DMITRY SERGEEVICH
ROZHDESTVENSKY

FROM THE HISTORY OF PHYSICS

DMITRII SERGEEVICH ROZHDESTVENSKII

(On the 75th Anniversary of His Birth)

S. E. Frish

Dmitrii Sergeevich Rozhdestvenskii was born in 1876 in Petersburg, in the family of a gymnasium teacher. He received his secondary education at the Petersburg Sixth Gymnasium, from which he graduated in 1894 with a silver medal. In the same year he entered Petersburg University, where at first he studied the natural sciences and only from his second year devoted himself to the study of the physical and mathematical sciences. He graduated from the university in 1900 in the mathematical division with a first-degree diploma and was retained at the university to prepare for the title of professor.

Both in his student years and in the years after graduating from the university, Dmitrii Sergeevich Rozhdestvenskii belonged to the progressive part of the Russian intelligentsia, which spoke out against the criminal policy of the tsarist authorities. In 1899 he joined the student strike, refusing to take the state examinations that were being held that year under police supervision. In 1905 he took part in strikes of university teachers, including the strike held as a sign of protest against the bloody events of January 9, 1905. He took part in a number of meetings that demanded the introduction of a constitutional system and served as secretary of the university group of “junior lecturers.”

D. S. Rozhdestvenskii began his scientific work while still a student, under the guidance of Prof. N. G. Egorov. After graduating from the university he entered the Military Medical Academy as a laboratory assistant, where, however, he worked for only two semesters, until the spring of 1901. Then, to continue his education, he went—as was customary at that time—abroad to Germany, to Leipzig. In 1903, having returned to Petersburg, he took the position of laboratory assistant at the University, corresponding to the modern position of assistant. From that time on he was connected with Petersburg—Leningrad University.

DMITRY SERGEEVICH ROZHDESTVENSKY

with the exception of a short interruption, until his death in 1940.

At the beginning of the present century there were as yet no permanent scientific schools of physics at Petersburg University. The professors of physics at the University at that time were O. D. Khvolson and I. I. Borgman. Both of them were widely known as persons who had done much for the development of the teaching of physics in Russia, but they devoted comparatively little attention to research work and did not direct scientific work. Dmitry Sergeevich had, to a considerable extent, to make his own way in the field of scientific research. He set about this task with enormous energy and persistence, choosing a theme that was very difficult experimentally—the investigation of the anomalous dispersion of light in sodium vapor.

As early as the second half of the nineteenth century, as a result of the theoretical work of a number of physicists, it became known that near a spectral absorption line the refractive index of light should increase to enormous values on one side of the line and take on very small values on the other side of it. This discontinuity in the course of the refractive index, which received the name “anomalous dispersion,” is a consequence of the resonant oscillation of the quasi-elastic system of the atom by the incident light wave. The quantitative study of anomalous dispersion was of great interest, since it not only provides a way to verify the theory of dispersion, but also makes it possible to determine the number of those electrons inside the atom which are responsible for the interaction of light with matter. As a result of long and persistent work, D. S. Rozhdestvensky achieved great successes, and on December 8, 1909, he was already able to give his first report on the results obtained at the Physics Section of the Russian Physico-Chemical Society; this report was published in 1910.

By combining two optical instruments—a diffraction grating and an interferometer—D. S. Rozhdestvensky obtained interference fringes that reproduced with extraordinary clarity the steep rise and sharp drop of the refractive index near an absorption line. The photograph he obtained of the anomalous course of dispersion in sodium vapor (Fig. 1) acquired unprecedented popularity in science: there is not a single detailed textbook on optics, nor a single article on the anomalous dispersion of light, in which it has not been reproduced.

The essence of the method used by D. S. Rozhdestvensky is as follows. Interference fringes obtained in white light are projected onto the vertical slit of a spectrograph. The position of the maximum corresponding to zero path difference does not depend on the frequency of the light. Hence, to this maximum in the continuous spectrum produced by the spectrograph there corresponds

horizontal bright band extending along the entire spectrum. The maxima corresponding to path differences of one, two, three, etc., wavelengths are located, for different frequencies of the incident light, at different heights of the slit. They correspond, in the continuous spectrum, to bright bands situated above and below the zero band and fanning out from the blue part of the spectrum toward the red.

Fig. 1.

Between the bright bands there are dark ones, corresponding to minima in the interference pattern projected onto the slit. As a result, the continuous spectrum is found to be ruled along its entire length by alternating bright and dark bands. If the substance under investigation is introduced into the interferometer, in the path of one of the rays, in the form of a plane-parallel homogeneous column of vapor, then the bands will be displaced by an amount proportional to the path difference introduced. The path difference, for a given thickness of the vapor column, is proportional to \(n-1\), where \(n\) is the refractive index of the vapor.

