1901-2.] Photography in Natural Colours. 381 



pass much broader bands of the spectrum than the latter. This 

 distinction is very important, and was first pointed out by Mr. F. E. 

 Ives, of Philadelphia, perhaps the most familiar name in the literature 

 of the three-colour process. It is mainly owing to his genius and 

 perseverance that photography in colours occupies the position it does 

 to-day. The necessity for this distinction can perhaps be most clearly 

 seen in attempting to make a colour photograph of the spectrum. If 

 the reproduction glasses are used as taking filters, the evident result 

 will be three narrow isolated bands of colour instead of the continuous 

 spectrum ; while if the taking screens are used as reproduction glasses, 

 unnecessary impurity and degradation of colour will result from the 

 mixture of colours, other than the primaries, in the taking filters. 



The negatives obtained through these filters will not be alike, but in 

 some cases the differences are not marked. This, of course, is due to 

 the fact that the colours in nature are, in general, of a very complex 

 character, and pass, when analysed by the spectroscope, nearly all the 

 spectrum colours ; hence all three negatives will frequently be 

 influenced by the same colour though not of course to the same degree. 

 The projection of three such negatives side by side upon the screen 

 [s/iozvn] will illustrate, more fully than can be described, the various 

 points of difference between them [Plate]. The differences are most 

 marked in negatives of the spectrum and least marked in outdoor 

 subjects where the colours are, as a rule, very impure or mixed in 

 character. The purer the colours the more the filters differentiate them 

 into their three classes, and the greater the differences in the negatives. 

 The negatives may be called colour-record negatives, and are quite 

 similar in appearance to ordinary negatives [Plate], possessing no colour 

 whatever in themselves. The slight variations in the densities of the 

 corresponding parts are the means employed, in the various methods of 

 synthesis, of obtaining the positives in colours; the same negatives being 

 suitable for every kind of synthesis. 



The two principal methods of obtaining colour positives from the 

 colour-record negatives are by positive synthesis and by negative 

 synthesis. Positive synthesis includes triple projection and the 

 Kromskop, and is the method followed by Mr. Ives. Triple photo- 

 graphic prints on paper, the three-colour photo-mechanical process, and 

 triple superposed transparencies all come under the heading of negative 

 synthesis. The latter subdivision embraces most of the work done by 

 the writer which has, so far as he knows, not before been undertaken in 

 Canada. Positive synthesis depends on positive colour mixture or the 

 superposition of coloured lights, while negative synthesis depends on 



