166 ANNUAL OP SCIENTIFIC DISCOVERT. 



applicability of salts of chromium and iron for photographic purposes, in 

 place of those of silver and gold. The salt of chromium bichromate of 

 potash is dissolved, and paper steeped in the solution. The salt thus 

 brought into contact with organic matter in the paper, enters into chemical 

 union with it where it is touched by light, and forms an insoluble compound. 

 So much of it as light has not touched is washed away after the picture has 

 been taken on this paper, which is, in the next place, soaked for a few 

 minutes in the solution of a salt of iron. The iron adheres firmly to the 

 mordant image, but is removed from the rest of the paper by another wash- 

 ing. Now dip the paper in a solution of gallic acid, add galls to the iron, 

 and a picture comes out with fine violet-black tints, which is, in fact, a pic- 

 ture in writing-ink, as permanent as writing-ink is known to be. This pro- 

 cess has held its ground, standing the test of wider practice, and by it 

 photographic pictures can be made that may be cheap as well as permanent. 



PHOTOGRAPHIC NOVELTIES. 



Among the new and useful applications of photography, is the taking of 

 copies of machines, in whole or part, as patterns, or advertisements, of the 

 manufacturers. Thus the leading tool and machine makers of New York 

 City take photographic views of every machine which they make, which not 

 only serve as records of their products, but the pictures are sent to persons 

 who wish to order similar machines, so as to give them a clear idea of the 

 article which they may wish to purchase. Several machine-shops, like that 

 of Messrs. Hoe & Co., of New York, have a photographic gallery connected 

 with the drafting department. 



Photography has also been applied to furnish a key for detecting fraudulent 

 bank bills. The counterfeit bills are copied by photography on prepared 

 stones, and from these lithographic prints of the bills are obtained at a 

 comparatively low cost. 



Pouncey' s Photographic Carbon Printing. This invention 'of Mr. Pouncey, 

 of England, consists in preparing the paper for printing on, by spreading 

 over it, by means of a hog's-hair brush, a mixture of finely powdered vege- 

 table carbon, and equal parts of a saturated solution of bi-chromate of pot- 

 ash, and a common solution of gum arabic the proportions being one 

 drachm of the carbon to four drachms of each of the solutions. After it is 

 dry, the paper is rea,dy for printing on, by exposure in a printing-frame, in 

 the usual manner the time of exposure being from four to five minutes in 

 the sun, and from ten to fifteen in the shade, but Tarying according to season, 

 character of negative, etc. In washing the picture, it must lie under water 

 for at least five or six hours, when the picture, of which previously scarcely 

 a trace was perceptible, will become visible. The principal difference in the 

 appearance between a carbon print and one prepared with silver, being, ac- 

 cording to Mr. Pouncey, " that one may probably fade, while the other 

 remains imperishable." 



Photographs, for engraving, upon Copper. The following method of taking 

 photographs upon copper, for the purpose of engraving therefrom, has 

 recently been brought out in England by Mr. Colin Smart. 



Take some perchloride of iron, and pour it over a plate of polished copper 

 (such as is used by engravers), when the plate will at once be affected, and 

 the color changed. It is now washed with cold water, and dried with a soft 

 cloth, when it is sensitive to sunlight. If a negative picture is placed upon 



