The photochrom process was a method by which black and white photographic negatives could be transferred to lithographic stones and composited to create color photographs. The process was invented by Hans Jakob Schmid in Switzerland in the 1880s. In the U.S., it was licensed exclusively to the Detroit Photographic Company, for whom William Henry Jackson became a photographer in 1897. Jackson went on to become the plant manager in 1903. Photochroms grew in popularity in the United States after the Private Mailing Card Act launched the domestic postcard industry. It is thought that it the peak of photochrom production, the Detroit Photographic Company was producing as many as 7 million per year.