A Manual Of Photography

166 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. kind, was more calculated to awaken attention than anything which has been brought before the public since the publication of Daguerre’s beautiful photographic process. Having instituted a series of experiments, the results of which appear to prove that these phenomena are not produced by latent light, I am desirous of recording them. I would not be understood as denying the absorption of light by bodies ; of this I think we have abundant proof, and it is a matter well deserving attention. If we pluck a nastertium when the sun is shining brightly on the flower, and carry it into a dark room, we shall still be enabled to see it, by the light which it emits. The human hand will sometimes exhibit the same phenomenon, and many other instances might be adduced in proof of the ab- sorption of light; and I believe, indeed, of the principle that light is latent in bodies. I have only to show that the conclu- sions of M. Moser have been formed somewhat hastily, being led, no doubt, by the striking similarity which exists between the effects produced on the daguerreotype plates under the influence of light, and by the juxtaposition of bodies in the dark, to con- sider them as the work of the same element. 1. Dr. Draper, in the Philosophical Magazine for September 1840, mentions a fact which has been long known,—" That if a piece of very cold clear glass, or, what is better, a cold polished metallic reflector, has a little object, such as a piece of metal, laid on it, and the surface be breathed over once, the object being then carefully removed, as often as you breathe on it again, a spectral image of it may be seen, and this phenomenon may be exhibited for many days after the first trial is made." Several other similar experiments are mentioned, all of them going to show that some mysterious molecular change has taken place on the metallic surface, which occasions it to condense vapours unequally. 2. On repeating this simple experiment, I find that it is necessary for the production of a good effect to use dissimilar metals ; for instance, a piece of gold or platina on a plate of copper or of silver will make a very decided image, whereas copper or silver on their respective plates gives but a very faint one, and bodies which are bad conductors of heat, placed on good conductors, make decidedly the strongest impressions when thus treated. 3. I placed upon a well-polished copper plate, a sovereign, a shilling, a large silver medal, and a penny. The plate was gently warmed, by passing a spirit-lamp along its under surface: when cold, the plate was exposed to the vapour of mercury: