578 PHYSIOLOGY 



cone, or by separate substances, each of which is affected by one of these 

 colours. 



Thus the rays with a longer wave-length excite chiefly the red fibres or 

 elements ; those of medium wave-length the green percipient element ; those 

 of short wave-length the violet percipient element of the retina. The excita- 

 bility of these three elements by the rays of different parts of the spectrum 

 are represented in the figure (Fig. 296). At the extreme red and violet end 



FIG. 296. Curves, showing sensitiveness of the three varieties of nerve fibres to 



different parts of the spectrum. 

 1, red fibres ; 2, green fibres ; 3, violet fibres. 



of the spectrum alterations of the wave-length cause no alteration in colour, 

 showing that these rays only excite either the red or the violet fibres. All 

 other parts of the spectrum excite the three sets of fibres simultaneously, 

 but to varying degrees. Thus the greater part of the red, e.g. about ' C,' 

 excites the red percipient element strongly, the other two only weakly ; the 

 result is a sensation of red. The yellow rays at ' D ' excite almost equally 

 the red and the green percipient elements, but the violet only slightly. 

 Yellow is therefore a mixed sensation. The green rays excite the green per- 

 cipient element strongly, the other two slightly, with green sensation as a 

 result. Blue rays excite green and violet percipient elements to a moderate 

 extent, and the red rays to a somewhat less extent ; blue is therefore a 

 mixed sensation composed chiefly of green and violet. Simultaneous 

 excitation of all three elements evokes a sensation of white or grey, 

 according to the intensity. 



The cases of defective colour- vision which occur the so-called colour- 

 blindness are regarded under this, theory as due to the absence of one or 

 other of the elements which determine the primary colour-sensations. The 

 normal eye is spoken of as trichromatic since it has three primary colour- 

 sensations. Two kinds of dichromatic eyes are described those in which 

 the red sensation is lacking, and those in which the green sensation is lacking. 

 In either of these cases the subject confuses red and green. The ripe cherry 

 on a tree they may distinguish from the leaves, not by their colour, but by 

 form or difference in their luminosity. It is only when they are tested by 

 means of the spectrum that we find that whereas in the first case (red blind- 

 ness) there is insensibility to the red end of the spectrum, in the second case 

 the red end of the spectrum is seen as well as by the normal person, so that 



