He used something called the C.P. Stern Concealed Vest Spy Camera, which was a flat camera that was hidden beneath clothes and had the lens come through a buttonhole. He had a string that went down into his trouser pockets, that he used to trigger the camera.
He’d spend his days greeting people on the street, and secretly take a photo. He’d be able to capture 6 images at at time, then he’d have to go home and change the plate. Starting in 1893, he took over 500 photos of people in their natural state.
These are an amazing glimpse into what life in Oslo, Norway was like back then.