NATURAL PHILOSOPHY. 163 



block, but this brittleness has been overcome by a slight wash of var- 

 nish. Engravings produced in this way are light-drawn pictures 

 indeed. In his execution of them, the engraver is freed from the 

 mannerisms, or imperfections of the artist, or mechanical draftsman, 

 escaping on the one hand the dangers of the lack of " life," or the 

 missing of expression ; and on the other avoiding all inconvenience 

 from chance, error, or neglect. 



Since the above was laid before the public, the process has been 

 still further improved, by taking the photographic picture directly 

 upon the wooden surface to be engraved. This is best effected by 

 drying on a coating of lamp-black and white of egg, and varnishing 

 this over with a coat of pure white of egg, before laying on the collo- 

 dion. After collodionizing the wood, it is dipped into nitrate of silver 

 and placed at once in the camera, the picture being subsequently 

 developed, by dipping in sulphate of iron, and nitric acid, washing in 

 pure water, and finally fixing with hyposulphite of soda. To preserve 

 the picture a final coat of mastic varnish is laid on. 



GAUDIX OX ENGRAVING FROM DAGUERREOTYPES. 



DAGUERREOTYPES combine all the necessary conditions for engrav- 

 ing by means of acids, being composed of an unchangeable coating 

 deposited upon a sheet of silver that several chemical agents easily 

 corrode. By this means, the parts protected correspond to the lights, 

 and those corroded, to the shades precisely as the picture requires, 

 and as is the case with aqua fortis engravings ; moreover, the impres- 

 sions taken upon paper would be in their right position. But practice 

 is far from realizing the promises of theory. The reason of this want 

 of success arises primarily from the extreme delicacy of the dotted 

 parts which form the half-tints, and again, from the deficiency of depth 

 in shades of any extent. In fact, an .engraving fit for taking impres- 

 sions is composed of deep furrows, produced, either directly, ty the 

 cutting of the graver, or indirectly, by the interstices of the grain 

 formed on the plate, or by the scraping of the point, as in etching. 

 The shades are owing to the vast number of these irregularities on the 

 same spot, while the biting of the acid on a daguerreotype may plough 

 deep without leaving any irregularities necessary to retain the ink for 

 the impression. For example, if there happen to be in the daguerreo- 

 type open windows, forming shades of some extent, the acid will 

 make a hollow of the same extent, but the bottom of it will be smooth, 

 and the ink will scarcely adhere. M. Donne was the first who thought 

 of engraving daguerreotypes. His first attempts were very remarka- 

 ble, and no one doubted their ultimate success. He employed a very 

 simple and expeditious process. Over a daguerreotype taken on a 

 double-silvered plate and carefully washed with hyposulphite of soda 

 and afterwards finished with distilled water, he poured a mixture 

 composed of three parts of pure nitric acid and four of distilled water, 

 having first covered the edges of the plate with a thick coat of engra- 

 vers' wax, so as to make a flat disc of it. In the course of a short 

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