152 
B. DAYDON JACKSON - -ADDRESS : 
as many different names. A plate of polished copper is first 
prepared with a grain to receive the picture, usually in this way. 
A box containing powdered asphalt has its contents agitated by 
a rotary brush within it, saturating as it were the air in the box; 
after the lapse of a few seconds to allow the grosser particles to 
settle, a door in the side is opened and. the plate put in for 
a definite time, usually three minutes. When the plate is with¬ 
drawn it is slightly heated over a Bunsen burner so as to cause 
the fine particles of asphalt to adhere to the plate, but not to run 
together. A somewhat similar effect may he obtained by spraying 
with a solution of resin in spirit, or by the old aquatint ground 
laid with a brush, which ground on drying shrinks into innumerable 
tiny cracks. 
The picture is obtained from a glass positive, or more generally 
a carbon transparency is especially made from the chosen negative ; 
this is put in the photographic printing-frame, then a mask to cut 
off clean edges all round the print, and above it another piece 
of carbon tissue, sensitized with bichromate salts and gelatine, 
to form what is termed the gelatine resist. When finished, it is 
carefully pressed down upon the previously-prepared copper plate, 
and then placed in water, when the paper hacking of the tissue 
can be removed, and the soluble gelatine washed off, leaving only 
the oxidised portion on the metal. The hack of the plate being 
coated with varnish, it is put into a hath containing a weak 
solution of perchloride of iron;, several bitings are required of 
varying strengths, and the resources of the etcher are brought 
into play, such as stopping out to prevent further action of the 
etching solution, removing the ground in places for deeper biting, 
and the like. On washing off the protective film, the plate is 
proved by the printer. 
The plate (Kerguelen Land, from Chun’s ‘ A us den Tiefen des 
Weltmeeres,’ 1903) here shown does not give the most satisfactory 
view of this process from a botanic point of view ; indeed, the 
botanic interest in the plate before you is hut slight, and is confined 
to the patches of Azorella in the foreground. 
The process far more commonly employed for wash or half-tone 
drawings is produced by means of a screen of finely ruled glass 
placed j ust in front of the plate in a camera. These screens are 
the key to the whole process, and consist of two, cemented together 
at right angles to the ruling, thus cutting up the field into minute 
squares. The glass is ruled by machine, from 70 lines to the 
inch for common journalistic work, to 200 to the inch for the 
finest results in magazine work. The squares of the screen are 