Fig. 2.

Thus, the displacement of the interference bands in the spectrograph directly reproduces the course of the dispersion over the entire observed region. If any absorption line falls within this region, then the interference bands bend around it, as shown in Fig. 2, a.

However, such a method of studying anomalous dispersion is still of little use for obtaining quantitative data; it requires measurement of a large number of points on the steeply rising or falling part of the interference band, where the measurements are laborious—

were and insufficiently accurate. Near the absorption line itself the fringes run so steeply and are located so close to one another that measurements become altogether impossible. In order to adapt the method to quantitative measurements, D. S. Rozhdestvenskii subjected it to a substantial and extremely ingenious modification. Into the second arm of the interferometer he introduced a plane-parallel glass plate of definite thickness. In the absence of vapor in the first arm of the interferometer, the introduction of such a plate would produce a displacement of the fringes representing the course of dispersion in it. In a transparent glass plate, far from absorption bands, the course of dispersion reduces to a monotonic decrease of the refractive index with increasing wavelength (“normal” dispersion), which leads only to an inclination of the fringes. With a large thickness of the plate the inclination is considerable. Finally, since the plate is introduced into the other arm of the interferometer, not into the one containing the vapor, the inclination of the fringes is in the opposite direction. The appearance of these fringes is shown in Fig. 2, b.

With the simultaneous action of vapor with anomalous dispersion and of a glass plate with a normal course of dispersion, the displacement of the fringes is determined by the algebraic sum of the displacements caused by the vapor and by the glass plate separately. Far from the absorption band of the vapor, where its refractive index depends only slightly on wavelength, the inclination caused by the glass plate is more strongly expressed. Near the absorption band, where the bending of the interference fringes caused by the dispersion of the vapor is very great, the bending produced by the vapor is stronger. As a result the fringes at a certain place bend over and give peculiar maxima and minima, schematically represented in Fig. 2, c.

Rozhdestvenskii called these bends “hooks.” For a given thickness of the glass plate, the position of the hooks is determined by the magnitude of the dispersion near the absorption line of the vapor under consideration. The stronger the dispersion of the vapor near the line, the farther from the line the hooks are formed. Thus, from the position of the hooks one can directly measure the magnitude of the vapor dispersion near the absorption line. By selecting a suitable thickness of glass, the hooks can always be moved away from the line and such a position of them obtained in which measurements will be easily feasible and accurate. This is the enormous advantage of Rozhdestvenskii’s method: the measurements are performed simply and accurately and can be carried out at such small distances from the absorption lines where all other methods cease to be applicable.

Figure 3 presents, in a somewhat simplified form, the scheme of Rozhdestvenskii’s apparatus: \(S\) is a voltaic arc, serving as a powerful source of a continuous spectrum; \(F\) is a water light filter for absorbing infrared rays; \(L_1\) is a lens giving a parallel beam of rays. The rays fall on an interferometer consisting

of four mirrors \(A_1,\ A_2,\ A_3,\ A_4\), two of which, \(A_2\) and \(A_3\), are solid, while the other two are semitransparent. The interferometer is a modification of the Jamin interferometer and makes it possible to separate widely from one another the light beams \(A_1A_3\) and \(A_2A_4\). Identical tubes \(B_1\) and \(B_2\) are introduced into both beams; to their ends are attached identical plane-parallel glass plates. Thus the tubes \(B_1\) and \(B_2\) by themselves introduce no additional path difference into the light beams. After passing through the tubes and being reflected from the mirrors \(A_3\) and \(A_4\), the light beams are brought together and imaged by the lens \(L_2\) onto the slit of the spectrograph \(C\). As the spectrograph, D. S. Rozhdestvenskii used a plane diffraction grating in an autocollimation arrangement. The dispersion of the grating in the second order, in which the photographs were taken, reached \(5\,\text{\AA}/\mathrm{mm}\).

Fig. 3.

For observing anomalous dispersion, the substance under investigation was introduced into one of the tubes, for example tube \(B_2\). The tube was heated by an electric furnace to a temperature sufficient for vapors to form in it at the required pressure. In order to form “steps” in the beam \(A_1A_3\), a plane-parallel glass plate was introduced.

