Now, it is surprising how much detail the trained eye of a sailor will grasp, even in the brief space of time occupied by a gleam of sheet-lightning; it is due in part, I think, though certainly not wholly, to what scientists describe as "persistency of vision," or the phenomenon which causes an image to remain imprinted upon the retina of the eye for a quite appreciable period after the object has vanished. But I am certain that there is more in it than that, though precisely what it may be I cannot tell; suffice it to say that I was able to answer unhesitatingly: