328 SALTS. 



with a coating of iodide of silver.* A picture or face 

 to be copied being presented before the prepared plate, 

 the light which proceeds from it acts chemically 

 upon the iodide of silver. It decomposes it, to a 

 certain extent, and separates the iodine, thus open- 

 ing the way for the mercurial vapor, which is afterward 

 to be employed. The light has this effect, just in pro- 

 portion to its intensity. That which proceeds from the 

 lighter portions of the face, or dress, has most effect ; 

 that from the black portions, none at all, and that from 

 the intermediate shades, an effect in exact proportion 

 to their brightness. When the plate is afterward ex- 

 posed to the action of the mercurial vapors, they find 

 their way to the silver surface and paint it white, just 

 in proportion as this chemical effect upon the iodine 

 has been produced, and the way has been opened for 

 their admission. The darker portions of the plate are 

 pure silver. They appear dark in contrast with the 

 white amalgam.! 



836. USE OF THE LENS. In taking da- 



What is the 



object of the guerreotypes, a lens is placed between the 

 object to be copied and the plate, in order 

 that the light which proceeds from the former may be 

 concentrated, and its effect thus increased. 



* Bromide and chloride of iodine, are employed to give additional 

 sensitiveness to the plate. The iodide is thus made to contain a por- 

 tion of bromide and chloride of silver. 



f The art of taking portraits from the life by the Daguerreotpe pro- 

 cess, was invented by Dr. J. W. Draper, of the University of New 

 York. 



