[129] The chemical evidence of this will be found in Sir John Herschel’s Memoir On the Solar Spectrum, and particularly as exemplified in the changes produced on the tartrate of silver. Similar influences are described as observed on a Daguerreotype plate, in a paper entitled Experiments and Observations on Light which has permeated coloured media, and on the Chemical Action of the Solar Spectrum; by Robert Hunt.—Philosophical Magazine, vol. xxvi. 1840.
[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.
[133] On Thermography, or the Art of copying Engravings or any printed characters from paper or plates of metal, and on the recent discovery of Moser, relative to the formation of images in the dark, by Robert Hunt: Reports of the Royal Cornwall Polytechnic Society for 1842, and Philosophical Magazine, vol. xxi. p. 462.—On the Spectral Images of M. Moser, by Robert Hunt: Philosophical Magazine, vol. xxiii. p. 415.
[134] Catalytic force, or attraction of surface concerned in the diffusive power of gases: an occult energy or power in saturated saline solutions; Prater.—Mechanic’s Magazine, vol. xlv. p. 106. Ueber elektrische Abbildungen; by G. Karsten.—Poggendorff’s Annalen, vol. lvii. p. 402.—Melloni and Brewster may be consulted for much that is most remarkable connected with radiation from coloured surfaces.
[135] Cornelius Agrippa is said to have possessed such a mirror. The Chinese make mirrors which, when placed in a particular light, show upon their polished faces the pattern on the back of the metal, although it is invisible in every other position. This is effected by giving different degrees of hardness to the various parts of the metal. In Natural Magic, by Sir David Brewster, several curious experiments belonging to this class are named.