1G6 DR. J. D. MACDONALD, I.H.R.N., F.E.S., ON 



Nor does the followinj^ gratuitous supposition do anything' 

 more than send the inquirer farther, wJiich is the usual effect 

 of all attempts to clear up ultimate facts. A Tricliromic 

 Colour Sense seems to have sprung out of the doctrine of 

 three primary or fundamental colours, though a tri})artite 

 constitution of light is untenable on scientific grounds. As 

 set forth by Professor Church, '" Young's theory of colom- 

 perception amounts essentially to this, that in each minute 

 elementary part of the retina of the eye there is at least one 

 set of three different nerve fibrils (whether ' cones ' or 

 ' rods '), each of the three fibrils of a set being especially 

 adapted for the production of its specific colour sensation, yet 

 ill a less degree of the two others. Thus, the receptive 

 structure of the retina as a whole may be said to consist of 

 an immense number of nerve fibrils of three orders, what we 

 may call red fibrils being particularly acted upon by such 

 long light waves as those in the red, but being also stimu- 

 lated in a minor degree by the shorter waves in the green^ 

 and still less by those in the blue. The green fibrils Avill 

 respond most actively to green waves and in some measm-e 

 also to red and to blue waves ; while the blue fibrils will be 

 most excited by blue waves, though not uninfluenced by 

 green and even by red Avaves. It follows that when all 

 three kinds of nerve fibrils are equally and simultaneously 

 affected, the complex sensation of white is alone produced." 

 In the above statement as far as Young was concerned 

 Violet should be substituted for Blue. In his selection he 

 obviously laid stress on the question of refrangibility, which 

 is least in lied, most in Violet^ and intermediate in Green; 

 but if the octave Red (7()0) were visible, Blue (570) would 

 undoubtedly be in the mean of refrangibility, and this would 

 be in keepijig with the musical analogy, the 5th G, hold- 

 ing an intermediate position between the 1st C (."580) and 

 its 8tli or octave C (7(30). But to return to the argument. 

 If the nerve terminals are accredited to be the recipients of 

 the three so-called primary colours, though in difierent 

 degrees, one having the predominance to aid tiie trichro- 

 matic theory, it Avould be just ;i.s reasonable to sui)pose that 

 each of those nerve terjiiinals were equally susceptible of 

 the impact, not even of all the three primary colour waves, 

 but also those of the intervening hues. This would do 

 away Avith the necessity for the co-operation of two or more 

 nerve elements in the perception of every ordinary mixture 

 of colours, a complex physiological condition Avhich would 



