents one of the manifestations of energy migration, i.e. its transfer over large distances without loss and without the intermediate stage in the form of radiation. Energy migration, which also appears in the phenomena of fluorescence quenching—concentration quenching and quenching by foreign substances—served as the subject of a large number of theoretical and experimental works by S. I. himself and his collaborators. Undoubtedly, this phenomenon of energy transfer over large distances plays an important role in various processes occurring in nature. Since, however, a separate article by A. N. Terenin in the present issue is devoted to energy migration, we shall not dwell further on this question here.

One of the most remarkable features of polarized fluorescence is the dependence, discovered by S. I., of the degree of polarization on the wavelength of the exciting light. Let us imagine that the linearly polarized light exciting the fluorescence, with oscillations of the electric vector directed along the \(Z\) axis, propagates along the \(X\) axis. Let us denote by \(I_Z\) and \(I_X\) the intensities of the fluorescence light corresponding to oscillations along the \(Z\) and \(X\) axes when observed along the \(Y\) axis; the quantity

\[ P=\frac{I_Z-I_X}{I_Z+I_X} \]

is called the degree of polarization or, when expressed in percent, the percentage of polarization. The quantity \(P\), extrapolated to infinite viscosity (in order to eliminate the influence of rotational depolarization), when the molecules are excited near the maximum of the long-wave absorption band, generally approaches the theoretical value for a linear oscillator, 50%. S. I. traced the percentage of polarization upon excitation over a broad region of the spectrum, including the ultraviolet. It turned out that, as the wavelength decreases, the degree of polarization falls more or less sharply and—what is especially remarkable—beginning with some definite wavelength, the polarization changes its sign, i.e. becomes negative. This shows that if in the long-wave absorption band \(I_z > I_x\), then in the ultraviolet, beginning with a definite wavelength, \(I_x > I_z\). At still shorter wavelengths the polarization first passes through a (negative) minimum, and then again becomes positive. If the dependence of the degree of polarization on wavelength is represented graphically, the result is a curve which S. I. called the polarization spectrum.

It is quite obvious that the polarization spectrum gives very valuable information about the structure of the molecule. In particular, the appearance of negative polarization in a definite region of the absorption spectrum indicates that the classical oscillators corresponding to absorption and emission do not coincide with one another. S. I.’s student P. P. Feofilov obtained polarization spectra for a large number of complex organic dyes and compared them with the absorption spectra of the same dyes. In this way P. P. Feofilov was able, in a series of examples, to demonstrate the fruitfulness of the method of polarization spectra for the study of the anisotropy of molecules and of their structure.

The study of polarization spectra, as we see, naturally leads to the question of the elementary oscillators of absorption and emission. The question of the nature of elementary emitters was the subject of a broad range of S. I.’s work. Such emitters are usually regarded as electric dipoles. However, this does not exhaust all possibilities. An elementary emitter may also be a quadrupole or a magnetic dipole, or, in general, a multipole of higher order. S. I. indicated an extremely elegant method for determining the nature of elementary emitters with the aid of so-called polarization diagrams (S. I.’s term). It turns out that the distribution of the degree of polarization as a function of the direction of observation and the orientation of the electric vector of the exciting light will be substantially different, depending on the nature of the elementary absorbing and emitting systems. If one restricts oneself only to dipoles and quadrupoles, then it may happen that both the absorbing

and the emitting system—both will be dipoles, but it may happen that the absorbing system will be a dipole and the emitting one a quadrupole. In general four cases are possible: dipole → dipole, dipole → quadrupole, quadrupole → quadrupole, quadrupole → dipole. For all these four combinations S. I. calculated and constructed diagrams showing the dependence of the degree of polarization on the angle of observation for a given position of the electric vector of the exciting light, or the dependence on the position of the latter for a given direction of observation. These diagrams for the four listed combinations of absorbing and emitting multipoles, as has already been mentioned, proved to be essentially different. Thus, in S. I.’s words, “by rotating the polarizing prism through which the exciting light passes, and making measurements [of the degree of polarization], for example along and across the exciting beam, we obtain ... characteristic curves that make it possible to draw conclusions about the nature of the elementary absorption and emission in various cases.” Similar diagrams can be constructed for cases in which absorption and emission are effected by magnetic dipoles or by combinations of magnetic and electric dipoles.