Using his method, D. S. Rozhdestvenskii first of all measured the course of anomalous dispersion near the sodium \(D\)-lines. The results of the measurements proved to be in excellent agreement with Sellmeier’s theoretical formula. Small deviations were observed only at very small distances from the lines; moreover, it could be shown that they were of an “instrumental” character: when the spectrum was photographed in different orders of the diffraction grating, they turned out to be different.

Thus, already D. S. Rozhdestvenskii’s first work showed the very exact applicability of Sellmeier’s formula and, as it seemed, confirmed the hypothesis of the quasielastic character of intra-atomic forces. Subsequently, at a session of the Academy of Sciences of the USSR in March 1936, Rozhdestvenskii said: “Hardly anyone has examined these curves more thoroughly than I have, both along and across, and my answer is: yes, these curves are hyperbolas and, consequently, the electron has a quasielastic bond. Fortunately, my inherent skepticism forced me to add: however, the linearity of the equations governing the oscillations of the electron is sufficient to produce a hyperbola, without entering into consideration of the question of the essence of the bond.” The last phra-

...is very remarkable: it characterizes Rozhdestvenskii’s scientific caution and perspicacity. He allowed the possibility of another interpretation of the laws of anomalous dispersion, different from the interpretation based on the visual model conception of quasi-elastic oscillations of the electron inside the atom. But even if the theoretical conclusions were not unambiguous, in any case one thing became clear after the work of D. S. Rozhdestvenskii: any future theory must reckon with the exact validity of Sellmeier’s formula.

The results obtained by D. S. Rozhdestvenskii served as the subject of his master’s dissertation, which he defended with exceptional success in 1912. This dissertation, entitled “A Study of Anomalous Dispersion in Sodium Vapors,” is a classical investigation in the field of anomalous dispersion. It received wide recognition immediately after its publication and was awarded the Mendeleev Medal of the Academy of Sciences. In the same year, 1912, D. S. Rozhdestvenskii received the title of Privatdozent.

The further scientific work of D. S. Rozhdestvenskii was devoted to the development of the “hook” method and to its application to the study of anomalous dispersion not only in sodium vapor, but also in the vapors of other alkali metals—potassium, rubidium, and cesium. These investigations led to a new and no less important result. D. S. Rozhdestvenskii measured the ratio of the number of oscillators for the yellow lines of sodium and obtained for it the value 2 with an accuracy of up to 2.5%. He showed that this ratio depends neither on the temperature nor on the vapor pressure and, consequently, is determined by the properties of the sodium atoms themselves. He obtained the same ratio, close to 2, for the components of the principal doublets of potassium, rubidium, and cesium as well. The results of these measurements thus proved to be very significant: they showed that the ratios of the intensities of spectral doublets are integral numbers. This fact, at that time completely incomprehensible, indicated the peculiar character of the laws to which intra-atomic processes are subject; it later found confirmation in quantum mechanics. The results of these new investigations, summarized in the work “Simple Ratios in the Spectra of Alkali Metals,” served as the subject of the doctoral dissertation defended by D. S. Rozhdestvenskii in 1915.

Immediately after the defense of his doctoral dissertation, D. S. Rozhdestvenskii was elected head of the Physical Institute, and in 1916—professor of Petrograd University. He brought a new, fresh current into the atmosphere of the Physical Institute, which until then had been stagnant. As early as the period 1910–1912, together with A. F. Ioffe and P. S. Ehrenfest, he organized a scientific seminar, which attracted the most talented and active young people and became the center of scientific physical thought in Petersburg. Having become professor, Rozhdestvenskii gathered around himself students...

…ists and begins to direct original creative scientific work. At the same time, breaking the “academic” traditions by virtue of which scholars did not go beyond the bounds of “pure” science, he, together with a group of other progressive scientists and engineers (I. V. Grebenshchikov, N. N. Kachalov, and others), began to engage in the production of optical glass, which the Russian army so badly needed at that time. Thus, the activity of D. S. Rozhdestvensky had already, in the pre-October period, attained broad dimensions and borne a deeply progressive character, but it could acquire its full scope only after the Great October Revolution.

D. S. Rozhdestvensky greeted the October Revolution with enthusiasm. He belonged to the number of those progressive Russian scientists who always strove to place science at the service of the people and who immediately understood what enormous and new possibilities the October Revolution opened in this respect. In the very first year of Soviet power D. S. Rozhdestvensky submitted to the People’s Commissariat of Education an extensive and thoroughly worked-out plan for the organization of an Optical Institute. The Optical Institute was conceived by him as an institution of a new type, in which science and practice would be closely combined. Later he himself wrote: “We were all inspired by one common task—the creation of the Optical Institute—an institution of a new type, in which scientific and technical tasks would be inseparably linked.”

