Becquerel developed the unexpectedly strong dark-drawer photographic plates on 1 March 1896. The result helped reveal that uranium emitted penetrating radiation without needing sunlight or another external source.
Becquerel had been investigating whether phosphorescent uranium salts might produce X-ray-like effects. He wrapped photographic plates in thick black paper and placed uranium compounds on them, expecting sunlight to activate the salts. Overcast weather on 26 and 27 February left several prepared plates in a dark drawer.
When he developed them on 1 March, the uranium-related images were strikingly strong rather than faint. The darkness itself was the crucial clue: the radiation came from the uranium, not from lingering phosphorescence. Becquerel initially described the phenomenon cautiously, before later experiments established spontaneous radioactivity.
A common mix-up is the date of the first report. Becquerel presented earlier experimental results to the French Academy on 24 February and 27 February, but the celebrated dark-drawer observation and plate development occurred on 1 March.