PROCESSES IN ILLUSTRATIONS 159 



chloride, which eats through the gelatin on the plate, its 

 action being carefully supervised by the workman in charge. 

 It is obvious that there is little mechanical certainty in the 

 working of this process, and not uncommonly several plates 

 have to be made to get a successful result. Each reproduction 

 has to be printed separately, as all copper and steel plates are, 

 and it is usual to employ a thick paper called " plate paper." 

 In printing the paper is damped, and forced into the hollows of 

 the plate, which has to be inked and wiped by hand. The cost 

 may be assumed as a minimum of from two to three guineas per 

 plate for a crown or demy 8vo book, according to the difficulty 

 of the subject and the skill and standing of the engravers who are 

 employed to finish the plates. Among these latter are, at least 

 in the case of one firm, exhibitors at the Royal Academy. 



Colour Processes 



If it is desired to reproduce an illustration in its original 

 colours a variety of processes are now at command. Though 

 chromo-lithography with a dozen or more printings for each 

 plate was, up to the last twenty years, the only means of re- 

 producing a picture, at the present time very good results are 

 obtained by the three-colour process, which is founded on the 

 fact that by the combination of the three primary colours — red, 

 green, and violet — any other colour may be produced. A pig- 

 ment must be distinguished from a colour : it is a substance 

 which absorbs all the coloured light that falls upon it, except 

 its own. The primary pigments are yellow, red, and blue, and 

 a perfect combination of them should produce black, just as a 

 perfect combination of primary colours should produce white. 

 If, therefore, three photographs could be obtained, one showing 

 only the blue elements of the coloured object, a second showing- 

 only the red, and a third showing only the yellow, and im- 

 pressions from each of them in a pure pigment combined into 

 a single picture, the colours of the original would be repro- 

 duced. All modern colour processes may be said to depend 

 on this principle. 



In the ordinary three-colour process three half-tone blocks 

 are made, one each for the yellow, red, and blue, all the other 

 colours being strained off by a light filter in the camera as each 

 negative is being made. The manufacture of the negatives 

 requires the most skilful manipulation, and the slightest defect 



