Becquerel wrapped the photographic plates in black paper during his uranium-ray experiments. The thick paper blocked ordinary visible light while allowing him to test whether an unseen radiation could still affect the photographic emulsion.
His setup used Lumière photographic plates coated with a bromide emulsion. He enclosed each plate in two sheets of very thick black paper, then placed a phosphorescent substance, including uranium salts, on the outside. If sunlight merely leaked through, the protected plate should not have darkened.
The uranium salts nevertheless produced recognizable silhouettes on the developed plates. This result initially seemed to connect phosphorescence with the recently discovered X-rays, but Becquerel’s later dark-drawer experiments showed that sunlight was unnecessary. Uranium emitted the penetrating radiation spontaneously.
The black paper is sometimes confused with the objects placed between the salt and plate, such as coins or metal screens. Those objects created shadow patterns; the paper’s job was to exclude visible light and test the radiation’s penetrating power.