THE SENSATION OF SIGHT. 213 



through the various hues of the rainbow to violet. The red 

 and violet are united by shades of purple, which on the one side 

 pass off to the indigo and blue tints, and on the other through 

 crimson and scarlet to orange. The middle of the circle is left 

 white, and on lines which run from the centre to the circumfer- 

 ence are represented the various tints which can be produced by 

 diluting the full colours of the circumference until they pass 

 into white. A colour-disc of this kind shows all the varieties 

 of hue which can be produced with the same amount of light. 

 It will now be found possible so to arrange the places of the 

 several colours in this diagram, and the quantity of light which 

 each reflects, that when we have ascertained the resultants of 

 FIG. 84. 



Green 



Blue 



Violet Purple 



two colours of different known 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 combina- 

 tion-colour at the ' centre of gravity ' of the two amounts of 

 light. 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 compound 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 saturated in nature 



