NATURAL PHILOSOPHY. 183 



" Carbon-printing," as introduced into the art of photography by 

 Mr. Pouncey, involved the use of the bichromate of potash and gela- 

 tine mixed with finely-divided carbon. Those portions of a surface 

 so prepared which were screened from solar influence would dissolve 

 entirely off; whereas the parts which had undergone actino-chemical 

 change remained permanent, giving a black picture. 



These are the steps which have led onward to the process " pho- 

 to-zincography," which has been brought to a high degree of per- 

 fection by Col. Sir Henry James, of the British Ordnance Survey, 

 who has recently published a work giving a full detail of all the steps 

 of the operation, with illustrations of the results obtained. The pro- 

 cess is substantially as follows : " If a solution of gelatine and bi- 

 chromate of potash is spread on paper, and when dry exposed to light 

 under a negative of an engraving or a plan, the lines of the drawing 

 will be represented by insoluble lines on the coated paper ; while the 

 ground, having been protected from the action of the light by the 

 dense negative, will remain soluble. If the paper is now coated with 

 greasy printer's ink and damped at the back, the soluble parts will 

 swell and the lines will be in intaglio ; and when rubbed gently with 

 a sponge dipped in gum and water, the ink overlying the soluble parts' 

 (now again viscid and in a dissolving state) will be removed, while it 

 will adhere to the insoluble parts. The engraving or plan will then 

 appear in black ink, which can be transferred to the surface of zinc 

 or stone. This is the gist of the whole process ; but, though it ap- 

 pears so simple, in practice it requires care and judgment, and many 

 difficulties were met and overcome before very good results were 

 obtained." 



Among the illustrations of photo-zincography given in Sir Henry 

 James ' book, there is, for example, a copy of a page from the folio 

 Shakspeare of 1623, which shows that rare and valuable printed 

 books may be reproduced with unerring accuracy. There is also a 

 page of the Domesday Book, which proves that by photo-zincography 

 the most faithful copies can be obtained of ancient manuscripts. Two 

 of Piranesi's engravings of antique vases, a part of one of the 

 engravings by Volpato of the panels in the Vatican painted by 

 Raphael, and a reduced copy of an engraving by Dorigny, from 

 the original, by Raphael, show how truthfully, with every touch of 

 the original engraver, such works can be reproduced. 



When we remember the facilities which are offered for copying, 

 and varying the scale as we please, of any printed or written matter, 

 of any map or plan, or of any engraving, we must be convinced of 

 the value of photo-zincography, 



PHOTOGRAPHY AND FORGERY. 



We derive from the London Photographic News the following arti- 

 cle on the above subject : The facilities afforded by photography, 

 and more especially by photo-lithography, for effecting forgeries of 

 bank notes and other documents, appear to have been considerably 

 overlooked by those who are, or should be, most concerned. The 

 sources of danger have been looked for in other directions, and it is 

 from the imitative skill of the skilful engraver that counterfeit pro- 



