ON THE SEVENTY-FIFTH ANNIVERSARY OF THE BIRTH OF T. P. KRAVETS

One of the oldest and most versatile figures in Soviet physics, Corresponding Member of the Academy of Sciences of the USSR Professor Torichan Pavlovich Kravets, has reached the age of 75. One of the first pupils of the remarkable Russian physicist, the founder of the first major Russian school of experimental physics, Petr Nikolaevich Lebedev, having worked under the latter’s direction longer than anyone else—about a decade and a half—T. P. Kravets is an outstanding bearer of the traditions of this scientific school. These traditions were creatively and widely employed by him in his many-sided scientific and scientific-organizational activity, which unfolded broadly after the Great October Revolution, which created unprecedented conditions for the development of science, and of physics in particular, in our country.

Torichan Pavlovich Kravets was born on March 22, 1876 (New Style), in the village of Volovo, Bogoroditsk district, Tula province, in the family of a zemstvo physician. Having received his secondary education at the Tula gymnasium, T. P. entered in 1894 the physics and mathematics faculty of Moscow University, in the mathematics division. At the university, brilliant with the names of remarkable Russian scholars, T. P. attended lectures by such luminaries of science as A. G. Stoletov, N. A. Umov, N. E. Zhukovsky, P. N. Lebedev, and the still-living N. D. Zelinsky. Characteristic of the gymnasium and university periods of T. P.’s life is the fact that he was always among young people, from whom a number of outstanding scholars later emerged; at the same time, Kravets’s personal qualities and uncommon ability placed T. P. to a considerable degree at the center of this circle. Already in his third year T. P. came to P. N. Lebedev—then still a very young thirty-year-old university lecturer—and began to work under his direction; he was the third, in point of time, practical student of what later became the famous Lebedev laboratory.

After graduating from the university in 1898, T. P. Kravets was retained there to prepare for the rank of professor (in modern terminology, in postgraduate study). Continuing to work under the direct guidance of P. N. Lebedev, T. P. carried out his first scientific work, devoted to the measurement of anomalous dispersion and of the dispersion of electromagnetic waves. In 1901, for part of this investigation, entitled “Dispersion and absorption of electric waves in water,” the Moscow Society of Lovers of Natural Science, Anthropology and Ethnography awarded T. P. a prize. The misfortune that befell T. P. in the second part of this investigation—a temporary divergence with his teacher in 1903—interrupted T. P.’s experimental work for a certain period, but did not affect his participation in the famous Lebedev colloquium, of which he was one of the most active members throughout its existence, up to the death of P. N. Lebedev.

In 1906, after returning from military service, on which he had been during the Russo-Japanese War, T. P. resumed work in Lebedev’s laboratory, at first undertaking, on his own initiative, the spectral distribution of the photoelectric effect, and then, on the advice of his teacher, turning to a subject logically connected with his previous work—namely, the study of the absorption of light waves. This work, carried out-

which occupied T. P. Kravets for six years, was one of the most outstanding investigations in optics in pre-revolutionary Russia and formed the subject of T. P.’s master’s dissertation, written and defended at St. Petersburg University after the death of P. N. Lebedev in 1913.*) Begun in Lebedev’s laboratory, this work, after P. N. Lebedev’s departure from the university together with other progressive professors, was completed by T. P. in the laboratory of the Moscow Higher Women’s Courses, where he taught physics beginning in 1907. A peculiar form of T. P.’s association, at that time not having any direct relation to Moscow University, with the protest of the leading professors of the University against the reactionary policy of the tsarist government in the field of higher education was T. P.’s refusal, from natural feeling, to defend his dissertation at his own university and his decision to defend it in Petersburg.

