138 ANNUAL OF SCIENTIFIC DISCOVERY. 



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to light, it is not reproduced. Certain engravings which have been exposed 

 to light are reproduced better than others, according to the nature of the 

 paper; but all kinds of paper, even the filtering paper of Berzelius and the 

 papier de sole, with or without a photographic design, and others, are repro- 

 duced more or less perfectly after exposure to light. Wood, ivory, parch- 

 ment, and the living skin, are reproduced perfectly under the same circum- 

 stances ; but metals, glass, and enamels, are not reproduced. If an engraving 

 is exposed to the rays of the sun for a very long time, it is saturated with 

 light ; and the intensity of the impressions obtained by contact in darkness 

 is so great, that M. Niepce hopes to arrive at a process by which, operating 

 upon very sensitive papers as paper prepared with the iodide of silver, for 

 example, or upon the dry collodion or albumen tablets, and developing the 

 image with gallic or the pyrogallic acid to obtain proofs sufficiently vigor- 

 ous to form an original, from which impressions may be taken. A new 

 means for reproducing engravings will thus be secured. 



Further results of M. Niepce's experiments are described by M. Chevreul 

 as follows : 



If we interpose a plate of glass between the engraving and the sensitive 

 paper, the whites of the engraving are no longer impressed upon it. The 

 same interruption of the radiations takes place if we interpose a plate of 

 mica, or a plate of rock-crystal, or of yellow glass stained with the oxide of 

 uranium. We discover, further, that these substances arrest equally the 

 impression of the phosphorescent rays when placed directly in front of the 

 sensitive paper. 



An engraving covered with a film of collodion or of gelatine, is reproduced ; 

 but an engraving covered with a layer of varnish or of gum, is not repro- 

 duced. An engraving placed at three millimetres' distance from the sensitive 

 paper, is very well reproduced ; and if the design is of a bold character, it 

 will be reproduced at the distance of a centimetre.* The impression is 

 not, then, the result of action of contact, or of chemical action. A colored 

 engraving of many colors is reproduced very unequally ; that is to say, the 

 colors imprint their image with different intensities, varying with their 

 chemical nature some producing an impression which is very visible, 

 whilst others scarcely tint the sensitive paper. 



It is similar with characters printed with different inks. Printers' ink, 

 whether it be such as is used with type or for copper-plate printing, and the 

 ordinary writing ink, formed of a solution of nutgalls and sulphate of iron, 

 do not give images ; while certain " English inks give impressions sufficiently 

 sti-ong." Vitrified characters, traced upon a plate of varnished porcelain, 

 or covered with enamel, are imprinted upon the sensitive paper without the 

 porcelain itself leaving any trace of its presence ; but a porcelain not covered 

 with varnish or enamel, such as biscuit china or " la pate, de, kaolin," produces 

 a slight impression. 



If, after having exposed an engraving to the light during one hour, we 

 apply it upon a white card which has remained in darkness during some 

 days, and if, after having left the engraving in contact with the card during 

 twenty-four hours at least, we put the card in, its turn in contact with a leaf of 

 sensitive paper, we shall have, after twenty-four hours of this new contact, 

 a reproduction of the engraving, a little less visible, it is true, than if the 



*The millimetre is 0-03937 of an English inch. The centimetre is 0-39371 of an 

 English iiich. 



