Maxwell's Curves to Threc-colmir Work, &c. 41 



and (supposing for the moment the plate to be equally sensitive to 

 all colours) will be proportional to the final opacity of the plate. If 

 the filter curves have been arranged so as to match the spectrum in 

 luminosity as well as hue, then the resulting print will reflect the sum 

 of light which is equivalent in ray composition to the sum of this 

 band of spectrum colour, and will therefore match it in dominant 

 hue. But if the spectrum has been matched in hue only, the 

 resultant colour will be the sum of the colours in this band mixed 

 in the wrong proportions, and will not have the same dominant hue 

 as the original. 



10. The Light-filters. 



If the inks are neither perfectly transparent or perfectly opaque 

 to each colour, so that there are no parts of the spectrum to which 

 they are only partially transparent, and if also the absorptions do not 

 overlap, the resultant colour will be that left after successive sub- 

 traction by the three inks, and will be the same as would result from 

 the successive addition of the three complementary lights in triple 

 projection; and then the filters can be found by determining the 

 dominant hue of the complementary colour, t 



The curves for the filters can also be calculated in this case by 

 finding what percentage area the inks should cover so as to transmit 

 light whose composition, in sums of the colour sensations, is everywhere 

 that given by Maxwell's curves. This is how I calculated the curves 

 for the hypothetical inks. 



But when we deal with practical inks these conditions are far from 

 being fulfilled, and the filters cannot be found by dealing with the 

 complementary lights. For, as we have seen, since the same colour is 

 absorbed, at least to some extent, by more than one ink, the arith- 

 metical law of absorption will not hold, and the colour resulting from 

 the printing with the inks is not that produced by the combination of 

 the complementary lights. So that in practice we shall have to adopt 

 some other method, which must be experimental, and must be founded upon 

 impressions of the actual inks. 



11. Depth of Colour and Process Printing. 



One of the greatest practical difficulties in printing with only three 

 colours, and especially with such strong colours as red and blue, is to 

 ensure that the quantity of ink shall not vary from impression to 

 impression. Usually, very slight variation in the amount of ink is 

 accompanied by a change in the whole tone of the picture. This is 

 because with most inks the curve of absorption is different for different 

 depths of colour, and thus the proportion of the three fundamental 



t Abney, 'Photographic Journal,' January, 1900, p. 121. 



