I90I-2.] Photography in Natural Colours. 385 



both the natural colours and stereoscopic relief, which can not be 

 excelled by any other method, yet some process of producing slides to 

 be projected by an ordinary lantern or viewed like an ordinary tran- 

 sparency, would be decidedly more useful. 



Such slides and transparencies are examples of the second method 

 of producing coloured positives, i.e. by negative synthesis. They 

 depend upon the superposition of coloured transparencies or pigments, 

 and the basis of the method is essentially different. It is, in fact, 

 exactly complementary to the former method, in which the effects were 

 produced by adding coloured light to coloured light, hence the name, 

 positive. This latter method depends upon the addition of absorption 

 to absorption, or the addition of colours (by negative mixture remem- 

 ber) which are obtained by subtracting the positive colours from white 

 light, and is called negative by reason of the subtractive nature of the 

 superposed absorptions. 



From the three negatives, transparencies are made as in positive 

 synthesis, but, instead of being in black and white, they are in colours 

 complementary to the reproduction colours. That is, from the red- 

 record negative, a transparency is made in minus red or blue-green, 

 from the green in magenta-pink, and from the blue-violet in yellow. 

 The colour and appearance of these three transparencies are represented 

 approximately, although not exactly, by the three coloured positives 

 shown in the Plate. The superposition and registration of the three 

 transparencies produces a slide similar in external appearance to an 

 ordinary lantern slide but exhibiting the colours of the object. 



Let us examine into the reason for the complementary nature of the 

 printing and reproduction colours ; that they are very nearly com- 

 plementary is at once evident by the spectroscopic test \sJiown\. 

 Consider the transparencies, by the two methods, from the negative 

 taken through the red filter. In positive synthesis its lights are 

 coloured red, or red is transmitted, while its shadows are dark or red is 

 absorbed. In negative synthesis its lights are white or red is trans- 

 mitted and its shadows blue-green, complementary to red, and hence 

 red is absorbed. Thus red is transmitted by the lights and absorbed 

 by the shadows in the two cases ; and the same thing will be true for 

 the green and blue-violet records, so that the superposition by the two 

 methods should give identical results. 



A simple concrete example may perhaps help to render this some- 

 what difficult point more clear. Consider the effect of photographing a 



