28 Dr. It. S. Clay. On the Application of 



curves, and thus we should have to print it with the red and green 

 inks. Now the original yellow was equivalent to the addition of a red 

 and green light, so that it ought to be brighter when both colours are 

 there than when only one. The reverse would be the case if printed 

 as above, and, of course, as a white object reflects all the colours, it 

 would be printed with all three inks. Now, though red, green, and 

 violet lights when added give white, red, green, and violet inks super- 

 posed on white paper certainly will not make white. 



In fact, inks produce colour by absorption. Thus, instead of succes- 

 sive inks adding to the light, they each reduce it. 



Suppose, then, we proceed the other way, and instead of looking for 

 those parts of the picture where red is reflected and printing with red 

 ink, we print those parts where no red is reflected with an ink which 

 absorbs red, but reflects all the other colours. This will be a bright 

 bluish-green ink. In the same way we will print those parts which 

 reflect no green with an ink which absorbs that colour ; this will lie a 

 bright magenta or almost a pink. And those parts which reflect no 

 violet we will print with a yellow ink. Now a yellow object reflects 

 red and green but no violet. Hence it will have to be printed with 

 the violet absorption ink only that is, the yellow and with neither 

 of the others. A red object reflects neither green nor violet, and will 

 be printed with the pink and yellow inks, which will leave red only. 

 As a yellow object is printed with only the one ink, but a red one 

 with two inks, the yellow will be the brighter, and this is as it 

 should be. 



As the inks are to absorb red, green, and violet respectively, they 

 will, roughly speaking, be complementary to those colours, that is, 

 when added to them either by a double-image prism, or by reflection 

 from a clear glass surface, they should give white. Still, comple- 

 mentary is a very vague description of a colour, and it must be our aim 

 presently to define it more precisely. 



The above theory is due to Mr. Ives. 



3. Application of Maxwell'* Curves. 



As we said at first, these curves were derived by adding three 

 monochromatic lights, and are only strictly true of such colours. If, 

 however, we use lights compounded of or, at least, capable of being 

 matched by one of these monochromatic colours and white, we shall 

 still obtain a result very nearly as perfect as with the pure colours. It 

 can easily be seen, however, that if any other colour than white were 

 combined with the pure colours the result would be spoilt. 



All colours, including white, can be matched both in hue and 

 luminosity* by combining the three monochromatic colours, red, green, 

 and violet, in proportions given by Maxwell's curves. It is easy to 



* See Note 1, p. 27. 



