rHOCEEDlNGS OF THE THIRD ENTOilOLOlilCAL MEETING 1043 



86.— THE PREPARATION AND REPRODUCTION OF SCIENTIFIC 

 ILLUSTRATIONS. 



Bii A. \V. Slater (Mutuu/er. Calcutta Pholotijpe CompcDuj). 

 (Plates 170—179.) 



There are many processes used in reproduction for printing, but in 

 ilie present day photo or process engraving has quite Superseded all 

 other methods, and it is quite rare to find any publication in which 

 the illustrations are produced other than by a photographic process. 



It is not intended in this article to give a history of the art of repro- 

 duction or a detailed treatise on every stage of each process but just 

 a general outline of the jJi'ocedure in the principal processes used and 

 to point out from the engraver's side little points that should be thought 

 of in preparing originals or in the choice of the process to be employed. 



Nearly all illustrations are in those days produced by either Line 

 engraving, Half-tone engraving or Three-colour engraving. 



These all come under the heading of process engraving which is 

 the process by which drawings or photogi-ajjhs are, by the aid of photo- 

 graphy, transferred to metal, afterwards being etched by chemical 

 action, the result being a block from which a very large mimber of 

 copies can be printed. 



The choice of the process to be used must of necessity depend on 

 the nature of the drawing, photo or picture to be reproduced. 

 Line Blocks. 



Line engravings or line blocks are made from pen and ink sketches, 

 i.e., black and white drawings in hne, without any shading except that 

 obtained by the varying thickness of the line of the drawing. Before 

 considering the points which go to make a good original a brief descrip- 

 tion of the process will be interesting. 



In the first place a negative is made somewhat similar to an ordinary 

 photographic negative except that the wet plate process, in which the 

 operator prepares his own plate, is used. 



This negative must be perfectly black and white with clear white 

 lines on a black ground. It is at this stage that the attempt to reproduce 

 the grey or faint lines, described later, afiects the result as, in the 

 endeavour to clear the negative by chemical action, all the lines are 

 liable to lose their true value and thicken up. 



The negative is then printed, a piece of sensitized metal (usually 

 zinc) being placed into contact with the negative in a printing frame 

 and exposed to the action of the light. The effect of the Ught passing 

 through the clear parts of the negative is to make the coating on the 

 plate insoluble in water. When sufficiently exposed the plate is covered 



