Schulze is best known for his discovery that the darkening in sunlight of various substances mixed with silver nitrate is due to the light, not the heat as other experimenters believed, and for using the phenomenon to temporarily capture shadows.

To provide an interesting demonstration of its darkening by light, he applied stencils of words to a bottle filled with the mixture and put it in direct sunlight, which produced copies of the text in dark characters on the surface of the contents.

Because they were produced by the action of light, an extremely broad and literal definition of what a photograph is may allow even these fluid, ephemeral sun printings to qualify as such, and on that basis many German sources and some international ones credit Schulze as the inventor of photography.

Thomas Wedgwood and Humphry Davy produced more substantial but still impermanent shadow images on coated paper and leather around the year 1800.

Nicéphore Niépce succeeded in photographing camera images on paper coated with silver chloride in 1816 but he, too, could not make his results light-fast.