He passed into the shop as he spoke, and the Professor could hear the sound of his footsteps as he climbed the stairs. The Professor, left alone in the room, proceeded to examine the titles of the books in the shelves. They were mainly botanical works, such as one might expect to find in the library of a herbalist. But here and there among them were a few volumes of general scientific interest. Dr. Priestley smiled as he saw, lying inconspicuously at the end of one of the shelves, a work he had published a year or so before, Fact and Fallacy . No doubt this had given Ludgrove the cue to his own interest in a problem so intricate as that of the Praed Street murders.