942 



A MANUAL OP PHYSIOLOGY 



mixture referred to above. When all the rays of the spectrum 

 act upon the retina together, the three components are about 

 equally affected, and this equal effect is supposed to be the 

 condition of the sensation of white light. When the green of the 

 spectrum alone falls on the retina, the ' green ' component is 

 strongly excited, the other two only slightly ; this is the relation 



FIG. 413. CURVES OF EXCITABILITY OF PRIMARY SENSATIONS FROM OBSERVA- 

 TIONS ON COLOUR MIXTURES (KoNic). 



The numbers give wave-lengths of the spectrum in millionths of a millimetre. 



between the amount of excitation in the three components 

 which is associated with a sensation of spectral green. When 

 two complementary colours, such as red and bluish-green, fall 

 together on the same portion of the retina, the three components 

 are excited in the relative proportions associated with the 

 sensation of white light. 



The colour triangle is a graphic method of representing various 

 facts in colour-mixture (Fig. 414). 



The chief points to be noted are the following : (i) On the curve the 



spectral colours are 

 arranged at such dis- 

 tances that the angle 

 contained between 

 straight lines drawn 

 from the point marked 

 ' white,' and inter- 

 secting the curve at 

 the positions corre- 

 sponding to any two 

 coloursis proportional 

 to their difference in 

 FIG. 414. COLOUR TRIANGLE. tone. (2) The dis- 



tance of any point of 



the curve from the point marked ' white ' is proportional to the 

 stimulation intensity of the colour corresponding to it. (If the 

 stimulation intensities of all the colours be represented by proper- 



