Betty Yao isn't sure what she finds more remarkable about the photographs of John Thomson: the fact that they have been so well preserved after 146 years, or the way a bearded, English-speaking Scotsman managed to so skillfully capture the personalities of ordinary Chinese people in the middle of the 19th century.

"Given the long exposure time, it's really unusual the way he captured people's eyes – you can almost see their inner feelings," says Ms. Yao, a Hong Kong-born, London-based curator. "It was the Qing...