= Becquerel's uranium-radiation experiment
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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.<Sur les radiations invisibles émises par les corps phosphorescents by Henri Becquerel>[2 March 1896 communication: copper screen, delayed development, deliberate dark exposure and glass-separated crystal]{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>.<Sur les radiations invisibles émises par les corps phosphorescents by Henri Becquerel>[Final two paragraphs: tentative invisible-phosphorescence hypothesis and limits of the inference]{ref}
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