80

MISCELLANEOUS PROCESSES.

complisliment; and from its extreme sensibility, complete in all thedetails of picturesque effect. It is neither the one nor the other. Thepreparation of the plate requires the skill of an artist, combined withthe knowledge of a chemist; and even these are not always sufficientto ensure a perfect surface. The revival of the silver is not to bodepended on: sometimes it does form a continuous sheet over the partsacted on by the light, but often it is only spangles; and frequently ametallic arborescence will commence in tho light parts, and run rapidlyinto the portions in shadow. The fact is, that light has the property ofeffecting the revival of tho silver spread upon any carbonaceous body,but caloric having the same effect, and being indeed rather more activein the operation than light is, any slight increase of temperature pro-duces a revival of the metal over the parts in shadow.

Reference to the early volumes of Nicholson’s Journal will affordample evidence of these facts, which I have also recently proved. Thesovolumes contain some papers by Count Rumford on the revival of goldand silver from their solutions, by light, when spread upon charcoal orcarbonaceous earth. This philosopher has conclusively shown, that thisrevival is more dependent on the action of heat than light, whichaccounts, in some measure, for the apparent effect of candle light. Itis, however, possible, that this process may, with some modifications,become of importance.

4.—A New Construction of tiie Photographic Camera Obscuiu.

A PHOTOGRAPinc camera should possess, according to Sir John llcr-scliel, “ the three qualities of a flat field, a sharp focus at great ijiclina-tions of the visual rag, and a perfect achromaticitg.” There can bo nodoubt but theso qualifications are very essential,—the two first particu-larly aro indispensable, and there is but one objection to the latter. Wocan only produce perfect acliromaticity by a combination of glasses,and my experiments go to prove that by increasing the thickness of theobject-glass, and the number of reflecting and refracting surfaces, weinterrupt a considerable portion of light, and consequently weaken theaction on tho photographic material, whatever it may be. It is withconsiderable reluctance that I express myself somewhat at variance withso high an authority as Sir John Herschel , gifted as he is with thehighest power of physical research; I am however satisfied, that wo mayto a considerable extent get rid of the difficulties of chromatic disper-sion, without having recourse to a combination of glasses of differentrefracting powers. I have long used myself, and constructed for others,a camera obscura, which appears to answer remarkably welL It is butright I should acknowledge that I am indebted to the suggestions ofDr. Wollaston, for part of my lenticular arrangement, a, Figure 26,represents the aperture of the lens; i i', a box sliding into an outer