darkening

“At last the first glimpse from a bridge of an open-top red bus, and a noticeable darkening of the atmosphere from the smoke of London: then the increasingly dingy stations with double-barrel names, set amid what has always been to me the outstanding feature of the "Premier Line" approach to London—the positively marvellous display of crazy chimney-pots on the grey inner suburban houses. As many as twenty, all of varying style, standing together like ranks of jagged teeth, and providing a Dickensian back-cloth which no other route can boast.”

“NEGATIVE FLASHES — Brief darkenings of a light that is otherwise steadily on.”