On the Cause of Dark Lightning and the Clayden Effect

Abstract

I HAVE been criticised in a letter which appeared recently in NATURE for not alluding in my letter on dark lightning to the peculiar photographic reversal known as the Clayden effect. I must confess that at the time of writing my letter I was unaware of this effect, a description of which has only appeared, so far as I know, in one of the photographic journals. Mr. Clayden has certainly explained dark lightning, and it only remains to explain his explanation. As I think that this effect is not generally known, I believe that it may be worth while to devote a few words to the statement of the case before describing the experimental work by which I have determined the factors which play a part in this very curious photographic phenomenon.