Longue lettre scientifique de l’astronome américain T. M. Donahue

Lettre dactylographiée signée de Thomas Michael Donahue avec schémas autographes, adressée à un confrère français. En anglais. Ajouts et apostille autographes.

Longue et belle lettre illustrée de 9 schémas, 3 formules scientifiques et un feuillet de graphiques autographes de Donahue. 

"[...] Your letter fascinated me. I was very pleased to learn of your results. I, too, am inclined to believe that the maximum you find ic an effect of resonance absorption. I have been working during the summer along with a very helpful graduate student to develop complete theory for the twilight (and daytime) radiation from sodium layer with takes into account absorption and radiation of the resonance photons from the sun with subsequent imprisonment of the resonance radiation. The first step in this calculation is to obtain the density of sodium atoms excited directly by sunlight as a function of altitude and solar depression from noon to the end of twilight [...]". Long et passionant exposé illustré de graphiques, schémas et formules.

La lettre est accompagnée d'un feuillet de papier millimétré avec courbes et calculs de Donahue.