Another, still more subtle method indicated by S. I. for establishing the nature of emitters consists in observing the interference of beams diverging at large angles. In elementary expositions of many physical questions, qualifications that are very essential for the correct understanding of a given phenomenon are often omitted. A good example is provided by interference. Usually various interference experiments (Young’s, Fresnel’s) are considered, but it is not stated that in these experiments beams very close in direction interfere, and that at large angles quite peculiar phenomena should be observed. S. I. exhaustively analyzed interference phenomena at large angles between beams, taking into account the nature of the elementary emitters. That the nature of these emitters must have a substantial influence at large angles, and that in general the character of the interference pattern must depend on the nature (more precisely, on the multipolarity) of the elementary emitters, is evident from the following considerations. As is known, the distribution of the radiation intensity of an electric dipole with direction is not isotropic; the amplitude of the electromagnetic wave emitted by such a dipole is proportional to \(\sin \vartheta\), where \(\vartheta\) is the angle between the direction of oscillation of the dipole and the direction of emission. Consequently, for example, the intensity of radiation of an electric dipole in the direction of oscillation is equal to zero, and is maximal at \(\vartheta = \frac{\pi}{2}\). On the contrary, in the case of a quadrupole the amplitude is proportional to \(\sin 2\vartheta\), as a result of which the quadrupole does not

radiates not only in the direction of the oscillations, but also in the perpendicular direction. It is therefore easy to see that the amplitudes of two coherent rays, emitted by one and the same radiator in different directions, will, generally speaking, be different, and moreover these differences are characteristic of the multipolarity of the radiator. Correspondingly, the “visibility” of the interference fringes also changes, i.e. the ratio of the difference of the intensities at the maximum and at the minimum to the sum of the same intensities. For a dipole, when the angle between the rays is changed from 0 to 180°, the visibility falls from 1 to 0; beams directed at very large angles, close to 180°, almost do not interfere. For a quadrupole the picture is still more complicated: at 60° the visibility becomes equal to zero, i.e. the interference disappears, and with a further increase of the angle the visibility becomes negative, i.e. in the place of a bright fringe a dark one is obtained, and conversely. These peculiar results of theoretical calculations for the case of dipole radiators were confirmed by refined experiments.

A large cycle of work by S. I. and his collaborators was devoted to the investigation of phenomena at extremely low light intensities. Let us note here first of all the works, already included in textbooks, on the detection of fluctuations of visible light due to its quantum nature. As is known, owing to the quantum nature of light, at very low intensities fluctuations must be observed, caused by fluctuations in the number of photons that enter the receiving instrument. These fluctuations were detected by various methods with X-rays or $\gamma$-rays, where the photons are very large and their number, at one and the same total energy, is correspondingly small. S. I. showed that the same fluctuations can also be detected with visible light, and for this purpose he made use of the following properties of the human eye: a) the exceptionally high sensitivity (not yet attained by any objective instruments) of a well dark-adapted eye, and b) the presence of a sharp threshold of visual sensation. The latter circumstance, i.e. the existence of a threshold, considerably facilitates these difficult measurements.

S. I.’s experiments were arranged in such a way that between a small incandescent lamp, whose light passed through a green filter, and the observer’s eye there was placed a rotating disk with an aperture. During its rotation the disk transmitted the light for 0.1 sec. and held it back for 0.9 sec. Thus, every second a flash of green light fell into the observer’s eye, which was fixed in a definite position by a weakly luminous red point situated to the side. If the energy of the flash does not reach the threshold value, then, owing to the existence of the threshold, the flashes are not visible at all. Thus there is a very sharp qualitative indication of fluctuations: the flashes either

visible, or not visible at all. The visibility of the flash is determined by the number of photons absorbed in the retina during the time in which the disk transmits light. If \(Z\) is the number of photons absorbed during the flash, and \(n_0\) is the number of photons corresponding to the threshold, then the flash will be visible only when

\[ Z \geq n_0 . \]

Thanks to this circumstance, the presence of a fluctuation in the number of photons is revealed by the fact that the observer either sees or does not see the flash. Each revolution of the disk with an aperture was automatically recorded by a mark on the tape of the chronograph; at the moment when the observer saw a flash, he pressed the key of an electrical circuit, and the chronograph pen likewise marked the flash. A comparison of the number of passages of the disk apertures, i.e. the number of flashes that actually occurred, with the number of flashes seen by the observer made it possible to judge the fluctuations. The results of many hundreds of series of such fluctuation measurements were then subjected to numerical analysis by the methods of probability theory, and the statistical character of the observed fluctuations was established, in agreement with the conception of the quantum nature of light. All possible sources of error in these difficult measurements were taken into account, in particular possible fluctuations in the number of photons \(n_0\) corresponding to the threshold under the influence of physiological causes.

