SAGNAC Georges De l’optique des rayons de Röntgen et des rayons secondaires qui en dérivent.

Walter. Scott (dir.). La Correspondance entre Henri Poincaré et les physiciens, chimistes et ingénieurs. Bâle, Birkhaüser, 2007. pp. 322-238. Christian Bracco, « Henri Poincaré : du « principe de mouvement relatif » au « principe de relativité » en passant par le « principe de réaction » et son rapport à la théorie de Lorentz », Philosophia Scientiæ, 27-2, 2023, pp. 35-62. 

First edition of this thesis. Copy nscribed to the mathematician Henri Poincaré (1854–1912) on the half-title page: ”  à M. Henri Poincaré, respectueux hommages “. 

Focusing on the optical properties of X-rays, discovered by Wilhelm Röntgen in 1895, this dissertation—defended in Paris in December 1900—examines their propagation, scattering, and absorption through a strictly optical approach. His research notably demonstrated the existence of secondary rays derived from X-rays.

The significance of this copy also lies in its provenance. A former student of Poincaré (1854–1912) at the École Normale Supérieure, where he notably took his courses on Hertzian oscillations between 1892 and 1893, Sagnac naturally dedicated his dissertation to the man he recognized as one of the leading scientific authorities of his time. Like many young academics, Sagnac wished to have his work recognized by his peers, whose approval was of paramount importance. The letter highlights an academic connection between the two men, a connection that would be sustained through several scientific exchanges.

This relationship is documented in particular by correspondence predating the submission of his dissertation, including a letter—now held in the National Archives—regarding the research conducted by W. Nicolaiève, dated 1899. In their correspondence, Georges Sagnac and Henri Poincaré discuss fundamental questions of electromagnetism, such as the role of the magnetic field, the mechanisms of electromagnetic action transmission, and the principle of action and reaction.

These exchanges took place within a particularly active scientific network centered around Poincaré, founded on interdisciplinarity and the sharing of knowledge. Thus, between 1878 and 1912, he corresponded with more than sixty scientists, physicists, chemists, and engineers, among whom was George Sagnac.

The debates between the two scholars found particular expression in the development of Einstein’s theory, to which Poincaré made a major contribution by introducing the principles of simultaneity and the relativity of time, notably in “La mesure du temps,” published in 1898 in the Revue de métaphysique et de morale.

If Sagnac’s name is particularly well known in the scientific community, it is primarily because of the discovery that bears his name, which is still used in engineering today. Convinced by Fresnel’s theory of the ether, Sagnac conducted optical experiments aimed at demonstrating the existence of a time delay in the reception of two signals traveling along opposite paths. He interpreted the results of his experiment as contradicting Einstein’s theory of relativity. In reality, Sagnac had just discovered a deeper aspect of the phenomenon of relativity, which would be correctly interpreted in 1921 by Paul Langevin.

Sending his work to Poincaré was thus part of a vital network of scholars, within which ideas and knowledge circulated, influenced one another, and helped foster a scientific debate essential to progress. This work, one of his earliest scientific publications, marked his first step toward joining the circle of scholars who shaped modern science in the early 20^th century.

Front printed cover and spine missing, paper loss on the remaining back cover.