230 



Prof. J. Joly. 



scotopic luminosity curves.* Now the latter according to the present theory, 

 can represent but one photon from each electron. We must conclude that the 

 former also represents one photon from each electron. 



(20) From this we must infer, as regards photopic luminosity, that the 

 number of fibres activated by one electron does not influence the luminosity 

 stimulus. The stimulus remains a unit stimulus, irrespective of whether it 

 originates from two, three, or four grouped fibres. 



If the stimulus value were proportional to tlie number of constituent 

 photons, the form of the resultant photopic spectral curve must differ 

 radically from that of the scotopic curve. It would rise towards the violet 

 end. This feature it certainly does not exhibit. Of course, the photopic 

 luminosity curve stands at a higher energy level than the scotopic curve. 

 This is because the numbers of quanta acting per second are greater in the 

 former than in the latter case. 



(21) On the other hand, with respect to the excitation of colour sensation, 

 the number of constituent photons entering into the stimulus is all important. 

 For it is by this number, i.e., by the form of the stimulus, that the funda- 

 mental colour sensations are evoked. 



The relation of luminosity to colour sensation is, therefore, according to 

 the present theory, as follows : — Luminosity is the more primitive sensation, 

 and at first was associated entirely with rod vision. The evolution of the 

 cone brought in multiple stimuli, and the sensation evoked became corre- 

 spondingly complex. The basal luminous sensation remained, excited as 

 before by a nerve stimulus from the retina, but it was accompanied now by 

 a new and additional sensation, that of colour. 



Colour sensation necessarily involves luminous sensation. It necessarily 

 involves it because the energy is there which excites it. The converse 

 proposition is not, however, true. It is not true, because there may be 

 sufficient energy to excite a luminous sensation and not sufficient to excite 

 colour sensation. The separation of colour sensation from luminous sensation 

 is therefore impossible. Colour sensation curves take both into account.f 

 Hence the energy relations revealed in the relative heights of the three 

 curves, as referred to above, is to be expected. 



It is easy to see that there is a balance of energy available for the colour 

 stimulus. The unit of luminosity is referable to the photon. Ked sensation 

 is seen when two photons act simultaneously. We may ascribe one to the 

 luminous and one to the colour sensation. As both arise together, and are, 

 in fact, superimposed, this allocation of the energy is perhaps a little fanciful. 



* Parsons, loe. cit., fig. 14. 



t Kcinig and Dieterici, loc. cit., p. 22. 