As the immediate aim of his work T. P. Kravets set himself “...the rapprochement of the doctrine of anomalous dispersion with that electronic theory from which it took its origin, but from which it had for some time as it were broken away.” In the introduction to the dissertation T. P. wrote, immediately following the phrase quoted above: “My work directly adjoins one of the last works of Drude and seeks to obtain, in the region of absorption, what he had worked out in detail in the region of dispersion, especially normal dispersion. The results obtained by me indicated the necessity of new additions to the theory.”

The object of study was weak solutions of strongly colored substances—dyes—possessing, as is known, absorption bands in the visible region of the spectrum considerably broader than the bands of vapors and gases. T. P. paid special attention to the structure of these bands; comparing their form with that which is derived for them from the theories of Drude and Planck, he came to the important conclusion concerning the complexity of these bands, which represent the result of the superposition upon one another of many much more elementary bands corresponding to different states of the oscillators in the dye molecule. S. I. Vavilov (1922) and D. S. Rozhdestvenskii (1934) subsequently addressed this question and confirmed this view.

One of the important results of the work is the derivation of a theorem relating the number of molecules \(N\), the molecular monochromatic absorption coefficient \(\varepsilon_\lambda\), the oscillator strength**) \(f\), and the refractive index of the medium \(n_0\):

\[ N f \frac{e^3}{m} = \frac{n_0 c^2}{\pi} \int k\, d\left(\frac{1}{\lambda}\right), \]

where

\[ f = n_0 \cdot 4.314 \cdot 10^{-9} \int \varepsilon_\lambda\, d\left(\frac{1}{\lambda}\right). \]

Taking into account the polarizing action of neighboring molecules, T. P. established the dependence of the oscillator strength on the refractive index of the solvent:

\[ f_1 = f \frac{9}{(n_0^2 + 2)^2}. \]

*) The opponents, who highly praised the dissertation, were Professors I. I. Borgman, N. A. Bulgakov, and D. S. Rozhdestvenskii. The last of these became only then acquainted with T. P., which afterward he himself studied the question of the nature of broad absorption bands.

**) The not very felicitous term “oscillator strength,” characterizing the intensity of absorption, was introduced into circulation much later by Pauli.

These formulas make it possible to determine the strength of a molecular oscillator. T. P. calculated this quantity for a number of dyes he had investigated and obtained values close to unity.

It should be noted that great importance is attached to the quantity \(f\) in the quantum theory of absorption; over the last 10–15 years experimental works have repeatedly appeared devoted to determining it in the molecular absorption spectra of organic compounds and to establishing its dependence on the solvent, which in its time was also the subject of a special study by T. P.

T. P. Kravets’s work, entitled “Absorption of Light in Solutions of Colored Substances. An Experimental and Theoretical Study,” was published only in a comparatively little-known organ—in the Proceedings of the Moscow Engineering School (Part II, No. 6, 1912). Therefore T. P.’s priority long remained little known. In the literature, however, reference is usually made to Ladenburg, who in 1914 obtained, close to T. P. Kravets’s equation, an expression for the spectral absorption lines of vapors, the transfer of which to liquids leads to erroneous results. In 1931 Chako derived formulas for \(f\) that are quite analogous to T. P.’s formulas, not only without referring to T. P., but with a direct indication that he had done this for the first time. After this the “Chako formula” appeared in the literature. It seems necessary to us to establish the priority of Russian science in this question.

After 35 years T. P. Kravets returned once again to questions of the theory of light absorption by solutions of dyes. In recent years he carried out a number of works that are a direct continuation and development of his dissertation.

During the period of work with P. N. Lebedev, T. P. devoted much time to teaching activity, which, after the expiration of his university scholarship, provided him with means of subsistence. From 1898 to 1913 he was a freelance instructor at the Moscow Engineering School (the future Institute of Railway Engineers), and after that, until his move to Kharkov, a staff adjunct of this school. Here he worked under the guidance of another outstanding Russian physicist, Prof. A. A. Eichenwald, with whom he was bound by close friendship and to whom, by his own admission, he owed much with respect to the art of lecturing, the demonstration of lecture experiments, and the experience of scientific-organizational activity.