D. S. Rozhdestvensky’s project met with full recognition in the People’s Commissariat of Education, and on December 15, 1918, the State Optical Institute was organized. D. S. Rozhdestvensky was director and active head of the Optical Institute until 1932, and did much to promote the development both of scientific optics in the USSR and of all branches of the optical industry. He took direct part in putting into operation the first Soviet optical-glass plant and devoted much labor and energy to the development of all the other branches of our now so powerful optical industry.

The activity of D. S. Rozhdestvensky was also closely connected with the Academy of Sciences of the USSR. In 1925 he was elected a corresponding member, and in 1929 an academician. On his initiative, the Commission for the Study of Rare Earths was first created at the Academy, and then the Spectroscopic Laboratory, which he headed until the end of his life. A brilliant organizer, a man able to infect those around him with his scientific enthusiasm, he gathered around himself a large number of young physicists, succeeded in training a whole constellation of talented pupils, and created a major school of Leningrad opticians.

Questions of the training of scientific personnel were always at the center of D. S. Rozhdestvensky’s attention. During the period of the creation and development

Optical Institute, with exceptional energy he continued his work also at Leningrad University. In 1919 he organized an independent physics division at the Faculty of Physics and Mathematics and carried out a fundamental reform of the teaching of physics. On his initiative, a special course in mathematics was developed, taking into account the interests of physicists; all the newest branches of physics were also included in the curriculum, and alongside lectures seminars were introduced, with their active form of student instruction. The reform carried out by D. S. Rozhdestvenskii significantly raised the level of students’ training and formed the basis of all the subsequent transformations that led to the modern physics faculties of the universities. D. S. Rozhdestvenskii himself taught a number of lecture courses—on optics, electricity, the electromagnetic theory of light—and conducted seminars. Dmitrii Sergeevich Rozhdestvenskii prepared all his lectures and reports with the utmost care; not infrequently, a four-hour lecture required almost an entire week of preparation. He thought through and wrote down the plan of each lecture, selected the literature, and devised new demonstration experiments. His lectures, though not distinguished by outward effect, were exceptionally rich in content and brought enormous benefit to the attentive listener. In the lectures, and especially in the reports, Dmitrii Sergeevich’s great and distinctive talent, the freshness and originality of his thought, and the abundance of large and new ideas were evident. D. S. Rozhdestvenskii wrote two large chapters in volume V of O. D. Khvolson’s Course of Physics, namely, “The Electromagnetic Theory of Light” and “Magneto-Optics and Electro-Optics.” For a long time both of these articles were the most complete and thorough expositions of electromagnetic optics available in Russian.

Investigations of anomalous dispersion constituted the most significant and extensive part of D. S. Rozhdestvenskii’s varied scientific activity. He returned to this theme many times over the course of his life, and to it he devoted his last major scientific work, carried out once again within the walls of Leningrad University, to which he returned in 1939 after a short interruption. This work, performed by D. S. Rozhdestvenskii together with his pupil N. P. Penkin, had as its goal the extension of the “hook” method to the investigation of vapors of refractory elements. The experimental apparatus used earlier had made it possible to study only comparatively low-melting elements. The substance was introduced into an evacuated quartz tube and heated by an electric furnace that allowed temperatures not exceeding \(1200^\circ\) C to be reached. In order to pass to higher temperatures, D. S. Rozhdestvenskii created a new apparatus with a vacuum furnace whose heating element was in the form of a graphite tube.

The work was connected with overcoming exceptional experimental difficulties. Into one arm of the interferometer, a most delicate optical instrument, it was necessary to introduce a bulky furnace, easily disturbing the strict temperature regime required for obtaining a stable interference pattern.

After a year of intensive work a new interferometer was built with posts measuring 30 cm, i.e., five times larger than in the previous interferometers. Such a large interferometer is extremely sensitive to the slightest vibration, and in order to obtain good interference fringes it had to be mounted on a double system of stone slabs, with a total weight of half a ton, on rubber gaskets. Into one of the arms of the interferometer a furnace was introduced, about 40 cm in diameter and 75 cm long. The furnace was supplied by a 30-kilowatt step-down transformer and, at a current strength of 1800 A, gave a temperature of 3000 °C. Such a high temperature led to new experimental difficulties. For example, at the junctions of the graphite tube with the lead-in electrodes, deformations arose during heating, leading to the destruction of the electrodes or to damage to the contact.