Work on the visual detection of quantum fluctuations of a luminous flux, in addition to its fundamental interest for physics, opens a new path for the study of the eye. These investigations made it possible to determine the true sensitivity of the retina at the threshold of visual sensation, indicated certain features of the functioning of the eye in various regions of the spectrum, etc.

Interesting observations were also made of the fluctuations of two coherent beams. For this purpose, a Fresnel biprism was placed between the rotating disk and the observer’s eye, which divided the luminous flux into two coherent beams. The observer’s eye then saw two spots. On reaching the threshold, both spots fluctuated quite distinctly independently of one another, and only very rarely were they visible simultaneously. This is in full agreement with the corpuscular picture of the nature of light and is in sharp contradiction with the wave picture.

The corpuscular picture of interference was revealed still more clearly in the following experiment. In the path of the rays, instead of the biprism, a double Young slit was placed, and in front of the eye a lens was arranged so that, with sufficient intensity of the luminous flux, an interference pattern appeared on the retina. Between the double slit and the lens a diaphragm with two round aper-

by apertures which, as far as possible, were located precisely, respectively, at the centers of the dark and light neighboring fringes. Therefore, when the intensity of the flux was sufficiently great, at each passage of an aperture in the rotating disk a flash appeared in one (the upper) aperture of the diaphragm, while the other (the lower) remained dark. If, however, the intensity is lowered to the threshold value, then the lower aperture remains dark, while in the upper one fluctuations are observed: the flash is now visible, now not visible. This means that no photons at all enter the place occupied by the lower aperture (coinciding with the center of the dark interference fringe), whereas at the place occupied by the upper aperture photons, with a very weak flux, enter sometimes in greater and sometimes in smaller numbers, and sometimes do not enter at all—the bright fringe is formed statistically: under the usual conditions of an interference experiment it is the result of averaging over an enormous number of photons. It is hard to devise a more vivid illustration of the statistical character of interference!

We have dwelt on these works of S. I. in such detail in view of their rare elegance and great fundamental significance. In conclusion to this far from complete outline of the scientific activity of Sergei Ivanovich, let us consider one of the most important discoveries to come out of his laboratory in recent years. We are speaking of the discovery of a special kind of luminescence, for which the name “Cherenkov radiation” has already become established in our scientific literature and abroad. In 1933 S. I. proposed to P. A. Cherenkov—then a graduate student of the Academy of Sciences—that he investigate luminescence under the action of hard γ-rays. Very soon, however, it was found that, alongside luminescence, γ-rays produce a glow which in its properties differs sharply from ordinary luminescence. This exceedingly weak glow arises in all pure liquids; its spectral composition is the same in all liquids; it is polarized in such a way that the electric vector oscillates in the direction of the exciting beam; finally, its distribution in space is sharply anisotropic: the glow is directed forward along the path of the beam and propagates within a narrow cone, while its intensity in the direction of the axis of the cone is diminished. The vast experience in the study of luminescence at once suggested to S. I. that this glow is by no means luminescence. This was also pointed out by S. I. in a note published simultaneously with Cherenkov’s report of the experimental results; in the same note S. I. expressed the supposition that the glow is caused not by the γ-rays themselves, but by fast electrons arising in the medium during the passage of γ-rays. By this, however, one should not understand some trivial effect, since in general the first result of the absorption of the enormous quanta of γ-rays is the ejection of fast electrons. S. I.’s idea was that the radiation

connected with the rapidly moving electrons themselves, and therefore it is precisely its properties that depend hardly at all on the properties of the medium in which the glow arises.

A complete theory of the phenomenon was given later by I. E. Tamm and I. M. Frank. Its qualitative foundations are now well known to physicists: the glow is produced by electrons moving uniformly in a medium with a velocity greater than the phase velocity of light in the given medium (i.e. greater than \(\frac{c}{n}\)); it is an electromagnetic analogue of the “bow wave” accompanying the rapid motion of projectiles in air and clearly visible in photographs of sound waves arising during the motion of projectiles.