From 1901 to 1914 T. P. lectured in physics at the Moscow Pedagogical Courses, and from 1907 to 1917 also at the Moscow Higher Women’s Courses. At the same time he gave many popular-science lectures at people’s universities, etc. Already as a student, having become acquainted with the remarkable lecturing talent of such professors as N. A. Umov, N. D. Zelinskii, and many others, T. P. adopted from his teachers and, during the course of his many years of teaching activity, finely honed the high art of lecturing, which even in those young years won him the glory of one of the best lecturers on physics in Russia, and in our time captivates everyone who has the opportunity to hear his presentations—whether systematic lectures, a major public report, or an episodic extemporaneous appearance.

At the end of 1913 Kharkov University elected T. P. Kravets professor in the department of physics. T. P. moved to Kharkov in the spring of 1914 and worked there for about six years. In Kharkov, alongside fulfilling his main duties, T. P. carried out extensive organizational work as secretary of the faculty, chairman of the university financial commission, and, finally, prorector. At the same time he lectured at the Kharkov Higher Women’s Courses and occasionally at other higher educational institutions.

During this period T. P. carried out theoretical work on light pressure, thereby paying tribute to the principal life’s work of his teacher

P. N. Lebedev. In this study, published in 1916, T. P. Kravets gave a formula for the light pressure for an individual oscillator, which subsequently proved essential for astrophysics, since it is now known that light pressure ejects molecules from stellar atmospheres and forms comet tails.

During the next several years T. P. was engaged almost exclusively in teaching at various institutions of higher education.

From 1923 to 1926 T. P. Kravets was a professor at Irkutsk University and head of the seismological station of the Academy of Sciences. The performance of these latter duties and the impossibility, in the absence of optical instruments, of doing experimental work in optics focused T. P.’s interests on geophysical questions. Having worked creatively for a quarter of a century in the field of physical optics, T. P. was deeply familiar with the theory of oscillations, one branch of which is, in fact, the theory of the propagation of electromagnetic, including light, waves. It is therefore natural that T. P. quickly found in the field of geophysics questions which, while of considerable practical interest, gave him an opportunity to display his creative powers. These questions were those of the mechanical oscillations of water basins—the theory of seiches and tides. In this field T. P. began working with students, i.e. in the same way as his teacher P. N. Lebedev had worked all his life. The first small scientific school of T. P.’s that arose in the course of this work was the prototype of those large creative collectives whose creation he subsequently carried out so successfully and became one of the chief and, perhaps, the most important concerns of his life. These students were: the now deceased V. N. Solov’ev, I. A. Parfianovich (now professor of physics at Irkutsk University), V. P. Dubov, and A. S. Toporets. The degree of T. P. Kravets’s participation in all these works may be judged from the inscription on an offprint of an article by V. N. Solov’ev published in 1925, made by its author: “To the highly esteemed Torichan Pavlovich, whose full right to head this work I might have asserted without any exaggeration.” Here we first encounter a feature characteristic of all T. P.’s subsequent activity—not to put his name on articles based on works carried out by his students on his initiative and under his immediate supervision, articles that in some cases had actually been written by him. T. P. undoubtedly borrowed this trait from his teacher P. N. Lebedev, who likewise, though he had dozens of students working with him for many years, did not publish a single article jointly with these students—all articles appeared only under the names of the latter.

T. P. Kravets proposed to V. N. Solov’ev, then still a student, that he investigate on a model the so-called seiches—the natural oscillations of Lake Baikal caused by the action of winds. A concrete model of the lake was made, reduced in comparison with the actual dimensions approximately 600,000 times horizontally and 11,000 times vertically. The model was filled with water and oscillations were excited in it, which were recorded with the aid of a float recorder. In this way, on the model, the distribution of the nodes and antinodes of the fundamental oscillation and its overtones, the relative magnitudes of the amplitudes, and the periods were found. Comparison of the periods obtained on the model with the periods of the seiches determined from limnograms at various points of Baikal revealed a very close correspondence between them. Together with this, knowledge of the distribution of the oscillation amplitudes made it possible to indicate the most favorable places for installing limnographs.

