...the difficult position in which this hypothesis found itself, caught between the Scylla and Charybdis of the experiments of Fizeau and Michelson. The point of view taken by Einstein on this question from 1905 onward in his theory of relativity was the denial of the ether. The lecture under review is a turning point in this respect. Einstein considers it possible to identify the ether with the physical space of the general theory of relativity: “The ether of the general theory of relativity is a medium which in itself is devoid of all mechanical and kinematic properties, but which at the same time determines mechanical (and electromagnetic) events.” The difficulties arising from the dual nature of the new ether (mechanical and electromagnetic ether) are then indicated. With this indication the brief lecture ends. In such a view there is nothing essentially new; the possibility of identifying the concepts of “ether” and Einsteinian physical space of variable curvature was pointed out, for example, by P. Lenard in 1918. Most remarkable is the “lifting of the ban” from the hypothesis of the world ether by the very author of this “ban,” which hypnotized science for fifteen years and undoubtedly impeded the natural development of a hypothesis valuable for physics.¹

The brochure is intended for readers already familiar with the foundations of the theory of relativity.

— S. Vavilov.

P. Lenard Über Relativitätsprinzip, Äther, Gravitation, 1920 Hirzel Verlag.

The brochure by P. Lenard consists of a series of remarks concerning the general principle of relativity of A. Einstein. The remarks almost do not touch upon the essence of the principle, and deal chiefly with broad generalizations—extrapolations and overly hasty “consequences” of Einstein’s theory. Fully reconciling himself with the old “special” principle of relativity, which gives a mathematically exact description of experimental results in the domain of rectilinear uniform motions, Lenard protests above all against the universality of the new principle, limiting its applicability to the domain of motions occurring under the action of forces proportional to masses (for example, gravitation). In such motions the effects of inertia disappear; in all other cases of motion, however, the effects of inertia make it possible to ascertain absolutely the presence of nonuniform motion. Lenard proposes to call Einstein’s new theory the “extended principle of relativity,” or the “gravitational principle.” Lenard’s second remark concerns the “elimination” of the concept of the world ether from physics—as a consequence of Einstein’s principle. Lenard considers this “elimination” a mere misunderstanding. Establishing the duality of scientific conceptions of nature, expressed either in the purely mathematical description of phenomena or in the construction

¹ Cf., for example, A. Sommerfeld, Atombau, p. 380, 1921, where the author apologizes for using the term “ether.”

mental models of the same phenomena, Lenard points out that in the mathematical method the ether had been eliminated long before Einstein, or rather had not even appeared there; and if it did sometimes figure, it did so in a purely external way, as a simple mathematical notation. On the other hand, the rejection of the ether in the method of models is evidently equivalent to the rejection of the method itself. Such a consequence, however, can hardly be drawn from the principle of relativity, even from the general one. Lenard remarks ironically that many properties of the world ether quite clearly appear in Einstein’s four-dimensional space of variable curvature.

In the third part of his article Lenard points to the possibility of constructing an electromagnetic theory of gravitation, proceeding from the idea of a discontinuous ether consisting of particles moving at the speed of light. The theory is sketched by Lenard only in the most general outlines and is not entirely clear. The basic propositions are the following: 1) matter is constructed from rotating elements (dynamids), 2) two dynamids act upon each other like two circular currents, 3) the planes of rotation of these currents can always turn, 4) the magnetic field of the elementary circular currents does not fill space continuously, but is discontinuous in space and in time. The author also describes a model of his dynamids, demonstrated at the physics seminar in Heidelberg.

S. Vavilov.

La découverte de l’Électromagnétisme faite en 1820 par J. C. Oersted
Copenhague 1920.

On July 21, 1920, one hundred years had passed since the publication of Oersted’s famous memoir describing the experiments that first established the connection between electrical and magnetic phenomena. The anniversary committee in Copenhagen issued, for this memorable date, three volumes of Oersted’s works, his correspondence with various scholars, and finally a small booklet with a facsimile of Oersted’s original Latin memoir and translations of this memoir into French, German, English, Italian, and Danish. The Latin memoir, “Experimenta circa effectum conflictus electrici in acum magneticam,” written with extraordinary concision, drew the most intense attention of physicists throughout the civilized world, and this explains the almost simultaneous translation of the memoir into the principal European languages. The discovery of the fundamental fact of electrodynamics placed Oersted, alongside Galvani, among the founders of the modern doctrine of electricity, and Denmark is rightly proud of this name, along with the names of Tycho de Brahe and Rømer.

The facsimiles in the edition under review are impeccably executed and will be duly appreciated by lovers of the history of physics. The book is adorned with an image of a medal in memory of Oersted, awarded in recognition of scientific work.

S. Vavilov.