Night at the Vulcan

Night at the Vulcan by Ngaio Marsh
My rating: 4 of 5 stars

Entirely set in a theater during the run up to a new play this one had me convinced I knew who-done-it and then totally surprised me.

"When she considered them all as they sat about their own working-stage, bruised by anxiety and fatigue, Jacko's ugly word sounded not so much frightening as preposterous. It was unthinkable that it could kindle even a bat-light of fear in any of their hearts." (chapter VIII)

This quote made me wonder what a bat light is. I looked around but couldn't locate an answer. It might be like a ghost-light, or there are high-power flash-lights called bat lights, but that makes no sense in the context. Any ideas?