In the summer of 1950—during his last vacation—S. I. wrote an excellent monograph, The Microstructure of Light, in which he summarized some of his work over a thirty-year period.

S. I. never confined himself to the sphere of problems of “pure science.” Being a major specialist in the field of luminescence, he devoted great attention to its practical applications. The work carried out under S. I.’s direction on applications of luminescence proceeded along two lines: 1. luminescent analysis, and 2. luminescent light sources. In the field of luminescent analysis, special methods of analysis were developed in the laboratories directed by S. I. (for example, the delicate method for analyzing ozone content in air, developed by M. A. Konstantinova-Shlezinger), as well as apparatus necessary for various kinds of luminescent measurements. An enormous amount of work preceding the production of domestic luminescent lamps was carried out under S. I.’s direction in a number of institutions (at the State Optical Institute, the All-Union Electrotechnical Institute, and the Lebedev Physical Institute). The highly economical daylight lamps created as a result of this work are already being mass-produced and are finding ever broader application.

S. I. always attached great importance to the training of personnel. Many young research workers brought up by S. I. now themselves occupy leading positions in research institutes and in departments of higher educational institutions. Such are Prof. V. L. Levshin, Corresponding Member of the Academy of Sciences of the USSR I. M. Frank, Prof. V. A. Fabrikant, Doctors of Physical and Mathematical Sciences E. M. Brumberg and P. P. Feofilov, Prof. M. A. Konstantinova-Shlezinger, B. Ya. Sveshnikov, N. A. Tolstoi, and others.

III

Throughout all his scientific activity S. I. devoted much attention to questions of the history and philosophy of science. He wrote a number of works on the history of physics in our country. Among them we shall note works on the history of physics in the Academy of Sciences, from

which the readers of our journal are familiar from S. I. Vavilov’s article “The Physics Cabinet.—The Physics Laboratory.—The Physics Institute of the Academy of Sciences over 220 Years” (UFN, 28, 1, 1946). The activity of M. V. Lomonosov always served as a subject of special attention for S. I. His articles devoted to Lomonosov’s little-known optical works are of great interest. Having recounted the sad history of the “night-vision tube,” the project of which, after Lomonosov’s dispute with Euler, was put “under the academy’s cloth,” S. I. notes: “In our time the night-vision tube (of course, in its modern form) has become a commonly used military instrument... Yet few know that the inventor of the ‘night-vision tube,’ or ‘night-glass,’ was M. V. Lomonosov.” Having described the catadioptric astronomical tube built by Lomonosov in 1762, with a slightly inclined concave mirror, S. I. draws attention to the fact that the same design idea underlies Herschel’s famous telescope, built in 1789. Therefore “it is high time to call this optical system the Herschel–Lomonosov system.”

A large number of S. I.’s articles were devoted to the history of the development of national science in our Soviet era. Let us note here his book and numerous articles devoted to the development of science in the USSR over thirty years, and articles devoted to the history of the State Optical Institute.

A number of very valuable works by S. I. were devoted specifically to the history of optics. Thanks to his labors, Newton’s optical works and his role in the history of optics have been illuminated in the USSR with considerably greater completeness than in Newton’s homeland—in England, where the last incomplete edition of Newton’s works (partly in Latin) was issued in 1779–1785 and has not been reprinted since! S. I. translated into Russian and annotated not only Opticks, but also the little-known and hard-to-obtain Lectures on Optics, as well as all the special optical memoirs. He wrote an excellent biography of Newton, which is the result of a many-sided study of Newton’s life, scientific works, and scientific-philosophical views, and which combines vividness and accessibility of presentation with thoroughness and depth.

This brief list gives only the faintest idea of the enormous labor invested by S. I. in the study, translation, and analysis of Newton’s works in the field of optics.

S. I.’s work “Galileo in the History of Optics” is of great interest and novelty. “In the history of optics,” writes S. I., “up to the present time Galileo is, at best, mentioned only in connection with his telescope and, at best, with the microscope. This brevity, however, is only a new example of the glaring discrepancy between school history and the actual process of development. In all

