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Prof. J. Joly. 



the exciting electron ; and evokes grades of sensation owing to the possession 

 of more than one fibre connection with the brain.* 



(4) It may be objected that the energy of the photo-electron is too small 

 to be divided. The answer is that it is not the energy of the electron which 

 travels to the brain. The " all or none " law and the fact of the refractory 

 period plainly indicate that the stimulus is a trigger action. And where we 

 are dealing with the equilibria of molecular systems of extreme delicacy we 

 are not entitled to limit the possibilities. True we are not in a position to 

 state the character of the operative mechanism. But here, as elsewhere in 

 physiological and physical science, we necessarily leave, and are clearly 

 entitled to leave much to the unknown. 



(5) Although the conditions involved appear to be very complex some 

 discussion of the bearing of this view of the structure of the cones on colour 

 vision may be permissible. It is evident that the activation of the nerve 

 fibres in definite numerical groups according to the energy of the photo- 

 electron may be regarded as realising Young's three kinds of nerve fibre or 

 the three " apparatus," which, by various writers have been postulated as 

 accounting for colour vision. 



(6) We are presented in the visible spectrum with a series of frequencies 

 ranging over about one octave. The corresponding quanta appear in the 

 photo-electron and must be reckoned with, whatever view we may hold 

 respecting the number of primary colour sensations. 



It will, however, conduce to clearness and at the same time, very probably, 

 lead us nearest to the actual facts if we assume three primary colour 

 sensations. 



Various spectral positions have been assigned by different authorities to 

 these sensations. The difficulty of the subject is revealed by these differences. 

 Helmholtz bases his selection on curves plotted to represent the rate of 

 change of hue attending change of wave-length. Such curves show . two 

 regions of special sensitivity respecting change of hue ; one in the yellow and 

 one in the blue-green. These positions of maximum change, although 

 originally derived by direct observations,! mark the intersections of the colour 

 sensation curves and may be referred to parts of the spectrum where the 

 ordinates of the colour curves are most rapidly changing in relative value. 

 Hehnholtz's fundamental green sensation is located between these points at 

 540 fifi to 560 fxjjb. Konig's green sensation curve attains its maximum 



* It seems not improbable that the evolutionary development of the cone originated 

 in the fusion of the rods to form a single organ. The rods are known to be anastomosed 

 in groups with a single conducting neuron. 



t Parsons' ' Colour Vision,' Cambridge, 1915, p. 30. 



