[130] 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.” [131] The details of this curious subject may be studied in the following memoir and communications:--_On vision and the action of light on all bodies_: by Professor Ludwig Moser, of Königsberg; from Poggendorff’s Annalen, vol. lvi. p. 177, No. 6, 1845. _Some remarks on Invisible Light_: by Professor Ludwig Moser, of Königsberg; from Poggendorff’s Annalen, vol. lvi. p. 569, No. 8. _On the power which light possesses of becoming latent_: by Professor Ludwig Moser, of Königsberg; from Poggendorff’s Annalen, vol. lvii. No. 9, p. 1. 1842. _On certain spectral appearances, and on the discovery of latent light_: by J. W. Draper, M.D., Professor of Chemistry in the University of New York; Phil. Mag. p. 348, Nov. 1842. _On a new imponderable substance, and on a class of chemical rays analogous to the rays of dark heat_: by Professor Draper; Phil. Mag., Dec. 1842. _On the action of the rays of the solar spectrum on the Daguerreotype plate_; by Sir J. F. W. Herschel, Bart.; Phil. Mag., Feb. 1843. See remarks in this paper on the use which Moser has made of coloured glasses: also a communication by Professor Draper, _On the rapid Detithonizing power of certain gases and vapours, and on an instantaneous means of producing spectral appearances_: Phil. Mag., March 1843; and _On the causes which concur in the production of the images of Moser_: Comptes Rendus, Nov. 1842. See _Scientific Memoirs_, vol. iii. [132] This fact was first observed by myself, and described in the paper already referred to, Philosophical Magazine, vol. xxii. p. 270. It does not, however, appear to have attracted the attention of any other observer.