242 RECENT PROGEESS OF THE THEORY OF VISION 



strength of light (in the same way as we might determine 

 the centre of gravity of two bodies of different known 

 weights), we shall then find their combination-colour 

 at the ' centre of gravity ' of the two amounts of light. 



FIG. 34. 



Blue 



Violet 



Pu rple 



Red 



That is to say, that in a properly constructed colour-disc, 

 the combination-colour of any two colours will be found 

 upon a straight line drawn from between them ; and com- 

 pound colours which contain more of one than of the 

 other component hue, will be found in that proportion 

 nearer to the former, and further from the latter. 



We find, however, when we have drawn our diagram, 

 that those colours of the spectrum which are most satu- 

 rated in nature, and which must therefore be placed at 

 the greatest distance from the central white, will not 

 arrange themselves in the form of a circle. The circum- 

 ference of the diagram presents three projections cor- 

 responding to the red, the green, and the violet, so that 

 the colour circle is more properly a triangle, with the 

 corners rounded off, as seen in Fig. 34. The continuous 

 line represents the curve of the colours of the spectrum, 

 and the small circle in the middle the white. At the cor- 



