Thom says most of the photos were taken by foreigners - missionaries, businessmen and journalists - who were visiting China at the time and took snaps to show to their fellow countrymen back home.
"They appeared to have more interest in the scenes that would be unfamiliar in their own countries, such as pigs running down the streets and people pushing single-wheeled handcarts."
Other images were taken by Chinese people in photo studios during the 1920s and 1930s when it was uncommon for people in the country to own a camera.
Thom says there is a huge difference between the photos taken by the visitors and the locals. While the former were looking to record Beijing, the latter tended to use photos to commemorate certain occasions or represent family life.
"The only thing in common is that no one, neither foreigner nor Beijinger, took photos of the hutong (alleyways). Why? Because at that time hutong were nothing special. Visitors to Beijing were even disappointed by them - the muddy roads, gray walls and shabby houses all looked the same to them.
"Now we like to paint hutong culture in a romantic light, but things were different 100 years ago."
