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the picture disappears, but this is only while wet, for on washing it withpure water and drying, it is restored, and assumes, when laid on a blackground, much the air of a Daguerreotype, and still more so whensmoked at the back, the silvered portions reflecting most light, so thatits character is changed from a negative to a positive drawing. Toobtain delicate pictures, the plate must be exposed wet, and when with-drawn must immediately be plunged in water, that the nitrate, which isliable to crystallize, may be abstracted.
Sir John Ilerscliel has made some experiments on thickening thefilm of silver, by connecting it, under a weak solution of that metal,with the reducing pole of a voltaic pile* “ The attempt afforded dis-tinct indications of its practicability, with patience and perseverance, ashere and there, over some small portions of the surface, the lights hadassumed a full metallic brilliancy under this process.” Glass coatedwith the iodide of silver, and treated as above, is more sensitive thanthe chloride.
“ When the glass is coated with bromide of silver, the action, per se,is very slow, and the discolouration ultimately produced far short ofblackness; but when moistened with nitrate of silver, sp. gr. 1.1, it isstill more rapid than with the iodide, turning quite black in the course ofa very few seconds’ exposure to sunshine. Plates of glass thus coated,may be easily preserved for the use of the camera, and have the advan-tage of being ready at a moment’s notice, requiring nothing but a washover with the nitrate of silver, which may be delayed until the image isactually thrown on the plate, and adjusted to the correct focus with alldeliberation. The sensitive wash being then applied with a soft flatcamel-hair brush, the box may be closed and the picture impressed, afterwhich it only requires to be thrown into water, and dried in the dark, tobe rendered comparatively insensible, and may be finally fixed withhyposulphite of soda, which must be applied hot, its solvent power onthe bromide being even less than on the iodide.”
Sir John Ilerscliel suggests a trial of the fluoride of silver uponglass, which, if proved to bo decomposable by light, might possiblyeffect an etching on the glass, by the corroding property of the hydro-fluoric acid.
Frequent trials of these and other methods, have not enabled me toadd any thing to the above directions. The bromide of silver used inthis way is capable of producing pictures of the most extreme delicacy;and as we are enabled to take a number of positive copies from anoriginal negative photograph on glass, it is a means which promises tobe exceedingly valuable in forwarding the most important branch ofthe photographic art, namely, publication.
See Spencer's Electrography.