804 Captain Ahney [Feb. 25, 



But the amount of white which is in the purest green sensation, 

 renders it desirable to choose a green which has less white inherent 

 in it, to prevent the mixture being pale. 



In 1861, Clerk Maxwell gave a lecture in this theatre, in which 

 the method of producing photographs in the colours of nature by 

 means of illuminating three photographic pictures, and combining 

 the images together, was foreshadowed, and it is to this, and to his 



Fig. 2. — Maxwell's Curves of Colour Sensations. 



original work on the mixture of colours, that we must turn. By 

 means of what he called his colour-box, he could mix any three 

 colours of the spectrum together, and, for reasons which appeared 

 adequate at the time, he took a bright red, a bright green, and a 

 bright blue of the spectrum as best representing the sensations. He 

 referred all other colours of the spectrum to these, and expressed 

 them as mixtures of the three. The diagram that he made is given 

 (Fig. 2). The heights of the different curves he obtained by 

 measuring the width of the three slits through which any three 

 chosen colours came, and making such widths the ordinates. The 

 standard red he chose was a red containing a little green; the 

 standard green near E is nearly free from white, but a glance at 

 the diagram (Fig. 1) will show it is mixed with a certain amount of 

 red ; Maxwell's blue contained a certain quantity of green. This is 

 merely history, but it may be remarked that where we are dealing 

 with colours, and not sensations, the colours he chose are probably 

 nearly the best for the purpose we have in view. I have reduced the 

 Maxwell curves so as to represent luminosity as well as colour, and 

 you will see that they all fit into the spectrum curve, and that the 

 great mass of brightness is due to the green and red. Of blue there 

 is but very little. These curves should be kept well in your mind. 



We need not trouble you much about colour mixtures. I have 

 an apparatus here which allows us to mix any colours together 