On this same model I. A. Parfianovich in 1926 studied tides; the work was published only in 1930. For this purpose the model was placed on a platform that was rocked with a definite period. The tidal amplitude obtained from the experiment and recalculated to the actual dimensions of Baikal ...

turned out to be approximately 19 mm. Processing the limnograms recorded on the lake by the method of harmonic analysis enabled T. P. Kravets and his collaborator A. P. Ekimov (1926) to establish with great accuracy the actual amplitude of the tidal wave caused by the diurnal rotation of the Moon around the Earth. The amplitude proved to be about half that found on the model. This discrepancy was readily explained by the fact that, simultaneously with the water surface, the earth’s crust was also oscillating.

The cycle of work on the study of water-level oscillations in Baikal ends with an investigation of the propagation of Baikal seiches along the Angara River. In this work, carried out already in Leningrad in 1935, T. P. theoretically solved the problem of the transfer of a seiche wave by water flowing along a narrow straight channel, and, together with A. S. Toporets, analyzed the available material on the registration of water level in the Angara at its various points. It unexpectedly turned out that in some, though not all, cases Baikal seiches propagated along the river to places 40 km distant from Baikal.

Subsequently, on T. P.’s initiative, the model method was applied by V. P. Dubov to the study of the seiches of the Baltic Sea, which also represents an almost closed water basin. The complicated branched form of the latter creates considerable difficulties for the theoretical investigation of the question. Therefore here the experimental model method proved especially fruitful. It was found that the waters of the Gulf of Bothnia participate in three seiches, and the waters of the Gulf of Finland in four. Particularly unexpected was the four-nodal seiche, in which the central part of the Baltic Sea performs transverse oscillations of considerable amplitude relative to the western side opposite the island of Gotland. The periods of the seiches determined on the model again agreed well with the periods determined from mareographic records.

Having studied the materials on floods in Leningrad, V. P. Dubov in an article of 1936 came to the conclusion that there is a connection between seiches and floods. It was found that the distribution of floods according to their duration is not uniform, but is grouped around the periods of seiches. The highest maximum of recurrence falls at a period of 25 hours, which exactly coincides with the period of the most widespread two-nodal seiche, and the next maximum in height with the period of the one-nodal seiche (30 hours). Analysis of the materials on the especially severe Leningrad flood of September 23, 1924, convincingly showed that such a large rise of water was caused by the superposition of two seiches: a two-nodal and a three-nodal one.

The cycle of work by T. P. and his pupils on the natural and forced oscillations of the water basins of the Soviet Union is the first and, apparently, the only one of its kind in the world.

In 1926 the then vice-president of the Academy of Sciences of the USSR, Academician V. A. Steklov, invited T. P. to work at the Physico-Mathematical Institute of the Academy, which he headed, in order to resume the activity and leadership of the experimental department of that institute. But by the time T. P. moved to Leningrad, V. A. Steklov had died, and the plans he had developed for the development of experimental physics in the Academy at that moment did not receive sufficient support. Meanwhile, Academician D. S. Rozhdestvensky, founder and at that time director of the State Optical Institute, having learned of T. P.’s arrival, invited him, beginning in October 1926 (at that time—initially without pay), to head the photographic laboratory of this institute. The needs of the domestic photographic industry, which was rapidly developing in the period of the Stalin five-year plans, the absence in the first years of any other scientific institution for photography in the country (the Scientific-Research Motion-Picture and Photographic Institute in Moscow was organized several years later), and the importance then attached in the Optical Institute to the creation within it, as in the основ-

Chronicle

... as the country’s scientific center for optics, a photographic laboratory. The reason was that this latter laboratory, headed by T. P. by combination of offices, grew much faster than T. P.’s main laboratory in the Academy of Sciences. The creation of the largest scientific school in the Soviet Union dealing with problems of photography, both from the theoretical and the practical side, was the great and nationally important achievement in the life of T. P. Kravets. The beginning of the history of this direction lies in the very first works undertaken by T. P. at the Academy of Sciences.

