1878.] to the Bcproduction of Maps and Ttans. 1 18 



Saturated solution of bichromate of ammonia,... 5 drachms. 



Honey, 8 „ 



Albumen, 3 „ 



Water, \\ pint. 



When the coating is thoroughly dry, the plate is exposed to light 

 under a transparency. A large camel's hair brush is charged with a mix- 

 ture of the two finest kinds of emery powder, and applied with a circular 

 whisking motion all over the surface. The powder attaches itself to those 

 parts of the plate on which the light has not acted, precisely in proportion 

 to the amount of protection they have received, owing to the unaltered 

 parts of the film attracting moisture and becoming 'tacky.' The most 

 ' tacky' parts, forming the deep shadows of the picture, will attract the 

 coarsest particles of the emery, the finer parts will take finer emery and the 

 highest lights will take none at all. When the picture is fully developed, 

 it is placed face to face against a polished plate of softer metal and passed 

 between a pair of rollers, so that the emery powder image may be indented 

 into the polished metal. The plate is then burnished in parts by a skilled 

 engraver and when the desired result is obtained, is printed in the usual 

 way in the copper-plate press. 



This process seems capable of rendering useful service in the repro- 

 duction of maps. 



Scamoni's method. — The last method is that recommended by Herr 

 G. Scamoni, of the Imperial Russian State-paper Office, and is fully described 

 in his ' Handhuch der Heliogra^hie' already referred to. The results are 

 exceedingly good, but the process is only suitable for line work. 



Herr Scamoni having obtained a suitable negative of a drawing or 

 other line subject, makes a positive copy of it in the camera by the wet 

 collodion process, and after fixing, treats this positive with various successive 

 intensifying solutions, so as to give it a very sensible relief. After drying 

 the plate is varnished with a thin varnish and coated with fine plumbago, 

 after which it is electrotyped in the usual way. 



In the specimens I have seen of this process the lines are exceedingly 

 sharp and fine, and it would seem well adapted for map-work. 



A photo-mechanical process has lately been introduced by Messrs. 

 Aubel and Kaiser of Lindenhohe, near Cologne, and called * Aubeldruck.' 

 The results for line-work are very superior but the process is a secret. 

 It is believed to consist in some method of etching a glass negative, so that 

 prints may be obtained direct from the glass surface or by transfer to stone. 



In all cases where the printing-plates are obtained by electro-deposi- 

 tion of copper, and many copies are required, it is necessary to protect the 

 engraved surface of the plate with a coating of iron by the process known 

 as ' acierage' or ' steel facing.' This enables a very large number of copies 



