1046 PROCEEDINGS OF THE THIRD ENTOMOLOGICAL MEETING 



stage of the process and it is almost impossible for a workman to keep 

 details which are only slightly discernable on the copy. 



Photographs of subjects with a lot of fine detail should be taken in 

 a good light and a white background used, care being taken to avoid 

 shadows. A good well-diffused light is preferable to sunlight. Avoid 

 unnecessary reduction in the case of originals with a lot of fine detail ; 

 on the other hand, a good bold subject will be improved by reduction. 



Photographs should be packed flat and should not be roughly 

 pasted on paper. Negatives are not necessary if good clear prints are 

 available. 



Three-Colour Engraving. 



The three-colour process is an adaptation of the half-tone process 

 and the procedure is the same except that special colour-sensitive plates 

 are used and greater skill is needed at all stages. 



The principle of the three-colour process is based on the theory that 

 all colours are composed of yellow, red and blue, which, while not 

 scientifically true as applied to light, is nearly correct when applied to 

 inks. 



Three-colour negatives are, briefliy, made as follows : — Each negative 

 retains only those rays of the desired colour. Thus the negative showing 

 the yellow values excludes the blue and red rays. The yellow rays 

 are excluded from the red and from the blue negatives. The rays not 

 wanted are cut off by means of Ught filters. The etching of three- 

 colour illustrations is similar to half-tone etching. 



Three-colour work, as the name implies, requires three printings. 

 First the yellow plate, then the red, lastly the blue, each colour " regis- 

 tering " on the previous impressions, thus completing the picture in 

 its natural colours. 



To print three-colour illustrations is an exceedingly delicate task. 

 The amount and colour of the ink must be carefully regulated and, 

 as many impressions are usually taken, the printer must be cautious 

 to maintain uniformity. As the colours fit exactly on top of one another 

 the merest stretching or shrinkage of the paper will be disastrous to 

 fine work. 



Originals for this process should be in colours and complete in all 

 detail and should be made on smooth card. The degree of reductiou 

 depends entirely on the original. A very well drawn original with a 

 lot of fine detail should be reproduced to the same size but as a rule it 

 is best to make the copy for reduction to two-thirds. 



