REPRODUCTION OF ILLUSTRATIONS 



If, therefore, a copper or zinc plate were covered with 

 a film consisting of a mixture of fish-glue, albumin, 

 and bichromate of potash, and portions of it so covered 

 that they were not acted upon by light, while other por- 

 tions were exposed to it, it was found that by treating 

 this plate chemically and then placing it in an acid bath, 

 the portions exposed to the light retained their original 

 surfaces while the covered portions were eaten away. 

 In other words, an engraving could be made in this 

 manner. It made no difference as to the size of the sur- 

 faces covered or exposed to the light, a thin line being 

 protected against the attack of acid by the hardened 

 bichromate mixture as readily as a white blotch. If, 

 for example, lines in black ink were drawn upon a 

 surface of glass, and this glass placed over a sensitized 

 copper plate and exposed to light, the lines would ap- 

 pear as depressions in the plate when treated with 

 the acid, as the ink would protect the thin film of the 

 sensitized medium from the light. On the other hand, 

 if this drawing upon the glass were reversed so that 

 the lines made by a dry pen or point appeared as 

 transparent lines, like scratches on smoked glass, such 

 lines, allowing the passage of light, form hardened 

 lines in the bichromate mixture, and, when the plate is 

 treated with the acid, appear as raised surfaces like 

 the printing-lines of a woodcut. In short, if the artist's 

 drawing were scratched upon a glass covered with an 

 opaque substance, an engraved reproduction of his 

 drawing could be made upon zinc or copper plates by 

 the process just described. 



In actual practice such a method of drawing is 

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