Difficulties also arose in creating an optical system that would provide sufficient luminosity in the presence of a long and relatively narrow tube. Nevertheless, all of them were overcome, and the new apparatus made it possible, at high temperatures (up to 3000 °C), to take “hook” photographs quite suitable for measurements.

As a result of the work carried out, the experimental possibilities proved to be greatly expanded, and D. S. Rozhdestvensky investigated anomalous dispersion in chromium vapors and found that the ratio of the intensities of the components of the narrow chromium multiplet lying in the violet part of the spectrum is integral, being equal to 3:5:7. He also outlined an extensive program of further investigations on anomalous dispersion, which he was not destined to carry out, but which is now being successfully carried out by his pupils and followers. The works of D. S. Rozhdestvensky on anomalous dispersion, even during his lifetime, were closely intertwined with the works of his pupils (A. N. Filippov, V. K. Prokofiev, G. S. Kvater, N. P. Penkin, and others). With the aid of D. S. Rozhdestvensky’s “hook” method, a large number of problems were posed and solved that are of primary importance for elucidating the laws of the interaction of light with matter and for the theory of the structure of atoms. The study of anomalous dispersion became a monopoly of Soviet physicists. The few works on anomalous dispersion carried out abroad were performed by the “hook” method with the aid of apparatus copied from Rozhdestvensky’s apparatus. Almost 40 years have passed since Rozhdestvensky’s first work, however

open method has not lost its freshness—a very rare case at the present very rapid pace of development of physics.

Rozhdestvenskii’s method gave a simple and exact means of comparing the intensities of absorption lines. The intensity of absorption lines, as will be shown below (see formula (2)), is determined by the number of absorbing oscillators \(N_i\). The number of oscillators \(N_i\), in turn, is directly connected with the constant \(a_i\) of Sellmeier’s formula, according to which, near an absorption line, the refractive index may be represented in the form:

\[ n - 1 = \frac{a_i}{\nu_i^2 - \nu^2}. \tag{1} \]

The theory of the “hooks” method shows that the value of the constant \(a_i\) is obtained by a simple measurement of the wavelengths at which the “hooks” are located. Such simplicity of measurement makes the “hooks” method the most convenient, rapid, and accurate for determining the intensities of spectral absorption lines. Other methods—the method of photographic photometry, the method of resonance absorption, and the method of rotation of the plane of polarization—are more complicated and less reliable.

Fig. 4.

The integral character of the ratios of the intensities of spectral lines, discovered by Rozhdestvenskii, first found its reflection in Bohr’s theory of the atom. According to the classical theory, an ensemble of oscillators with natural frequency of oscillation \(\nu_i\) absorbs the energy

\[ \Delta E = N_i \frac{\pi e^2}{m}\,\rho(\nu_i), \tag{2} \]

where \(N_i\) is the number of absorbing centers, and \(\rho(\nu_i)\) is the energy density in the incident light flux of frequency \(\nu_i\).

Bohr’s theory considers the absorption and emission of light by an atom as the result of the transition of the atomic system from one discrete energy state to another. If \(E_k\) is a level poorer in energy, and \(E_l\) is a level richer in energy, then the act of absorption of light corresponds to the transition from level \(E_k\) to level \(E_l\), shown by the arrow in Fig. 4. The frequency of absorption of light \(\nu_{kl}\) is determined, as is known, by Bohr’s frequency rule

\[ \nu_{kl} = \frac{E_l}{h} - \frac{E_k}{h}, \tag{3} \]

where \(h\) is Planck’s constant. If the absorbing vapor contains, per unit volume, \(N_k\) atoms in the state with energy

\(E_k\), then, according to Bohr’s theory, the absorbed energy is equal to

\[ \Delta E = N_k \frac{g_l}{g_k}\,\frac{c^3}{8\pi \nu_{kl}^2}\, A_{kl}\rho(\nu_{kl}), \tag{4} \]

where \(c\) is the speed of light, and \(A_{kl}\), \(g_k\) and \(g_l\) are constants determined by the properties of the atom.

