Paper negatives look like the images taken by pinhole cameras – everything comes out the opposite. This includes colors (a black sky, and a white spot where the tree casts a shadow) and writing (which looks like you’re reading it in a mirror). In the early development of cameras, they didn’t have strips of film so often took pictures directly on photo paper.
The Metropolitan Museum of Art (most commonly known as the Met) has a great archive of paper negative photographs, as well as positive images printed from paper negatives. You can see the collection of images by clicking here.