HISTORY OF BOTANIC ILLUSTRATION. 
153 
reproduced as dots in the finished plate, which is sensitized by 
a compound of fish-glue, chromic acid, bichromate of ammonia, 
and water. Three to ten minutes under the arc-light is enough 
time to print the picture from the negative, and then the unattacked 
albumen is washed off under a spray. At this stage the picture 
is almost invisible ; it is therefore dipped into a solution of methyl 
violet, which stains the film and brings np the picture. The 
plate is next heated by a Bunsen burner, the so-called enamelling, 
till the picture becomes of a rich brown tint, when the heating 
has to be discontinued. The plate is then etched with perchloride 
of iron, at first all over, then treated by stopping out, or cutting 
out the high lights, by special craftsmen, to obtain the desired 
Fig. 23.— Galls on tlie leaves of Salix Cciprea. From Connold’s 1 British Galls,’ 
1904 (reduced in size). Half-tone. 
effect; and mounting on wood, as in other processes, completes 
the task of the workman.*' The four slides now shown display 
the scope of the half-tone process: a group of flowers, from 
a recent number of ‘ The Garden ’; Pecopteris plumosa (a fossil 
fern), and a view of preparations in a botanical laboratory with 
a black background, the last two from the current (1904) volume 
of the 1 Berichte der Deutschen botanischen Gesellschaft 1 ; and galls 
* Plate III, illustrating this Address, has been produced by this process, with 
a screen ruled to 150 lines to the inch. With a screen ruled to the fineness of 
200 lines to the inch, the impression can only be satisfactorily printed on highly 
glazed paper, the surface of which (being coated with China-clay) is perishable. 
VOL. XII.—PART IV. 
11 
