From the Play Gas Light:
The original title stems from the dimming of the gas lights in the house that happened when the husband was using the gas lights in the flat above while searching for the jewels belonging to a woman whom he had murdered.
The wife correctly notices the dimming lights and discusses it with her husband, but he insists that she merely imagined a change in the level of illumination.
Ahhhh ... humans can be so resourceful in behalf of evil.