HISTORY OF BOTANIC ILLUSTRATION. 
151 
the use of the camera, and frequently upon the electric arc-light, 
together with the action of bichromate salts upon colloids. The 
drawing is made in liquid Indian ink on card, and from it a dense 
negative is taken. It may surprise some to hear that for this 
the old wet collodion process is still used and preferred, for 
more than one reason; not only can a negative of satisfactory 
density be procured by its means, but as the negative has to be 
reversed the film may be stripped and turned over. There are 
other ways of achieving the reversal of the negative, such as 
placing a right-angled prism or mirror in front of the lens, or 
focussing through the plate exposed. The zinc to form the block 
is sensitized by either a solution of bitumen asphalt in benzol, 
or else a compound of white of egg, water, and bichromate of 
ammonia. This must be dried in the dark, as it becomes sensitive 
to light when dry. The prepared negative is then placed over 
it in a printing-frame, and a few minutes’ exposure suffices to 
oxidise the albumen where the lines permit the passage of light; 
the surface is then rolled with printer’s ink, dipped into a bath, 
and gently rubbed with cotton-wool; the unhardened albumen 
coming off with the ink, leaves the design on the plate in an 
insoluble condition. The back and sides of the plate are then 
varnished, and the first etching takes place, for a few minutes 
in very weak acid. Next the design is inked with a roller, and, 
while the ink is still wet, finely powdered resin or asphalt is 
dusted over the face of the plate, and then heated, so as to melt 
and thus cover the lines of the drawing with a protective film. 
In this way the etching proceeds, the plan detailed being many 
times repeated, till the plate has been bitten to a sufficient depth; 
then the whole of the etching protective is removed, merely the 
tops of the lines are re-inked, and a final immersion in the acid 
bath brings the whole of the lines to a true V-shape. The last 
stage is trimming the plate and mounting it on wood the height 
of type. I have here a slide from a line-block thus made.^ 
One drawback to this process is that it can only be used in 
cases where the drawing to be copied is in positive black and 
white, that is, either in line or dots on the white ground: it 
cannot represent gradation of tone. The method of photogravure 
which I have now to describe does that excellently, but the cost 
of it is too great for the ordinary work of botanic illustration. 
In the following account of the process you must bear in mind 
that there are many modifications of it practised, and known under 
* Plates I and II, which illustrate this Address, have been produced by this 
process. 
