154 
R. DAYDON JACKSON-ADDRESS: 
on the leaves of sallow, S a lice Caprea , from Connold’s new book 
on British galls, plate 110. With regard to the last plate notice 
the pseudoscopic effect due to the negative having been taken 
with the light from below, or more probably, the specimens 
inverted; on reversing the slide in the lantern you see that the 
galls now show as prominences, whereas before they resembled 
hollows (see Fig. 23).* 
While the screen produces the smoothest and best result, it is 
not indispensable. Here is a slide from a German source; the 
drawing has been made on a paper with a strong grain, with 
the outline and lettering in ink, and the shading in lithographic 
chalk. This is then transferred to a lithographic stone, so as to 
preserve the actual drawing; the rest of the process is by transfer 
and as in half-tone or line blocks. It is a simple way of producing 
a given result, but as the work is shallow it is apt to be soon 
clogged in printing. There are many kinds of grained paper used 
for this method, according to the fancy of the draughtsman, but 
the coarser patterns make a clean outline or delicate lettering 
almost impossible. 
An older process than some of those just detailed is that usually 
termed in this country Collotype or Heliotype. It is based upon 
the well-known property of light on bichromatized gelatine, that 
of hardening it and at the same time rendering it insoluble in 
water. The printing-plate is a piece of glass about half an inch 
thick, ground on one side the better to hold the film. On this 
is spread a basis of albumen, water-glass, and water, over which 
is laid a fluid film of gelatine, albumen, and bichromate of potash 
in water. This compound film has to be dried by heat, requiring 
about four hours in a special oven to effect this ; when cool it is 
ready for receiving the print. The margins are masked by strips 
of tinfoil, and, the negative being applied, the printing takes 
place, requiring from twenty minutes to half an hour ; but the 
progress cannot be looked at during the printing. Next follows 
washing for half an hour, to remove all traces of unutilised 
bichromate, and then the plate is dried naturally. Printing is 
usually done in a lithographic press; the film is wetted with 
a solution of salt, glycerine, and water, and allowed to soak 
a short time, then inked by a leather roller which inks the 
stronger parts of the picture, and this is followed by a gelatine 
* This was photographed through a screen ruled to 200 lines to the inch, this 
extreme fineness being necessitated by the large plate from which it was taken 
being already in half-tone. It is a severe test of the skill of the producers of the 
block, Messrs..Andre & Sleigh, and of our printers. See preceding footnote. 