Introducing the quantity

\[ f_{kl}=\frac{g_l}{g_k}\,\frac{mc^3}{8\pi^2 e^2 \nu_{kl}^2}\, A_{kl}, \tag{5} \]

we rewrite formula (4) in the form

\[ \Delta E = N_k f_{kl}\frac{\pi e^2}{m}\,\rho(\nu_{kl}). \tag{4a} \]

It differs from the classical formula (2) only in that, instead of the number of absorbing vibrators \(N_i\), there stands the product of the number of atoms \(N_k\) and a certain constant \(f_{kl}\), dependent on the properties of the atoms. This means that, from the point of view of Bohr’s theory, all atoms can absorb, but transitions between different energy levels, and consequently the corresponding acts of absorption, occur with unequal frequency. The quantity \(f_{kl}\) determines the probability of the given transition. These “transition probabilities” \(f_{kl}\) are determined directly by Rozhdestvenskii’s “hook” method. Indeed, the equality

\[ N_i=N_k f_{kl} \]

gives

\[ f_{kl}=\frac{N_i}{N_k}, \]

where \(N_i\) is found, as was said, from the measurement of the position of the “hooks.” To find \(f_{kl}\), it remains only to determine the number of absorbing atoms \(N_k\) per unit volume. This can be done if the experiment is carried out with a homogeneous column of vapor and its elasticity is known. In many cases, however, it is sufficient to know the relative values of the transition probabilities \(f_{kl}\); then there is no need to determine \(N_k\), and all measurements reduce to finding the position of the “hooks.”

Let two spectral absorption lines with frequencies \(\nu_{kn}\) and \(\nu_{kl}\) arise in the transition between one lower and two upper levels. The ratio of their intensities, by formula (4a), is equal to

\[ \frac{I_l}{I_n}=\frac{f_{kl}}{f_{kn}}, \tag{6} \]

i.e., it is determined only by the ratio of the transition probabilities \(\dfrac{f_{kl}}{f_{kn}}\). This confirms that Rozhdestvenskii’s method makes it possible in a particularly simple way to determine ratios of line intensities.

For the case of components of one and the same spectral doublet \(A_{kl}=A_{kn}\); moreover, if the doublet is narrow, then the frequencies \(\nu_{kl}\) and \(\nu_{kn}\) may approximately be set equal to one another; then, from (6) and (5),

\[ \frac{I_l}{I_n}=\frac{g_l}{g_n}. \]

In Bohr’s theory the quantities \(g_l\) and \(g_n\) have the meaning of statistical weights of the energy levels \(E_l\) and \(E_n\), and are equal to \(2J+1\), where \(J\) is the “inner” quantum number of the given level. For levels corresponding to the doublets of the alkali metals, \(J\) is equal to \(\dfrac{3}{2}\) and \(\dfrac{1}{2}\), whence

\[ \frac{I_l}{I_n}=\frac{g_l}{g_n}=\frac{2J_l+1}{2J_n+1}=\frac{4}{2}=2. \]

Thus Bohr’s theory explains the integer character, discovered by Rozhdestvenskii, of the ratio of the intensities of the components of the spectral doublets of the alkali metals.

The mechanism of emission of spectral lines is determined by two quantities characteristic of the atom: the energies of the levels \(E\) and the transition probabilities \(f\).

As is clear from what has been said, Rozhdestvenskii’s method proved applicable to the measurement of this new and most important characteristic of intra-atomic processes—the “transition probability” \(f\). Hence the significance of Rozhdestvenskii’s works for the entire subsequent development of the doctrine of atoms becomes understandable.

At the same time as the successes of Bohr’s theory, its major shortcomings were also revealed. In explaining the integer ratios of line intensities, it—with its conceptions of unchanging stationary states—could not explain the very fact of anomalous dispersion. Moreover, Rozhdestvenskii’s further measurements showed that the intensity ratios are not always integers; for the second doublet of rubidium the ratio obtained is 2.6, and for the third doublet of cesium 7.4. The deviation from whole numbers here is undoubtedly greater than the errors of observation. Thus the rich experimental material provided by Rozhdestvenskii’s works was invariably interwoven with all the details of the development of atomic theories.

A way out of the difficulty was provided by quantum mechanics, which again led to Sellmeier’s formula and showed that deviations from the integer character of the intensity ratios of the compo-

...spectral multiplets. At the same time, quantum mechanics advanced the requirement of systematically determining, for atoms, the transition probabilities \(f\) as one of the principal characteristics of their properties. Hence the further path of development of experimental work on anomalous dispersion became clear: using Rozhdestvenskii’s method, to determine transition probabilities for as large a number of different atoms as possible, and, for each atom, for as large a number of lines as possible. In those cases where the transition probabilities are already known, D. S. Rozhdestvenskii’s “hook” method makes it possible to determine the number of absorbing atoms.

The solution of this latter problem is especially important for investigating the mechanism of a gas discharge and many questions connected with it. Work in this direction is now being conducted by many Soviet physicists, who make extensive use of Rozhdestvenskii’s scientific methods.

