of 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 occasionally figure, it was in a purely external way, as a simple mathematical designation. On the other hand, the rejection of the ether in the method of models is evidently tantamount to a rejection of the method itself. Such a consequence, however, can hardly be drawn from the principle of relativity, even from the general one. Lenard notes ironically that many properties of the world ether are quite clearly manifest 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 conception of a continuous 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. Its basic propositions are as follows: 1) matter is built from rotating elements (dynamids); 2) two dynamids act upon one another 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 the 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
Copenhagen, 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. For this significant date the jubilee committee in Copenhagen issued 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, attracted the most intense attention of physicists throughout the civilized world; 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 may rightly be proud of this name along with the names of Tycho Brahe and Rømer.

The facsimiles of the publication under review have been executed irreproachably and will be duly appreciated by lovers of the history of physics. The book is adorned with an image from the medal in memory of Oersted, awarded in recognition of scientific work.

S. Vavilov.