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Henri Poincaré Papers (Université de Nantes): Poincaré – Le Bon correspondence

Visit: 2026-10-08 (Investigator, Step I, first run). Site: https://henripoincarepapers.univ-nantes.fr/ How found: Google query "Le Bon" "lumière noire" 1896 "Comptes rendus" returned letters 2-35-2 and 2-35-3. Letter pages lebon01-lebon16 were then fetched by script (lebon04 and lebon17+ returned 404). Saved copies: research/pages/poincare-lebon*.html.

Many letters are summaries ("Résumé d'après les notes prises par M.-J. Nye d'une ALS"), i.e. the editors' summary of Mary Jo Nye's notes on autograph letters; several originals are in a private collection (Paris 75017; one in "la collection de Pierre-Sadi Carnot"). Where the page gives a summary rather than the letter, that is said below.

What was found (all [web], URL pattern https://henripoincarepapers.univ-nantes.fr/chp/text/lebonNN.html)

  1. 2-35-2, Poincaré to Le Bon, c. 28.01.1896 (lebon09). Summary: Poincaré "ne voit pas d'explication de l'expérience de Le Bon, si ce n'est une diffusion par l'air"; asks whether it works with an ordinary metal, whether the metal was cleaned, and the exposure time; proposes a lead-plate test geometry (comparing intensity at P and Q). Editors' note: the set-up is that of Le Bon's note presented by Arsène d'Arsonval to the Académie on 27.01.1896. Also: Le Bon and Th. Ribot ran Wednesday lunches that Poincaré attended.
  2. 2-35-3, Poincaré to Le Bon, c. Feb 1896 (lebon06). Summary: Poincaré "valide le protocole d'expérience proposé par Le Bon", except he would drop the photographic negative; suggests an oblique silver plate to study how the shadow edge sharpens with distance. Editors: d'Arsonval presented Le Bon's first notes. Le Bon (1896b) "Nature et propriétés de la lumière noire", CR 122, 386-390.
  3. 2-35-4, Poincaré to Le Bon, c. April 1897 (lebon08). Summary: "Il a vu avec Lippmann M. Berget, qui a reproduit l'expérience de Le Bon, mais les effets obtenus à distance, annoncés par Le Bon, lui paraissent plus intéressants." Editors' note: Le Bon's note of 05.04.1897 (presented by d'Arsonval) says "son expérience sur l'action de la lumière sur divers métaux a été reproduite à la Sorbonne dans le laboratoire de Gabriel Lippmann par ses préparateurs"; "Il s'agit peut-être d'Alphonse Berget (1860-1934)."
  4. 2-35-5, c. end April 1897 (lebon07). Summary: Poincaré asks whether a metal struck by ultra-violet discharges the electroscope at a distance without the electroscope itself being lit, and whether the property persists after the light is removed. Note: Le Bon (1897) "Sur les propriétés électriques des radiations émises par les corps sous l'influence de la lumière", CR, presented 26.04.1897 by d'Arsonval.
  5. 2-35-8, before 01.01.1899 (lebon12). Summary: Poincaré's list of questions on "lumière noire": does every body store it or only fluorescent ones; CaS/ZnS versus marble; temperature; heating; does the residue fade faster when warm; and on light "transformed by passage through metals": how Le Bon established no refraction and no polarisation, whether the plate must be pre-fogged as in the ebonite experiments, etc. Le Bon answered most in the January 1899 notes.
  6. 2-35-9, c. 01-05.01.1899 (lebon03, full letter text). Poincaré: "Vous m'avez montré hier une plaque photographique qui m'avait paru curieuse. Cette plaque voilée avait été exposée au spectre solaire, et s'était dévoilée dans le violet & le bleu. Une partie de cette plaque avait été recouverte d'une lame d'ébonite très mince et s'était dévoilée, non plus dans le violet et le bleu, mais dans l'infrarouge. J'ai oublié de vous demander si la durée de pose avait été la même..." Le Bon annotated "Jamais" in bold. This is the infra-red "dévoilement" (de-fogging, i.e. the Herschel effect in later terms) through ebonite.
  7. 2-35-6, Le Bon to Poincaré, 5.1.1899 (lebon02, full text). Le Bon answers on Poincaré's own letter, encloses experiments "Ébonite sur et sous verre rouge", and announces two notes: "Sur la très grande transparence pour certaines radiations du spectre de corps réputés très opaques" (presented by Poincaré 30.01.1899) and "Durée de l'émission et propriétés optiques de la lumière obscure émise par certains corps après leur phosphorescence" (presented by Poincaré 16.01.1899). Address: 29, rue Vignon (Madeleine), Paris.
  8. 2-35-11, c. 23-29.01.1899 (lebon11). Summary: Poincaré gladly agrees to present the note; would not use the words "lumière obscure" "parce que certains feraient la confusion avec ses premières recherches sur la lumière noire"; asks for any older experiments on months-long persistence; is troubled that radiation de-fogs a plate after passing through ebonite but not without it; regrets the lack of sun.
  9. 2-35-10, Le Bon to Poincaré, 9/1/99 (lebon01, full text). Le Bon reports arc-lamp and sunlight tests: "Le maximum d'action est dans l'infra rouge aux environs de 1 μ et dans cette région la sensibilité est de l'ordre de celle du gélatine bromure pour le bleu", with a sketch of a sulphide screen under a projected spectrum, half masked by black paper and ebonite.
  10. 2-35-12, after 27.03.1899 (lebon10, full text). Poincaré on reflection experiments: first find the cause of weakening with distance (inverse square, absorption by air, or diffusion by air), describing H. Becquerel's hemispherical-mirror test with uranium salt.
  11. 2-35-17, c. 07.07.1902 (lebon13). Poincaré: Le Bon's two notes will appear; "la seconde note a été plutôt nuisible, quelques personnes prétendent qu'elle annulait la première." Editors: the first note reported measurements of light acting on bodies; the second claimed priority (Le Bon 1897) over Nodon (1902); the first was referred to commissioners Sarrau, Poincaré and H. Becquerel (CR 134, 1387-1388, séance 09.06.1902).
  12. 2-35-18, Le Bon to Poincaré, 29.11.1902 (lebon05). On discharges produced by reactions of enclosed bodies: quinine sulphate in a closed box, "la décharge sera énormément réduite. Les choses se passent comme s'il fallait que les ions puissent circuler pour agir"; thorium-oxide plate facing the electroscope versus facing the air; refers to his memoir figure (p.624) in Revue scientifique 1902 (pp.614-625, 15.11.1902).
  13. 2-35-19, c. June-Oct 1903 (lebon14). Summary: Poincaré says an English author's sentence on ether density "n'a aucun sens précis"; also comments on N-rays ("La différence d'intensité ... est à peine sensible"). Le Bon's memoir in Revue scientifique from 17.10.1903 cites Larmor, Maxwell and W. Thomson.
  14. 2-35-20, after 31.10.1903 (lebon15). Summary (partly from Le Bon 1905 p.65, which printed an extract): Poincaré contrasts Le Bon's view of solar heat with Helmholtz-Kelvin: nebula -> sun is exothermic; "sous-atomes isolés -> atome" would be endothermic, storing potential energy that reappears as heat when the atom breaks up (radium). His question: "si cette combinaison est endothermique, comment peut-elle être si extraordinairement stable ?" He says Berthelot will not be pleased, that such questions are good to raise, and that Le Bon should leave his memoir as it is.
  15. 2-35-23, 26.11.1911 (lebon16). Summary: on Le Bon's Les opinions et les croyances (1911). Editors' note: Le Bon there recalls H. Becquerel's belief in N-rays; "Ce dernier a suggéré que la 'lumière noire' de Le Bon n'était autre que la lumière infrarouge (H. Becquerel 1897)."

