i62 SCIENCE PROGRESS 



good results are obtained by using a two-colour process by 

 which a warm tint is added to the original half-tone block. 



Other combinations are sometimes found necessary to pro- 

 duce the best effects or to remedy the faults of the original 

 drawings, but all such cases add very largely to the cost of 

 reproduction, and can only be suggested by the experience of 

 the blockmaker. 



Large Impressions 



Of recent years photogravure printing has been applied to 

 the production of large impressions in what is known as the 

 Rembrandt intaglio process. In ordinary photogravure printing 

 the plates are filled with ink and wiped by hand, and the paper 

 is put over the plate. In this process the plate is prepared with 

 a coarser grain than that of the bitumen dust, obtained by the 

 use of a very fine screen negative. The plate is bent into a 

 cylinder, which rolls over the paper, and leaves a print upon it. 

 The cylinder is inked mechanically and wiped as it revolves 

 under a series of flexible knives which scrape the ink from its 

 surface without removing any from the hollows. It will be seen 

 that this is only possible when the cylinders are turned absolutely 

 true and the machinery is perfect. This process in one form or 

 another is now being adopted by a number of weekly news- 

 papers for their more important illustrations, and an illustrated 

 newspaper abroad is entirely produced by it. The prints at 

 their best are softer and smoother than ordinary photogravures, 

 but lack their brilliancy and depth. The great initial cost of the 

 process is, however, a barrier to its use under present conditions, 

 unless a very large number of prints are required. An initial 

 cost which when spread over 100,000 copies is trifling is pro- 

 hibitive when only 1,000 are required. It is a process which 

 will certainly play a great part in the book illustration of the 

 future. Colour tints are often added to it by combination with 

 other processes. 



Offset Process 



Another process which seems destined to affect the illustra- 

 tion of books is the " offset " process. An ordinary offset is 

 caused when the impression on one sheet of paper is imprinted 

 on another facing it ; it is usually due to over-inking. In this 

 process the impression from the block and the type is not made 

 on the paper directly, but on an india-rubber plate or c} r linder ; 



