1 60 SCIENCE PROGRESS 



or inaccuracy is fatal to a good result. When the blocks are 

 completed proofs are taken, which are supplied with them to 

 the purchaser. These are usually a proof in the proper yellow 

 ink of the yellow block, a progressive proof of the red block 

 in red, and another of it superimposed on the yellow impression, 

 and of the blue block in blue, and another as printed over the 

 yellow and red. Between each of these printings several hours 

 must elapse. These proofs are the models for the colour printer 

 to work up to. 



When making water-colour sketches for reproduction the 

 drawing should be kept within well-defined limits, avoiding 

 vignetting. The colours should not be mixed with white, and 

 Chinese white should never be used for the high lights. 



With good blocks and good printing every colour and every 

 shade of colour in the illustration can be reproduced with great 

 exactness, though any three fixed colours will not suit equally 

 all subjects, while the paper on which it is printed must have 

 been in store for a considerable time to become thoroughly 

 seasoned. 



The price of blocks made from a painting or sketch may 

 be taken as about 35. per square inch (15. each block), with a 

 minimum of about £3. If the photographs have to be made 

 directly from the objects themselves the cost would be at least 

 one-third more, say 4s. per inch. 



The principle of making three negatives by the light filters 

 for the primary pigments can be applied to the collotype process 

 with the most exquisite results, some of the most artistic repro- 

 ductions of the day being made by pure collotype methods. 

 Coloured photogravures, like engravings, are inked in colour 

 by the hand on the plate itself, and are so extremely costly as 

 to be usually out of the question for book illustration. 



Lithography 



Before passing to some modifications of the colour processes 

 in use we must glance at the method of lithography. By it we 

 are able to produce facsimiles of drawings made on a prepared 

 transfer paper, or directly on a surface of zinc or stone. The 

 drawing is made with a fatty substance, which combines with 

 the surface of the stone, which is then damped. When an inked 

 roller is passed over the stone the lithographic ink does not 

 adhere to the moist parts, but only to the dry marks of the 



