228 



Prof. J. Joly. 



discharged at the visual centre of the brain. That is, we see it at a point oi> 

 the spectrum where the quanta activate both two-fibre and three-fibre groups 

 in about equal numbers. Somewhat nearer to the red end the quanta, owing 

 to their lesser energy, affect the three-fibre groups in less numbers than they 

 affect the two-fibre groups. So that there are, say, n x 2 (f) + m xS^ {n being 

 greater than m) discharged in the visual centre. This excites an orange 

 sensation. Such an expression as I have just written down is at the basis of 

 colour sensation equations. 



The hue at any point, therefore, depends on the destination of the spectral 

 quanta radiated at that point ; that is, upon their allocation among the fibre 

 groups which they are competent to activate. 



There are a considerable number of distinguishable hues, an appreciable- 

 amount of sensation being evoked by a very few quanta. 



(15) The stimulus value of the three colour sensations in such proportion& 

 as to give white light is nine photons. Two colours are complementary to 

 each other when the sum of their stimulus values is equal to 9^. Thus- 

 red = 2 <^ is the complement of blue-green = 4^-1-3 Again, green = 3 



is complementary of 2 ^ + 4<^, which is a colour not found in the spectrum, 

 i.e., purple. Yellow = 2 4>+ 3 </> is complementary of blue = 4</). 



(16) It is a fact that at their achromatic scotopic thresholds all lights are- 

 of equal brightness.* This is explicable on the view that rod stimulus is 

 conveyed through a single fibre, and that a one-fibre stimulus is the minimum 

 wherever it originates in the retina. For we are here brought into contact 

 with the " all or none " law. It is not the quantum of energy which is trans- 

 mitted : the quantum plays the part of the force applied to the trigger. 

 What is transmitted is that unit of energy which the visual nerve generates 

 and discharges into the cerebral cortex. The statement is an assertion of the 

 existence of the photon. 



(17) The basis of colour vision may, according to the foregoing views, be- 

 stated as follows : — 



(a) The number of spectral quanta converted to electronic energy and 

 thereby rendered capable of exciting vision is controlled by the light 

 absorption and bleaching of the visual purple (or substance possessing a 

 similar spectral absorption curve). 



(b) The quanta, increasing in energy from the long to the short wave- 

 lengths, stimulate two, three and four fibres of the cone according to their 

 energy, as shown by the colour sensation curves. 



(c) The simultaneous stimulation of two fibres is attended by the red 



* Parsons, lor. at., p. 61. 



