358 SCIENCE PROGRESS 



success in many departments of colour photography, does 

 practically the same, working according to Maxwell's curves. 

 On the other hand it has been stated that the colour screens 

 used by various workers do not always conform to their theories. 

 It must be borne in mind, however, that the dyes and pigments 

 used have to be selected from those that are available, and that 

 the best of them are only more or less close approximations 

 to what is really sought for. Thus the theories as to the selection 

 of the colours are more diverse than the practice. 



Having obtained the red, green, and blue records, there are 

 two distinct methods by which the colours may be combined 

 to form the picture. Each may be caused to give its appropriate 

 •colour separately, as by the use of a set of three optical lanterns, 

 each giving one of the required colours (the photograph regu- 

 lating its distribution and intensity), the three coloured images 

 being superposed on the screen. The three colours are added 

 together, light is added to light ; the combined effect of all three 

 is white and their absence is black. In the other method the 

 three colour prints are superposed to form a single print or 

 transparency. Here the combined effect of all three is black, 

 their absence is white, and each colour subtracts whatever it 

 can from the light — the same light that passes through all of 

 them. In the first case the lights are added, and in the second 

 the absorptions. The first is sometimes called the additive 

 method and the second the subtractive method. The colours 

 transmitted by each of the three prints in the second method 

 must be the complementaries of those transmitted in the first 

 case. Taking • for example a red patch in the original, this 

 will give a transparent or depositless place on the print that 

 represents the redness of the original, which, by the triple 

 lantern method, would give a red patch on the screen as required. 

 But for printing or staining purposes this holds no colour, 

 and so remains colourless. The prints that represent the 

 greenness and the blueness of the original will be coloured 

 at this part, the former by a colour that absorbs green and 

 transmits red and blue, the latter by a colour that absorbs blue 

 and transmits red and green. Thus the only colour that can get 

 through the three superposed coloured images is the red, as 

 required. In the additive method each image contributes one 

 of the three colours, in the subtractive method each image 

 subtracts or absorbs one of the three colours. These are the 



