Sagnac thanks Becquerel for having sent him his volume on the radioactivity of matter and for having cited his name in the work.

“[…] I have already read a large part of it. I am much obliged to you for having so kindly mentioned my name. On the subject of the role of the secondary rays in the production of the maxima of impression of which you speak, in particular p. excited secondary rays in the [?] of fig. 30, the following phenomenon that I studied in the first part of my doctoral thesis: [diagrams of Sagnac's hand]. If the photographic impression were at each point uniquely defined by the illumination produced at this point by the primary rays, it would be constant from x to a, it would increase from a to c (according to a law which I represent arbitrarily by a straight line) […]”. Following a long and fascinating scientific demonstration.

Henri Becquerel had published in 1903, by Firmin-Didot, a work entitled: " Research on a new property of matter: spontaneous radiant activity or radioactivity of matter“, which is in question here.

That same year, Henri Becquerel as well as Marie and Pierre Curie won the Nobel Prize in Physics for their research on radioactivity.