74 
The modern processes had been rendered possible by the 
discovery in 1814, by Niépee, that bitumen prepared in a certain 
way became insoluble after being acted on by light. The modern 
process had revolutionized the appearance of our books and 
periodicals. The great bulk of the reading public had probably 
not noticed that any change had taken place. The term “ half 
tone’ conveyed no meaning to the general public. The “ line 
process ”’ was the antithesis of ‘‘ half tone.” But there was no 
real difference between a ‘‘line’”’ and ‘half tone” block. The 
two processes could be and were frequently used on the same 
block. They differed only in method of production. After 
explaining these methods and the wonderful effect of the ruled 
glass screen and dots, examples were shown of the “ half tone” 
process, by whose aid the printing of a scene or object had been 
brought out of the region of laboratory experiment and made 
commercially successful, so that we had now our books and 
magazines illustrated in very fair approximation to nature’s 
colours. The beautiful collection of colour pictures (exhibited) 
showed a c uns‘ erable degree of success by the workers in this, 
the latest phase of book illustration. He was expecting before 
long, to be able to make coloured lantern slides with as much 
facility as he did the black-and-white ones. The process was 
known as the “ three-colour-process,” a photographic method 
which selected the primary colours, each on a separate plate, 
from a coloured object, so that printing-blocks might be made 
from the three negatives, and prints from these blocks made in 
yellow, red, and blue ink; one impression over the other, on 
white paper; the coloured original being thus reproduced in its 
natural hues. The process was based on the fact that any 
colour of the spectrum conld be matched by mixtures of three 
colours of the spectrum itself—red, green, and violet. For each 
of the three negatives required for the set, a piece of coloured 
glass of the exact corresponding shade, and called a ‘colour 
filter,” was placed either before or behind the lens. The function 
of these filters was to sort out the relative proportion of the 
three primary colours in the object photographed. He believed 
the three-colour process would be an immense factor in the 
illustrative work of the not very distant future. The work would 
certainly improve owing to the more perfect means of colour 
selection, and more skill in the block-making and printing ; while 
ink makers could do a great deal by providing the more perfect 
pigmentary inks. The illustrations of the immediate future, 
whether for books, periodicals, or commercial catalogues, would 
undoubtedly be in colour, and he hoped they would now be 
better able to appreciate the difficulties and the triumphs of this 
most wonderfnl process. 
