1898.] on the Theory of Colour Vision, dc. 809 



would be white, and what ought to be white would be black, and the 

 colours shown would be complementary. A yellow hy projection we 

 know is caused by a full mixture of red and green light, but by 

 superposition the red would cut ofif all the blue-green light, and the 

 green all the purple light, and the image would be nondescript, and 

 so with other colours. If we dyed the second set of gelatine 

 negatives with the complementary colours a very different state of 

 things would be found. Taking the yellow, for example, which in 

 the " red " and " green " negatives would be shown by great opacity 

 and in the blue by total transparency, the part of the print in the 

 " red " negative would be represented by very feeble sea-green, and 

 that in the green by very feeble purple, whilst in the blue negative it 

 would be represented by full yellow. From the first two the only 

 light penetrating would be the blue, and the only colour reaching 

 the eye after passing through the third gelatine transparency would 

 be the yellow, and so for other colours. Hence, for superposed 

 pictures, either for the lantern or for prints, the complementary 

 colours to those of the viewing screen should be used. This is the 

 foundation of most of the three-colour printing processes extant. 



We have three such prints in the three colours, lent me by Messrs. 

 Waterlow & Sons, and here they are superposed to make the final 

 coloured print. This triple printing can be done either by litho- 

 graphy or by printing in colour from gelatine films. 



I have endeavoured, by a brief sketch, to show you the principles 

 on which photography in colour has been based — principles which 

 are truly scientific — and which my friend, Mr. Ives, has adopted in 

 all his work. The rule-of-thumb man, who works according to his 

 own sweet wdll, is a man to whom a certain amount of success will 

 be given, but it is to him who works on the true principles of science 

 that the highest success must accrue. I have endeavoured to show 

 you that Young's theory of Colour Vision, though a theory, is yet of 

 supreme use in this particular branch of industry. I have purposely 

 omitted to mention many of the glaring mistakes which have been 

 made by the rule-of-thumb man, both at home and abroad, in regard 

 to it. 



[W. DE W. A.] 



