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In his 2 March 1896 report, Henri Becquerel described uranium salt crystals acting on a photographic plate through opaque wrapping. A copper cross weakened the exposure behind it. Plates prepared during intermittent sunshine were left in a drawer and developed on 1 March; unexpectedly strong images prompted deliberate dark tests. He placed a plate and crystals inside two opaque boxes in a drawer; one crystal was separated from the emulsion by glass. After five hours, both produced images. The separation tested transmission without direct chemical contact.^[ref]

The result challenged a dependence on immediate illumination. Henri Becquerel still considered long-lasting invisible phosphorescence a possibility, and explicitly withheld a final explanation. The photographic readout established an effect through intervening material; it did not count nuclear transformations or measure a nuclear decay constant.^[ref]

A physicist whose uranium experiments helped establish radioactivity. His 1896 photographic plate tests separated exposure to the material from immediate illumination; the cautious interpretation in his report is part of the story of Becquerel's uranium-radiation experiment. He held the chair of applied physics at the Paris Museum of Natural History.^[ref]

One half went to Henri Becquerel for discovering spontaneous radioactivity. Pierre Curie and Marie Curie each received one quarter for investigating that radiation.^[ref] Becquerel's uranium-radiation experiment and Curie ionization-current measurement expose two complementary experimental routes into radioactivity.

By Henri Becquerel. Académie des sciences communication, 2 March 1896. Cédric Van Rompay hosts the inspected French transcription.

Cited passages: opaque screens, dark storage, five-hour controlled exposures, and the closing cautious hypothesis.