^^<lccU's Curves to Three-colour Work, &.c. 33 



5. Printing with Black. 



As we shall obtain all our colours diluted with white, one is tempted 

 to ask whether printing with a black ink will not enable us to get 

 them pure ; that is, as we can copy any colour, except for the addition 

 of white, by the three printings in yellow, blue, and pink inks, could 

 we not, by the addition of black, match the colours exactly 1 



Suppose it is the red we are matching. We shall get a red with a 

 certain percentage of white. For instance, for 100 parts of red light 

 reflected there may be 10 green and 10 violet. If we now print 

 10 per cent, black what will be the result 1 If the black is a good one, 

 z.<?., if it absorbs uniformly all along the spectrum, we shall now have 

 reflected 90 parts red, 9 green, and 9 violet. In other words, we 

 merely darken the whole, without in the least altering the proportion 

 of white. The addition of black will then not improve this purity of 

 the colours ; it will only make them " dirty." 



6. Production of Colour by successive Absorption of Light. 



There is a very fundamental difference between this and the produc- 

 tion of colour by successive addition, as in Ives's triple projection. In 

 the latter case, if any spectrum colour is thrown on the screen by one 

 light say, the green and the same spectrum colour is transmitted 

 also by a second light red, for instance the amount of that particular 

 colour is doubled, and the result is an arithmetical addition of 

 luminosity. 



But in printing, if a spectrum colour is completely absorbed by one 

 ink say, the pink and also absorbed by another perhaps the blue 

 the total light of that colour absorbed by the superposition of the 

 inks is not twice that removed by a single one. The absorptions do 

 not successively subtract light. If two inks each would separately 

 transmit one-tenth of the light of a given wave-length, the two inks 

 together would transmit not one-twentieth, but only one-hundredth, of 

 the light of that colour. The law is a geometrical and not an 

 arithmetical one. 



In " process " three-colour printing this is of the utmost importance, 

 for there the inks are always printed full strength, and the tint is 

 regulated by the size of the dots, that is, by the percentage area of the 

 paper which is covered by the ink. The dots are produced by placing 

 a ruled screen in front of the negative while it is being exposed, and 

 the dots at any part, when developed and etched, have areas very 

 nearly proportional to the intensity of the light which fell upon that 

 part. Each colour is produced by similar dots, and when the three 

 inks are printed these dots partly overlap one another. As the ruling 

 is very fine and the dots are very closely spaced, it is impossible in 



VOL LXIX. D 



