Tradesmen selling a selection of vases.

In later years, Thomson went on to document the lives of London's homeless, while serving as the official photographer of the British royal family. After his death in 1921, his glass negatives were bought by the pharmaceutical tycoon Sir Henry Wellcome.

The purchase was fortunate, as the Wellcome Library was one of the very few places that could store such fragile glass negatives, though at the time no one understood how valuable Thomson's negatives were.

William Schupbach, iconographic collections librarian for the Wellcome Trust, first came across the three crates in the 1970s, and noticed they were labeled "scratched negatives".

He studied the images in detail and realized they were of historical value. His team then made prints from the negatives and catalogued each photograph. They have since been digitalized.

In 2006, Yao came across them in the Wellcome Library. Upon discovering that they had never been exhibited in China, Yao volunteered to curate the exhibition, and the project went from strength to strength.

In 2009, Yao took the exhibition to four Chinese museums: Beijing World Art Museum, Fujian Museum, Guangzhou Museum and Dongguan Exhibition Center. Subsequently the photographs toured internationally, including to Britain, Sweden, Ireland and the United States.

The photographs attracted widespread international acclaim.