The origin of blue sun drying

An elegant gentleman used them to make photos more than 100 years ago to save good times. This is the flower juice printing method: use crushed petals, a little alcohol and 2 to 3 weeks of sunshine to make photos. Is this John? Herschel's oldest photographic darkroom experiment.

1842, John Herschel invented an imaging process, which can make blue photos that can be preserved for a long time. This is a process of imaging by sun exposure, which uses the characteristics that iron ions can generate Prussian blue-tone substances under ultraviolet irradiation to produce blue photos, which are called "blue type".