Already during his investigation of dye solutions T. P. encountered the question of the properties of colloid-colored media, taking as an example the gold sols that were then of great interest to everyone. At the suggestion of P. N. Lebedev, T. P. Kravets prepared a series of colloidal gold solutions possessing different coloration, which he demonstrated at the first Mendeleev Congress (1907).

It was clear to T. P. Kravets, who was interested in the problem of optical absorption in its whole scope, that the question of the color of colloidal solutions had a very general significance. Therefore, as a first work at the Physico-Mathematical Institute, he proposed to his colleague M. V. Savostyanova the task of verifying the Rayleigh–Mie formulas for a number of substances with strong and spectrally selective absorption. Having solved this problem for a dye solution (fuchsin), M. V. Savostyanova turned to other objects and, first of all, to sodium in a crystal lattice of sodium chloride, explaining the nature of the coloring of rock salt encountered under natural conditions and showing methods for precalculating the light absorption in it. This investigation directed T. P.’s interests toward metals found in the finest state of subdivision in a crystal lattice. From there followed the natural transition to silver in a lattice of silver halide and, as a result, to the problem of the latent photographic image, which naturally interested T. P., who turned to the problems of photography.

T. P. Kravets and M. V. Savostyanova carried out experiments on the coloring of crystals of alkali-metal halide salts under different conditions of their illumination, deformation, and heating, and on the spectral study of their light absorption. It was established (and published in 1930) that the whole set of observed colorations of crystals can be explained by differences in the sizes of the particles of alkali metal formed in the crystal lattice of the salt: the observed change in the color of rock salt from yellow to blue corresponds to a transition from atomic absorption centers to particles of comparatively large size. This was accomplished by applying to such systems the Rayleigh–Mie theory of light scattering by turbid media and by establishing good agreement between calculated and experimental data on the spectral absorption curves. From this it became possible to solve the inverse problem—to determine particle sizes from the absorption curve.

These investigations were extended to crystals of silver halide, to the analogy between whose photochemical coloring and the coloring of alkali-halide salts T. P. had already pointed in 1928 in his speech at the anniversary annual meeting of the State Optical Institute on the subject “Modern Views on the Nature of the Photographic Process.” In this report T. P. for the first time indicated that it is precisely here that the explanation of the nature of the latent photographic image is concealed.

When light acts on crystals of silver halide, an additional absorption band appears in the visible region of the spectrum. As measurements and calculations by M. V. Savostyanova showed, both the scattering of light by the centers causing this absorption and the Rayleigh–Mie theory are applicable; this testifies to the fact that, in crystals of silver halide under the action of light, extremely small (colloidal) particles of silver are formed. It should be noted that Hilš and Pohl, who were occupied at the same time in Göttingen with the same question, initially...

initially believed that photochemical coloration was caused by the formation of stable atomic centers of silver, similar to the atomic centers of sodium in the lattice of yellow rock salt. Only after the publication of the work of M. V. Savostyanova (1930), where for the first time the point of view was expressed (although not very clearly) that both atomic and colloidal absorption centers arise under the action of light, did the above-mentioned authors also begin to regard these latter as the result of the process.

Later experiments by P. V. Meyklyar (1939—1940), also carried out on the initiative of T. P. Kravets, showed that, as the duration of illumination of a silver chloride crystal increases, the maximum of the curve of additional absorption shifts toward longer wavelengths (the color changes from red to blue), i.e., the sizes of the silver particles increase. The level of illumination has the strongest effect on the coloration: the higher it is, the larger the number of ever smaller particles formed. The last observation made it possible to approach, from a new point of view, the question of the deviations from the reciprocity law known for photographic layers. Finally, it was discovered that when a colored crystal is heated, absorption decreases and shifts toward shorter wavelengths, i.e., thermal dissociation of silver particles takes place.

