Note 130 — Chapter VIII

This peculiar continuance of an effect has frequently been observed in many of the photographic processes. In a note to a memoir On certain improvements in Photographic processes, Sir John Herschel thus refers to this property:—“The excitement is produced on such paper by the ordinary moisture of the atmosphere, and goes on slowly working its effect in the dark, apparently without other limit than is afforded by the supply of ingredients present. In the case of silver it ultimately produces a perfect silvering of all the sunned portions. Very singular and beautiful photographs, having much resemblance to Daguerreotype pictures, are thus produced; the negative character changing by keeping, and by quite insensible gradations to positive, and the shades exhibiting a most singular chatoyant change of colour from ruddy-brown to black, when held more or less obliquely. No doubt, also, gold pictures with the metallic lustre might be obtained by the same process, though I have not tried the experiment.”