What is strange about the single light in the room Dr. Archer enters?

The murder scene as well as light are connected. The motif continues shortly thereafter to tremendous effect when Dr. Archer arrives at the behest of those who reported a suspicious death. Notice the way in which the truly oppressive darkness of the room is offset by the potential for a clarifying illumination:

“The chamber was almost entirely in darkness. The walls appeared to be hung from ceiling to floor with black velvet drapes. Even the ceiling was covered, the heavy folds of cloth converging from the four corners to gather at a central point above, from which dropped a chain suspending the single strange source of light, a device which hung low over a chair behind a large desk-like table, yet left these things and indeed most of the room unlighted.”