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]
Sur les radiations invisibles émises par les corps phosphorescents by Henri Becquerel. Final two paragraphs: tentative invisible-phosphorescence hypothesis and limits of the inference
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