Work on anomalous dispersion by no means exhausts the scientific activity of D. S. Rozhdestvenskii. A large number of other questions of theoretical and applied optics attracted his attention. Moreover, in any of the directions he touched upon he was able to be original, to say his own new word. Dmitrii Sergeevich never considered the problems of anomalous dispersion narrowly, in isolation from other tasks, and this predetermined his interest in the theory of atomic structure and in spectral regularities. As early as January 1916, delivering the chairman’s address at a meeting of the physics section of the Russian Physico-Chemical Society, he devoted his report to series regularities and to work, new at that time, on the spectroscopy of X-rays.

In 1919, at the celebration of the first anniversary of the State Optical Institute, D. S. Rozhdestvenskii delivered a major report, “Spectral Analysis and the Structure of Atoms.” In this report he set forth the basic ideas on the structure of atoms and on the structure of line spectra, which he developed somewhat later in a whole series of special articles. For a proper assessment of the significance of these ideas, one must recall what was known at that time in Soviet Russia—cut off by the blockade of the imperialist powers from world science—about atoms and the nature of spectra. Knowledge was limited to the hydrogen atom. In the field of the systematics of the spectra of the remaining atoms, only the old empirical formulas of Rydberg were known. With remarkable scientific insight, D. S. Rozhdestvenskii formulated the fundamental propositions that made it possible to extend the theory then known to the spectra of any atoms. These propositions are as follows:

1) spectral series determine the orbits of the valence electrons in the atom;

2) the distant orbits in the atoms of the alkali metals are similar to the distant orbits of hydrogen;

3) the number of orbits of the valence electrons in the atoms of the alkali metals is exactly equal to the number of orbits in the hydrogen atom;

4) doublets and triplets are produced by the internal magnetic field of the atom;

5) the cause of the anomalous Zeeman effect likewise lies in the existence of an internal magnetic field in atoms;

6) the spectrum of the ion of a given element should resemble the spectrum of the neutral atom preceding that element in Mendeleev’s periodic system.

Thus D. S. Rozhdestvenskii expressed all those fundamental ideas which made it possible to extend Bohr’s theory, which initially applied only to hydrogen and ionized helium, to all other atoms. These ideas, partly simultaneously and partly somewhat later, were put forward in the West by a number of the foremost physicists and formed the basis of the modern theory of atomic spectra. Many of the ideas expressed by D. S. Rozhdestvenskii are fundamental discoveries underlying the whole of spectral systematics, and fairness requires that they be associated with the name of Dmitrii Sergeevich Rozhdestvenskii.

D. S. Rozhdestvenskii quite definitely expressed the idea of the magnetic nature of spectral doublets and triplets, to a considerable degree cast it in the form of a genuine mathematical theory, and connected it with the theory of the Zeeman phenomenon. At that time this idea was disputed by many prominent physicists, but for D. S. Rozhdestvenskii, who was able to generalize varied experimental material, it was clear that only forces of a magnetic character could be the cause of the existence of doublets and triplets. In defending this theory, he at the same time clearly saw the insuperable difficulties which it then encountered. It is now known that a complete theory of spectral multiplets can be constructed only by taking into account the electron’s own mechanical and magnetic moments (spin), then still unknown.

In addition, D. S. Rozhdestvenskii was the first to give an analysis of the spectrum of the magnesium ion; this work served as a model for the numerous analyses of ion spectra carried out both by Rozhdestvenskii’s pupils and by a number of physicists abroad. Finally, of special interest is his work on the analysis of the neon spectrum. Here, more fully than anyone before him, he advances the hypothesis of the possibility of the emission of a single monochromatic line in the simultaneous transition of two electrons. This hypothesis not only has great fundamental significance from the standpoint of the further development of the theory of atomic structure, but is also a key to

unraveling the structure of the complex spectra of atoms and ions with many valence electrons.

D. S. Rozhdestvensky’s works on spectroscopy likewise did not remain without successors. Thanks to the discoveries he made, Russian physics not only did not fall behind in the field of spectroscopy during the imperialist blockade of 1918–1921, as compared with foreign physics, but in many respects outstripped it.

