Symbolism, Imagery, Allegory
This one's a pretty popular metaphor in literature, and we find it everywhere in this novel. We all know that light usually represents reason, safety, happiness, and hope. And darkness means fear, superstition, mystery, and despair. Every horror film director knows this.
Sir Henry Baskerville knows it, too. He thinks the solution to all his problems is calling up the local power company and installing some high-wattage bulbs (6.46). That's pretty dim. See what we mean?
Dartmoor could just as well be called Darkmoor in this novel, because it's absolutely soaked in gloomy darkness. And with than gloom comes superstition and fear. See our "Setting" section for moor—sorry, we meant "more"—on this.