No. 56. — 1905.] PHOTOGRAPHY OF COLOUR. 
439 
The most essential feature in the practical application 
of this three-colour photographic process is necessarily 
the acquirement of the three images possessing the parti- 
cular colour values required for the composition of the 
perfect picture. These three colours may be selected from 
slightly varying areas of the solar spectrum, but the most 
accurate results have been obtained when shades of 
Prussian blue, majenta pink, and bright yellow have been 
used as the component colour factors. These three accurately 
tinted colour positives having been obtained, it is only 
necessary to superimpose them, sandwich fashion, in perfect 
registration to produce the finished natural colour picture. 
As compared with the Lipmann system recently referred 
to, the natural colour pictures built up by this trichromatic 
method are necessarily the result of an indirect process. 
That is to say, the coloured pictures produced are not taken 
directly in the camera, but in the form of monochrome 
or black and white negatives which yield complimentary 
monochrome positives which possess the requisite light and 
shade or intensity values of the three enumerated tints. 
These tints have to be imparted to the positives by dip- 
ping them into suitable dyes of the three respective colours, 
blue, pink, and brilliant yellow. 
In order to obtain the three positives possessing these 
respective colour values for the composition of the com- 
plete synthesis it is necessary to employ special colour 
♦ screens when taking the negatives that respectively absorb 
all the supplementary rays of the solar spectrum and only 
permit those three required for the composition of the 
resulting positives to pass through. Thus, when obtaining 
the negative for printing the Prussian blue positive a 
spectrotopically adjusted screen has to be employed that 
absorbs every other colour of the spectrum excepting this 
particular, blue tint. This screen has consequently to be of 
a ruby red hue. In like manner for the negative which 
only allows the majenta pink colour value to be represented, 
a green screen absorbing all the blue and yellow rays is 
