Dark Lightning

Abstract

I HAVE been greatly interested by some photographs showing the rare phenomenon of dark lightning, which have recently been sent to me. So far as I know the only explanation that has ever been offered to account for them is photographic reversal, due to extreme brilliancy. This appears to me to be wholly out of the question for two reasons. In the first place, a dark line on the picture, resulting from over-exposure of a very brilliant line, would be surrounded by bright edges due to the lesser photographic action in the halation region. This is never present so far as I know, the dark flashes being minute black lines ramifying from, or in the neighbourhood of, the main discharge. Secondly, from what evidence I can gather, the dark parts of the flash are not those which appear most brilliant to the observer. Mr. Jennings, of Philadelphia, who in 1890 secured a remarkable picture (reproduced in Photog. Times Annual, 1891) showing a very brilliant flash with countless dark flashes covering the sky around it, tells me that the appearance to the eye was a brilliant white discharge, with fainter rose-coloured ramifications, the latter developing in the negative, or rather positive, as dark flashes.