442 JOURNAL, R.A.S. (CEYLON). [VOL. XVIII . 
or in combination with a modification of the customary 
lantern slide formula. In the former instance the prints 
from the three negatives are made on three bichromated 
films, washed out in hot water, and then dipped into their 
respective dyes. It is found most convenient, however, to 
use glass as the main supporting basis, and as the requisite 
possession blue tint can be most effectually obtained by the 
chemical conversion of an ordinary monochrome lantern 
slide from black to blue by what is known as the ferro- 
prussiate process, this chemical method may be niost 
advantageously adopted for the production of this 
print. 
Some small amoimt of personal judgment and tentative 
experiment is necessarily required in order to arrive at the 
precise shade of the several tints of the three colours 
employed, but as soon as a correct standard has been 
obtained the multiplication of duplicates becomes a mere 
matter of mechanical detail. In order to impart to the com- 
pleted picture the maximum of transparency the three 
positives may be varnished and amalgamated with 
Canada balsam into one optically homogeneous whole. 
In addition to the composition of lantern slides, larger 
transparencies can be readily constructed on this three-colour 
photographic system by either the direct method or by the 
ordinary enlarging process. It is also possible from the 
original three negatives to construct process blocks from 
which pictures in their natural colours can be printed on 
paper. The chief difficulty encountered in this direction 
is the acquisition of sufficiently pure coloured and yet 
transparent printing inks, but this difficulty has been sur- 
mounted to such an extent that many English and European 
printing firms are now substituting this three-colour print- 
ing method for the much more complicated and expensive 
one of chromo-lithography, in which as many as eighteen or 
twenty or more lithographic stones are required for pro- 
ducing less correct effects than are obtainable with three 
photo-process works. 
