Dark Images of Photographed Lightning Discharges

Abstract

A VERY clear illustration of the reason why some of the lightning discharges in a photographed thunderstorm appear dark was afforded me at Wednesfield, Staffordshire, about mid-day on Thursday, July 19. There were a number of double flashes, that is, two discharges occurring rapidly in the same apparent region, but following different courses, and separated in time by from one-eighth to one-half of a second. But one flash, quite near to where I stood (one second and a half between flash and sound), gave a repetition following absolutely the same path as the first flash and practically as bright. The only difference was that two faint branches of the first flash were not repeated in the second discharge. The second flash followed so quickly (about an eighth of a second, I estimate), that the impression on the retina of the first discharge had not died out when the second exactly covered it, so that I could appreciate the absolute coincidence. A few kinematographic records of thunderstorms would show whether or not such repetitions are common, and whether they are the cause of dark flashes on the photographic plate.