4 
should also be well proportioned as regards their relative colouring power. 
The yellow, being less transparent than the other two colours, should 
be printed first. The map paper should be perfectly white, free of chem- 
icals, and well calandered. 
For average maps, perfect colour results may be obtained in one, two, 
or three printings, but for intricate maps it is in some cases advisable to 
make use of a fourth colour printing — grey for instance, which will har- 
monize with the other colours, For the reproduction of special features 
that require to be shown prominently, a still larger number of printings 
become necessary; in such cases, so-called body pigments — pure vermilion, 
ultramarine, gamboge, etc. — may be used. Ruled tints in body pigments 
are not, on account of their opacity, altogether satisfactory in combination 
printing. 
Difficulty is in some cases encountered when a fixed series of colours, 
must be rigorously followed. The process is then only partly applicable and 
flat printing in separate press runs has to be resorted to. It is seldom, 
however, that a scheme of colouring — that is to say, the selection of colours 
— is so rigid that by judicious substitutions the process is not adequate. 
Some advantage is gained, in certain cases, in modifying the hue of the 
primary pigments by admixture in small proportions, of other colours or 
black. 
The advantages of this process over ordinary lithographic colour 
printing, particularly when large editions of maps are to be printed, are, 
mainly, uniformity of results, accurate registration or fitting of colours 
owing to the paucity of printings, and economy in the cost of production, 
the press work being reduced to a minimum in time and labour. 
REFERENCES 
Abney, Capt. W. de W Colour measurement and mixture. 
“ “ Colour vision. 
Chevreul, M. E The principles of harmony and contrasts of 
colours. 
Earhart, J. F The harmonizer. 
Horgan, Stephen H Halftone and photomechanical processes. 
Luckiesh, M.. Colour ard its applications. 
Taylor, J. Scott Field's “ Chromatography for Artists.” 
Rood, Ogden N Colour, 
Zander, C. G. Photo-trichromatic printing. 
The accompanying chart. Plate IV, shows the result of superpositional 
printing in the three fundamental colours, yellow, blue, and carmine, in all 
possible combinations, from three lithographic stones prepared, each with 
one full tint and one line-tint. The twenty-six colour distinctions thus 
produced have, in practice, been found to meet all ordinary requirements 
for geological maps. 