How it bears on the claims

  • Claim 4 (black light / infra-red): Le Bon's January 1899 work, presented to the Académie by Poincaré, is the same programme as Evolution of Forces Book IV: infra-red action on sulphide screens peaking near 1 μ, transparency of ebonite and black paper to long waves, months-long invisible afterglow. Poincaré's questions are a contemporary, expert list of the controls (equal exposure times, de-fogging only after ebonite, light leaks). H. Becquerel's 1897 suggestion that "lumière noire" was infra-red is, in effect, the reading Le Bon himself later adopted for part of it (Forces p.318).
  • Claim 5 (priority and reception): the record shows reception that was neither silence nor simple rejection. Poincaré (the leading French mathematical physicist) read Le Bon's plates, questioned the protocol, presented four of his notes, and dined at his Wednesday lunches. One documented reproduction: Berget in Lippmann's Sorbonne laboratory reproduced the 1897 metal-light experiment (Poincaré's letter summary, and Le Bon's own CR note of 05.04.1897). A 1902 priority dispute with Nodon is documented and was "plutôt nuisible" per Poincaré.
  • Claim 2 (intra-atomic energy): Poincaré's 1903 remark (sub-atoms -> atom endothermic; then why so stable?) is a contemporary physicist's reading of Le Bon's energy reservoir in potential-energy terms, which Le Bon published (1905, p.65).

Leads not yet read: the editors' chapter introduction (Springer chapter "Gustave Le Bon", pp.215-233 of La correspondance entre Henri Poincaré et les physiciens, chimistes et ingénieurs, Birkhäuser 2007; paywalled; abstract read, see springer note); the site's calendar of correspondence (letters numbered up to 2-35-23, so about 23 items; lebon04 and others not reached).