All of the following explanations for the passage below are supported by the text below EXCEPT
Shaking my hair from my eyes, I lifted my head and tried to look boldly round the dark room; at this moment a light gleamed on the wall. Was it, I asked myself, a ray from the moon penetrating some aperture in the blind? No; moonlight was still, and this stirred; while I gazed, it glided up to the ceiling and quivered over my head.
I can now conjecture readily that this streak of light was, in all likelihood, a gleam from a lantern carried by some one across the lawn: but then, prepared as my mind was for horror, shaken as my nerves were by agitation, I thought the swift darting beam was a herald of some coming vision from another world.