Direct observations of the separation of the smallest silver particles during the illumination of single crystals of silver halide were made with the aid of an ultramicroscope by students of T. P. Kravets—M. V. Savostyanova and A. S. Toporets (1934). It was found that the separation of particles occurs to the greatest extent along cracks and other inhomogeneities of the crystal; this indicates the significance of the latter for the photographic process.

In the same work it was shown that illumination of a previously photochemically colored crystal with red light, i.e., light corresponding to the additional absorption band, leads to the dissociation and destruction of these particles. These last experiments were direct evidence that the phenomenon known in photography as the Herschel effect is, by its very nature, a process opposite to the coloration process. Another confirmation of this was the work of Yu. N. Gorokhovskii and S. A. Shestakov on the study of the spectral distribution of the Herschel phenomenon in ordinary photographic layers (1938). It was established that the spectral distribution of the scattering action of long-wavelength light for a photographic layer and for single crystals of silver halide is approximately the same, and that one and the same radiation can simultaneously both create (being absorbed by the crystal lattice) and scatter (being absorbed by silver centers) a latent photographic image. Desensitization of a photographic layer, i.e., suppression of the first phenomenon, leads to a stronger manifestation of the second phenomenon, the maximum of whose spectral activity curve is shifted toward shorter wavelengths.

Finally, the late S. V. Cherdyntsev, at T. P. Kravets’s suggestion, carried out a number of studies of dichroism arising in colored crystals of silver halide when they are acted upon by polarized red light (1933—1941). This phenomenon, known in the literature as the Weigert effect, long remained mysterious. S. V. Cherdyntsev for the first time explained and experimentally proved that it is caused by oriented scattering of silver particles, which are transformed from the likeness of spheres into ellipsoids of identical shape and identically oriented.

Clarifying the nature of the centers of the latent photographic image and the character of their scattering brought to the fore the question of the mechanism of formation and destruction of the latent image. To explain scattering, T. P. proposed the hypothesis that when a crystal of silver halide absorbs quanta of red light, an internal photoeffect occurs with the transfer of an electron from the metallic particle into the crystal lattice. In this T. P. proceeded from the fact that the work function of an electron from the surface ...

of a metallic particle located in a dielectric is less than that from the surface of the same particle in a vacuum. This hypothesis is in good agreement with the experimental data obtained by E. A. Kirillov and his school in Odessa. T. P. himself set out the corresponding detailed study under the leadership of N. Kafedrov at Leningrad State University (postgraduate student O. V. Novikova), but the war prevented the completion of his plans.

On the other hand, concerning the mechanism of formation of centers of the latent image, T. P. Kravets, in an extensive report at the First All-Union Conference on Scientific Photography (of which he was an organizer), held in Leningrad in November 1932, said: “I believe that, if not as a finally accepted picture, then as a working hypothesis underlying our modest and small school, one may consider that this is ordinary photochemical coloration, leading first of all to the formation of an atomic phase, and then, if not to a colloidal phase, then to centers composed of a large number of atoms joined together.” This consistent conception—that photochemical processes in alkali-halide and silver-halide crystals proceed in the same way, from metal atoms to more or less large particles—could for a long time not be directly verified experimentally. Whereas in alkali-halide crystals absorption bands of both atoms and colloidal particles are observed, in silver-halide crystals it was long impossible to detect the atomic phase. Therefore a number of foreign scientists (first of all Gurney and Mott) adopted the view that the process of formation of the latent photographic image, unlike the coloration of alkali-halide crystals, does not pass through an “atomic” phase, but at once forms a large “colloidal” particle. However, it turned out that it was precisely T. P.’s point of view that was correct. Indeed, as early as 1935–1937 A. S. Toporets discovered that in alkali-halide crystals with a small admixture of silver there are absorption bands of both atoms and colloidal particles. And in 1950 P. V. Meyklyar and E. K. Pushcheiko showed that, in silver bromide, under suitable measurement conditions, an “atomic” phase can also be detected, through which the process of formation of comparatively large silver particles proceeds.

