The dead of night evoked a fear in our ancestors that’s hard for us to imagine today. Most of us would be hard-pressed to recall when we were last engulfed by absolute darkness. Imagine a corner with shadows so dark that a stranger could lurk there without our knowing. With access to manmade illumination around the clock, we don’t worry ourselves. Our ancestors nightly locked doors and barred windows against wild creatures and things that howled at night—or against rough folk who waited in alleyways to do them harm.

Terrible things happened in the dark. Conversely, some monsters hide in dazzling light.