[SERGEI IVANOVICH VAVILOV]

during the existence of optics as a science, a time spanning millennia, it received its greatest stimulus precisely from Galileo.” In order to prove this thesis, S. I. had to do a great deal of work, since “in Galileo’s scientific printed and manuscript legacy there is not a single work devoted specifically to optics.” The historian of science, in which capacity S. I. appeared in this case, had to “reconstruct Galileo’s deeds and thoughts in the field of the doctrine of light,” making use for this purpose of individual pages of Galileo’s works and of his correspondence. S. I. drew the material for this reconstruction from the so-called National Edition of Galileo’s works. But in order to carry out this difficult task, it was necessary, after all, to study thoroughly all 20 volumes of this Edizione Nationale, and S. I. made use of this work in order to write further interesting pages about Galileo’s Italian predecessors in the sixteenth century, in particular about the optical works of Leonardo da Vinci! It would hardly be possible to find a better example to illustrate the astonishing breadth of S. I.’s erudition and capacity for work, for he could devote only his hours of rest to work in the history of science! Limitations of space do not allow us to recount even briefly the content of this work. Nor are we able to dwell on his other historical works, such as the vivid article “Lucretius’ Physics,” devoted to a detailed analysis of the physical views of this remarkable materialist philosopher of the first century before our era, whose didactic poem On the Nature of Things, in S. I.’s words, “determined many features of the world-outlook of Newton and Lomonosov, delighted Herzen, deeply interested the young Marx, and served as a banner of mechanistic materialism for L. Büchner.”

Amid his intense scientific work and responsible organizational and public activity, S. I. never forgot the importance of broad popularization of science. Already in his youth he wrote the splendid book The Actions of Light (1922), in which the classical and quantum conceptions of the nature of the actions of light are presented in an accessible form (photoelectric effect, photochemistry, dispersion and absorption of light). At the same time he succeeded in giving a clear conception also of such complex questions as are usually not touched upon in popular expositions (for example, the electron theory of the dispersion and absorption of light).

Another popular book by S. I., The Eye and the Sun, which went through 4 editions, is distinguished not only by the accessibility of its exposition, but also by the originality of its formulation of the problem. In many respects it is therefore of interest not only to the insufficiently prepared reader, but also to the specialist. S. I. also wrote a large number of articles in popular journals, Science and Life, Nature, and others.

S. I.’s literary activity was, in general, exceptionally broad and fruitful. He wrote many articles for Soviet encyclopedias (GSE, TSE), including such important ones as the article “Physics” in volume 57 of the GSE. He himself translated and edited a number of scientific and popular-science books and articles (A. Einstein, “On the Special and General Theory of Relativity”; N. Bohr, “Three Articles on Spectra and the Structure of Atoms,” and many others).

From the very foundation of our journal he was, and until his very end remained, one of its most active contributors. In the cumulative index of articles, abstracts, and reviews published in 1947, the mere list of what S. I. had done takes up 3 pages. Already in the post of President of the Academy of Sciences of the USSR, burdened to the utmost with highly responsible work, S. I., as before, remained the most active member of the Editorial Board and a great friend of our journal.

IV

Public activity occupied a large place in the intense labor of Sergei Ivanovich, an ardent patriot of the socialist Motherland.

He assumed the high scientific post of President of the Academy of Sciences of the USSR in the most momentous period in its historical significance—the period of our Motherland’s transition from socialism to communism. In his historic speech before the voters on February 9, 1946, Comrade Stalin outlined the great program for building communism and appealed to scientists with the call to “surpass in the near future the achievements of science beyond the borders of our country.” Comrade Stalin’s call became the program of S. I.’s activity. On behalf of Soviet scientists he declared in print: “scientists will justify Comrade Stalin’s trust” (article under the same title, 1946). Guided by the instructions of the Party and of Comrade Stalin, S. I. directed the activity of the Academy toward the cause of building communism.

In a number of articles and collections (the collection Soviet Science at a New Stage, 1946; the collection Science of the Stalin Era, 1950; Thirty Years of Soviet Science, Science and the People, and others) S. I. tirelessly propagated the profound ideas and instructions of Comrade Stalin concerning the special features of socialist science, its connection with the people, the unity of theory and practice, the significance of science in the work of building communism, and the methods of developing Soviet science through free discussions, through the expansion of criticism and self-criticism. S. I. strove to put these ideas and instructions of the leader into practice. At the same time S. I. presented the image of Comrade Stalin as the greatest genius of science (“The Scientific Genius of Stalin,” and others).

S. I. devoted much energy to questions of the connection of science with industry, to the introduction of the achievements of science into the practice of sociali-

SERGEI IVANOVICH VAVILOV

...the socialist economy, which was reflected in a number of his articles (“The Reorganization of the Photographic Industry Is a Task of the Day,” 1935; “Soviet Science and the National Economy,” and others).