Closely connected with all these studies are the works initiated by T. P. at the end of the 1930s (subsequently candidate dissertations) by A. T. Aleshkova on the optical properties of thin layers of silver and gold, L. S. Druskina, who discovered, under the action of light on “atomic” centers in alkali-halide salts, the appearance of new absorption bands, and Z. L. Morgenstern on the luminescence of alkali-halide crystals activated by gases.

T. P.’s interests in the field of photography were by no means limited to questions of the theory of the latent photographic image that were logically close to him. It would be long and difficult to enumerate all the questions of scientific photography and photographic technique that he raised and entrusted to his collaborators. We shall point out only a few.

Striving to transfer the bridge from phenomena occurring in models (large single crystals or fused films of silver halide) to phenomena occurring in a real photographic layer, and considering the latter as a complex heterogeneous system in which very diverse physical and physicochemical processes take place, T. P. attracted to his laboratory K. S. Lyashkov (for many years his closest assistant in directing the laboratory, and now professor at the Leningrad Institute of Motion-Picture Engineers) and entrusted to him the development of the theory of photographic emulsions and of their necessary constituent part—photographic gelatin. K. S. Lyashkov carried out a number of very capital investigations of the process of the so-called physical (or primary) ripening of photographic emulsions. The work begun by him on

photographic gelatin were subsequently continued and developed by G. P. Faerman and at the time had major technical significance.

Clearly understanding that the essential link of the photographic process lies not in the photochemical process itself (there exist many photosensitive inorganic salts), but in the possibility of sharp chemical amplification of the result of the action of light—in the selective reduction of the photosensitive substance exposed to light—T. P. drew two conclusions from this. On the one hand, in the early 1930s he undertook preliminary attempts to obtain photosensitive layers based on salts of mercury and lead, close in character to silver salts, and, on the other hand, he suggested to one of his closest collaborators, G. P. Faerman, that he take up the problem of photographic development, which led to the latter’s development of an electrochemical theory of development, which has now acquired general recognition.

Considering it very important that in photography precise, scientifically grounded methods for testing photosensitive materials be used, ones reflecting their practical properties as fully as possible, T. P. initiated the broad deployment of work on the standardization of sensitometric tests and on the creation of new kinds of such tests (in particular spectral sensitometry, worked out in detail by Yu. N. Gorokhovskii). Many years of work carried out in this direction at the State Optical Institute by a large team of collaborators led to the creation of a large set of sensitometric apparatus, now manufactured industrially, and to the development of the corresponding all-Union state standards. For this work T. P. Kravets, together with its main participants—Yu. N. Gorokhovskii, F. L. Burmistrov, S. S. Gilev, G. I. Kireev, and I. A. Chernyi—was awarded the Stalin Prize, Second Class, in 1946.

Throughout all the years of his leadership of the laboratory he had created at the State Optical Institute (from 1938), T. P. Kravets persistently emphasized and carried out in practice the idea of the necessity of a close connection between theory and practice, of the necessity of scientifically interpreting technological processes and directing scientific research to assist and develop the domestic motion-picture and photographic industry and to create new methods of photography, educating the large scientific collective he led in the spirit of selfless service to the motherland.

From 1932 to 1934 T. P. was a professor and head of the chair of optics at the Military Electrotechnical Academy of the Workers’ and Peasants’ Red Army; from 1934 and, with some interruption, to the present time he has been a professor at the A. A. Zhdanov Leningrad State University, where he once headed the chair of general physics, at which he widely developed the scientific work of his collaborators, interrupted by the war, and where for a number of years he taught courses in general physics and the electromagnetic theory of light.

