18e7.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 715 



points of various sizes, which reaiove the fihn at every stroke. The 

 result is a photographic negative, iu which the transparent lines are the 

 direct work of the artist, instead of a facsimile made by the sun. This 

 negative is used like any other negative for the production of sun prints 

 on sensitized paper. 



Frame 33.— Xo. 1G3, by Corot. Xo, 1G4, by James Hamilton, of 

 Philadelphia (1819-1878). 



13. MACHINE ENGRAVING. 



Machines are largely used for ruling skies and other parts of engrav- 

 ings, either iu straight or waved lines. In the engravings here shown, 

 Xos. 165-167, the medallions are entirely the product of a machine. A 

 point is made to travel over the medallion of which an engraving is to 

 be made, and by an ingenious arrangement a second point, governed by 

 the movements of the hrst, traces a series of lines, now nearer together, 

 now farther apart, according to the variations of the original, upon a 

 metal plate covered with an etching-ground. The drawing thus ob- 

 tained is then bitten in. A number of such machines have been in- 

 vented — one by Achille Collas, Paris (patented about 1830); another 

 by Joseph Saxton, in the United States. 



Frame 34.— No. 165, by A. Collas. No. 166, by Joseph Saxton. 

 No. 167, from Auer, " Der polygraphische Apparat," Vienna (1853). No. 

 168: The ornamental lines in this specimen are also the result of a 

 machine, the Guilloche machine. From Auer. . 



14. GALVANOGRAPHY, 



A picture is painted upon a metal plate, with colors laid on thinly iu 

 the lights, and more and more thickly as the shadows increase in depth. 

 The colors used must dry with a lusterless or granulated surface. Pen- 

 and-ink and crayon drawings can be made in a similar way. The fin- 

 ished picture, slightly black-leaded if necessary (although this, it is 

 claimed, is only occasionally the case), is then placed into an apparatus 

 for electrotypiug, and the copper deposit obtained is used as a printing 

 plate on the roller press (intaglio printing). Invented by Franz von 

 Kobell, 1839. 



Frame 34.— No. 169, by F. von Kobell. From his "Die Galva- 

 nographie," 2d ed., Munich (1846). No. 170, by Schouinger, of Vienna^ 

 from the same treatise, llouletting and galvanographj' combined. 

 No. 171, from Auer (1853). 



