THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



295 



consists in cleaning and polishing- tlu' plate to fit it for receiving the 

 sensitive coating on which. the lighr forms the picture. The second 

 is the formation of the sensitive ioduret of silver over the face of the 

 tablet. The third is the adjusting of the plate in the camera obscura 

 for the purpose of receiving the impression. The fourth is the bring- 

 ing out of the photographic picture, which is invisible when the plate 

 is taken from the camera. The fifth and last operation is to remove 

 the sensitive coating, and thus prevent that susceptibility of change 

 under luminous influence which would otherw'ise exist and quickly 

 destro}' the picture. 



First operation. — A small phial of olive oil, some finely carded cot- 

 ton, a muslin bag of finely levigated pumice, a phial of nitric acid, 

 diluted in the proportion of 1 part of acid to KJ parts of water, are 

 re(|uired for this operation. The operator must also provide himself 

 with a small spirit lamp and an iron-wire frame, upon which the 

 l^late is to be placed while being heated over the lamp. 



The plate being first powdered over with pumice, by shaking the 

 bag, a piece of cotton dipped into the olive oil is then carefully 

 rubbed over it with a continuous circular motion, commencing from 

 the center. When the plate is well polished, it must be cleaned by 

 pow'dering it well over wdth pumice and then rubbing it Avith dry 

 cotton, always rounding and crossing the strokes, it being impossible 

 to obtain a true surface by any other motion of the hand. The sur- 

 face of the plate is now rubbed all over wdth a pledget of cotton, 

 slightly w-etted witli the diluted nitric acid. Frequently change the 

 cotton and keep rubbing briskly that the acid may be equally dif- 

 fused over the silver, as, if it is permitted to run into drops, it stains 



the table. It will be seen when the 

 acid has been properly diffused 

 from the appearance of a thin film 

 equally spread over the surface. 

 It is then to be cleaned off with a 

 little pumice and dry cotton. 



'i'he plate is now placed on the 

 wire frame, the silver upW'ard, and 

 the spirit lamp held in the hand 

 and moved about below it, so that 

 the flame plays upon the copper. 

 This is continued for five minutes, 

 when a white coating is formed all 

 over the surface of the silver; the 

 lamp is tiien withdrawn. A charcoal fire may be used instead 

 of the lam]). The plate is now cooled suddenly by placing it on a 

 mass of metal or a stone floor. When perfectly cold it is again 

 polished with dry cotton and i)umice. It is necessary that acid be 