In a number of philosophical articles Sergei Ivanovich demonstrated the creative role of dialectical materialism in the development of modern physics (“The Triumph of the Dialectical-Materialist Teaching,” 1937; “The New Physics and Dialectical Materialism,” 1938; “V. I. Lenin and Modern Physics,” 1944, 1947; “The Development of the Idea of Matter,” 1941, and others) and subjected to sharp criticism foreign scientists (Eddington, Jeans) who had slipped into the positions of idealism. In this way S. I. gave a practical example of how a Soviet scientist should combine work in his special field of science with the study of Marxism-Leninism and its application to concrete scientific problems.

The most important events in the life of the country found an immediate, vivid reflection in S. I.’s political articles and in his reports before large audiences (“The Fascists Reveal Their Nature as Cannibals,” 1938; “A Grievous Loss” (on the villainous murder of S. M. Kirov), “A Friend of Science” (on A. A. Zhdanov), 1948; and others). Needless to say, every major event in the cultural life of the country—whether connected with science, literature, or art—was answered by S. I. with a speech, always interesting, substantial, and original.

Sergei Ivanovich was an ardent fighter for peace. In his article “Science in the Service of Peace” (1949) he wrote: “It is the duty of conscience and honor of scientists and of the intelligentsia in general, by all means available to us, to oppose the capitalist subordination of science to the aims of preparing war against free democratic countries. The unification of scientists for the noble and great purposes of putting an end to the use of science for the preparation of new wars is one of the important instruments of the policy of peace.”

In 1949 the Council of Ministers of the USSR appointed Sergei Ivanovich editor-in-chief of the second edition of the Great Soviet Encyclopedia. The enormous socio-political significance of this undertaking is evident from the tasks set before the Encyclopedia: the second edition of the GSE has as its aim broadly to illuminate the decisive victories of socialism in our country, the achievements of the USSR in the fields of economics, culture, science, and art; convincingly and fully to demonstrate the superiority of socialist culture over the decaying culture of the capitalist world, relying on Marxist-Leninist theory; to expose imperialist aggression and to provide a Party-minded critique of contemporary reactionary bourgeois currents in the various fields of science, technology, and culture.

Sergei Ivanovich was one of the initiators of the organization of the All-Union Society for the Dissemination of Political and Scientific Knowledge. As chairman of this society he devoted...

devoted much energy to the cause of raising the culture and the political and scientific enlightenment of the broadest masses of the population.

S. I. was repeatedly elected a deputy to leading Soviet bodies (to the Leningrad City Soviet of Workers’ Deputies in 1935, to the Supreme Soviet of the RSFSR in 1938, and to the Supreme Soviet of the USSR in 1946). In recent years S. I. was a deputy of the Supreme Soviet of the USSR and of the Moscow City Soviet of Workers’ Deputies. In all public and state posts he maintained the closest ties with his voters.

S. I.’s outstanding scientific and scientific-organizational activity was repeatedly recognized by high government awards. In 1939 he was awarded the Order of the Red Banner of Labor “for the fulfillment of government assignments and the mastering of new types of armament, and for strengthening the combat power of the Red Army and the Navy.” In 1943 he was awarded the Order of Lenin “for successful work in developing the national optical-mechanical industry and fulfilling Government assignments for the development of new types of optical instruments,” and in 1945 he received a second Order of Lenin in connection with the 220th anniversary of the Academy of Sciences, “for outstanding services in the development of science and technology.”

S. I.’s scientific works were twice awarded Stalin Prizes: the first time in 1943, for work in the field of luminescence and quantum fluctuations of light, and the second time (jointly with I. E. Tamm, I. M. Frank, and P. A. Cherenkov) for the discovery of a new kind of radiation.

Sergei Ivanovich has passed away...

He departed in the prime of his creative powers, at the height of his scientific and organizational activity. To the last minute he remained at his post, despite a grave illness that swiftly, all too swiftly, led to the fatal outcome.

Sergei Ivanovich has died, but the memory of him will not die. The Soviet people will sacredly preserve the memory of the talented scientist who devoted all his strength to serving the Motherland, to the flourishing of its science, and to the great cause of building communism. The image of Sergei Ivanovich, dear to us all, will remain alive for us; it will serve us as an example and inspire us to unremitting labor for the good of our great Motherland.