A professor vanishes in front of his class while proving that the occult can be measured, leaving behind a blue jewel and a promise; a young man spends years in the house where it happened trying to open the same door. Hall and Flint wrote it for Argosy in 1921, and the pulp readership argued about the ending for a decade. The first half is one of the best mysteries the magazines produced; the second half explains it, which is the risk such books run. Fifty-eight plates by Hammershøi and Grimshaw trace the hush of empty rooms and lamplit streets, the same quiet threshold the novel dares its characters to cross.