In the difficult days of the blockade of Leningrad by the fascist hordes, T. P. remained in Leningrad, serving as the university’s representative for the Physical Institute. By his unfailing cheerfulness and optimism he encouraged everyone with whom he worked and met at that time, in faith in a speedy victory over the enemy.

From the end of 1942, the principal activity of T. P. Kravets was again concentrated at the State Optical Institute; at the same time, for several years, he was a professor at the Leningrad Air Force Engineering Academy.

In 1943 T. P. was elected a corresponding member of the Academy of Sciences of the USSR in the Division of Physico-Mathematical Sciences. In the Academy he performs the duties of deputy chairman of two actively working commissions: the Commission on the History of the Physico-Mathematical Sciences (since 1945) and the Commission on Scientific Photography and Cinematography (since 1948).

The popular-scientific, historical-scientific, and editorial activity of T. P. Kravets is enormous. Being one of the most highly educated—

one of the most prominent Soviet physicists and an excellent orator, he has spoken many times before broad audiences with a great variety of lectures: among them a series of lectures on P. N. Lebedev, delivered over the course of almost 40 years; the lectures “Physics of the Twentieth Century” (1925), “Children and Physics” (1932), “The Creative Path of P. P. Lazarev” (1943), “Newton and His Studies in Russia” (1943), “N. I. Lobachevsky and His Time” (1944), “Fifty Years since the Discovery of Radioactivity” (1946), and many others. He is deputy editor-in-chief of the complete collected works of M. V. Lomonosov, now being published by the Academy of Sciences of the USSR; he edited E. H. Lenz’s Selected Works, translated Faraday’s Experimental Researches, Abraham-Becker’s Theory of Electricity, and Drude’s Optics, as well as many other books. In addition, he has written numerous review and popular articles on physics in the journals Priroda, Uspekhi fizicheskikh nauk, Fizika v shkole, Vestnik Akademii nauk SSSR, and others. These articles were written with deep knowledge of the subject and great mastery. Among T. P. Kravets’s literary works, however, one may also name a few unsuccessful ones that received a negative appraisal from the scientific community. These isolated failures, however, cannot overshadow his positive activity as a whole.

T. P. Kravets has devoted and continues to devote much effort and attention to scientific and public activity. He was at one time one of the organizers of the Moscow Physical Society named after P. N. Lebedev. In 1927–1928 he served as chairman of the physics section and president of the Russian Physico-Chemical Society (now the D. I. Mendeleev Chemical Society), and in 1935–1937 as chairman of the Leningrad Scientific Engineering-Technical Society of the Cinema and Photo Industry. He was the chief organizer of several scientific conferences: the First All-Union Conference on Scientific Photography (Leningrad, November 1932), the Conference on the Sensitivity of Photographic Materials, the Conference on the Photochemistry of Crystals (Leningrad, April 1941), and several others.

T. P. Kravets knows how to gather capable young people into scientific collectives, to guide their activity gently and carefully, and to educate enthusiasts of science who serve the cause of building communism in our country. The most recent convincing example of this gift is the new scientific collective he recently created—when T. P. had already entered the eighth decade of his life.

The Soviet government has highly valued T. P. Kravets’s scientific and pedagogical work, awarding him the Stalin Prize, decorating him with three orders—the Order of the Red Banner of Labor, the Order of the Red Star, and the Badge of Honor—and with three medals.

Despite his 75 years, Torichan Pavlovich Kravets retains undiminished vigor, mobility, and energy. He works a great deal, is full of creative plans, and, one may hope, will actively participate for many more years in the development of Soviet physics.

Yu. N. Gorokhovskii,
P. V. Meiklyar,
M. V. Savostyanova,
A. S. Toporets