D. S. Rozhdestvensky never confined himself within the framework of abstract theory. He always clearly had in mind the practical application of new physical discoveries and theories. The matter was not one of narrow, particular practical applications, but of broad prospects, of innovative ideas in the field of technology. In the field of applied optics he did not limit himself to organizational work, but carried out brilliant investigations on the theory of the microscope. As is known, the theory of the microscope was first given by Abbe, who took account of the role of diffraction in the formation of the image in the microscope. However, Abbe’s theory referred to the case in which the object is illuminated by coherent rays, which in practice usually does not occur. L. I. Mandelstam analyzed another limiting case—the case of a self-luminous object, when the rays are incoherent. D. S. Rozhdestvensky analyzed the most general case and, what is most important, the case corresponding to real conditions of illumination. He introduced the concept of the “degree” of coherence of light. Further he showed that, when considering more or less transparent objects (and biological objects are always such), interference phenomena play an essential role. He himself made observations of various biological objects and showed that the microscope can be used much more effectively than is usually the case. His works on the theory of the microscope are classics and await further development and application.

In the development of D. S. Rozhdestvensky’s work on microscopy, no small role was played by his interests in the field of biology. Having begun, while still in his first year at the university, to study biology, he retained an interest in it throughout his life. He was a deeply educated botanist and worked a great deal with the microscope, observing biological objects. The successes he achieved in the theory of microscopy were determined to a considerable extent by the fact that in his person the physicist-optician was united with the biologist-microscopist. Dmitrii Sergeevich, in the article “What the Microscope Has Mastered and What It Must Conquer,” wrote of Leeuwenhoek: “He himself melted the glass, himself ground it, himself polished it ... and, most important of all, himself sought and found objects for observation. Let us especially remember this, because in microscopy what is new and perfected is created by the one who knows for what he creates and what he seeks.” These words are characteristic of Dmitrii Sergeevich himself as well. In any field that he began to engage in—

to learn, he went into it “to the end,” mastered all the methods and the entire technique of the work. This applied both to the technique of physical experiment and to production. His participation in the development of the optical industry by no means proceeded solely along the lines of “scientific consultations” and measures of an organizational character. He entered deeply into all the details of optical production and himself learned from the craftsman Aleksandrov how to grind precise optical surfaces.

Among scientists—sometimes even prominent ones—there are those who follow a well-trodden, familiar path, generalizing and developing already known scientific propositions and methods. There are others—true innovators, following their own path, able at every step of their own to be original. Dmitrii Sergeevich Rozhdestvenskii was precisely such a scientist. A profound erudite, able to make use of everything advanced that existed in the science of his time, he at the same time always approached generally recognized authorities critically and was able, in everything, to say his own new word. He always preached the close unity of science with practice and was not afraid to make sharp critical remarks when it seemed to him that the problems of the interrelation of science and technology were being posed and solved incorrectly. Such was his deeply principled address at the March session of the Academy of Sciences in 1935, when the activity of the Optical and Physico-Technical Institutes was discussed. The ultimate goal—the welfare of the people—was always clear to him. Even in those periods when D. S. Rozhdestvenskii was occupied with what seemed to be the most abstract theories of modern physics, he dreamed of applying spectral analysis in the search for new useful minerals, of the future “spectroscopist-engineer,” and said in an engaging form that there must arise “concrete chemistry, understanding the process in the finest details and controlling the atom as a chauffeur controls an automobile.” Principled and consistent in his actions, Dmitrii Sergeevich Rozhdestvenskii placed all his scientific activity at the service of the people and of his Soviet Motherland.

LIST OF THE PRINCIPAL WORKS OF D. S. ROZHDESTVENSKII

1. Anomalous dispersion in sodium vapors. St. Petersburg, 1912.
2. Simple relations in the spectra of alkali metals. Petrograd, 1915.
3. Spectral analysis and the structure of atoms, Transactions of the State Optical Institute 1, issue 6, 1920.
4. The significance of spectral series, Transactions of the State Optical Institute 2, issue 7 (1921).
5. Terms of high order and the similarity between the spectra of one-electron and complex atoms, Transactions of the State Optical Institute 2, issue 8 (1921).
6. Two independent series in the neon spectrum, Transactions of the State Optical Institute 3, issue 18 (1923).

1. Note on optical glass, Proceedings of the State Optical Institute 8, issue 84 (1932).
2. Mendeleev’s periodic law on the basis of spectral analysis, Proceedings of the Mendeleev Congress, 1936.
3. Illumination of the microscope, Reports of the Academy of Sciences 25, No. 2 (1939).
4. Coherent and incoherent rays in the formation of an image in the microscope, Journal of Experimental and Theoretical Physics 10, 360, 805 (1940).
5. On the question of the image of transparent objects in the microscope, Proceedings of the State Optical Institute 14 (1941).
6. Determination of oscillator strengths in atomic spectra (with N. P. Pechkina), Izvestiya of the Academy of Sciences of the USSR, Physical Series 5, 97 (1941).