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Prof. J. Joly. 



neighbouring inferior points on the spectrum. The symmetrical curve of 

 the green sensation perhaps best reveals what is happening, for in the ease 

 of the red and blue sensation curves, end conditions imposed by the failing 

 absorption of the visual purple intrude themselves and largely define the 

 external slope of the curves. In the case of the green curve we find a 

 rounded crest of maximum green sensation, and in the slopes leading up 

 to it we perceive the increasing number of quanta which go to activate 

 three fibres and thus create in our cerebrum the green colour sensation. 

 At the same time we see by the over-lap of the red curve that this region 

 also pays toll to the red sensation ; many quanta degrading to stimulate 

 but two fibres. 



In those cases where stimulation is attended by a misfit there may also 

 be one-fibre stimuli. The one-fibre stimulus simply creates the luminous 

 achromatic sensation proper to the rods. 



The stimulation of the red sensation — i.e., of two fibres — is displayed the 

 whole length of the spectrum. It falls off rapidlv after the demand of the 

 three-fibre sensation begins to make itself felt, but continues right into the 

 violet. According to Abney's measurements, the violet sensation includes 

 72 per cent, of red and 28 per cent, of blue sensation. Here the value 

 of the quantum has risen to fully twice that required for a two-fibre 

 stimulus, and in this way, possibly, excites in tlie cerebrum the sensation 

 of red. 



The blue colour sensation curve — according to Konig — gives away 

 relatively few quanta to the green sensation curve. This may be explained, 

 perhaps, in this way. The stimulation of four fibres leaves five vacant 

 fibres. If a second stimulus of four fibres takes effect there is not room for 

 a three -fibre stimulus. iSTow in the case of the " green " quanta spending 

 themselves on two-fibre stimuli the element of chance is more in favour 

 of the degradation. Two successive three-fibre stimuli leave room for a 

 two-fibre stimulus. 



(9) On the present theory the points of maximum change of hue for a 

 given displacement along the spectrum is physically explained as follows : — 

 Where colour sensation curves over-lap we know that quanta are being 

 distributed among the fibres in two or three different ways. Thus, for 

 instance, some may go to create two-fibre stimuli and some to create three- 

 fibre stimuli. At some point on the spectrum, where the two-colour curves 

 intersect, an equal number of quanta stimulate each sensation. Here a 

 small change of the energy value of the quantum determines a flux of 

 quanta to one fibre system more than to another. At this point there is 

 a maximum rate of change of hue for a given change of wave-length. 



