168 DR. J. D. MACDONALD, I.H.R.N., F.R.S.^ ON 



Bmall enough to fall on only one cone, can be seen of a 

 fixed and definite colour that does not alter when the 

 position of the eye is changed and the image shifted from 

 one to another on the yellow spot." Tliat fact alone seems 

 to him sufficient to show the necessity for supposing that 

 each cone is capable of stimulation by all visible undulations 

 of light, and transmitting such nerve vibrations as are 

 capable of inducing all the colour sensations. Moreover, 

 when the image of a coloured star is made to fall on parts ot 

 the retina peripheral to the yellow spot, its colour does not 

 disappear and reappear when the eye is moved as would 

 inevitably be the case if the rods were not terminals 

 concerned as well as the cones in colour sense. It is indeed 

 obvious that if the colour of the star were made to pass 

 through all the hues of the iris, the facts would be 

 practically the same. There is, therefore, no standing room 

 here for the Young-Helmholtz theory, Avhile the JN^ewtonian 

 view is in touch with every part of the argument. 



Though we are not at all prepared to say tliat the number 

 seven does not reign in the vibrations of taste and smell in 

 keeping with the properties of their appropriate stimuli,* 

 no one has ever thought it necessary to invent special nerve 

 terminals for the perception of the leading notes of the 

 musical scale, or for particular tastes and odours, which are 

 far more diverse and indescribable than colours and sounds. 



Further, as to the perception of Mdiite and grey in the 

 absence of all objective colour, it is hard to accept the 

 gratuitous and complex doctrine that it requires the bal- 

 anced stimulation of all three coloured terminals for red, 

 green, and violet, to oftect this. Of course the principle 

 would be the same if red, yellow, and blue, or any other 

 colours were selected as primitives. It is rather curious to 

 notice how scientists differ in their choice of such colours, 

 while each suggestion is supposed to be satistactorily set 

 forth by its propounder. Thus 1. — Newton and Brewster 

 adopted red, i/elloiv, and blue ; 2. Young and Hehnholtz, reJ, 

 green, ixnd violet; 3. Ma-iLweW, 7'ec/,(jreeii, nnd blue; 4. llering, 

 red, yelloio, green, and blue; 5. Rosenstiehl, onuuje, ijelloio, 

 green, and blue ; and 6. Roechlin takes i/ellow and blue, while 

 a third principle is supposed to be present, but always 



* Messrs. Picsse and Lubiii distinguish tlieir perfumes by musical 

 notes, and tlio former gentleman informed tlie writer that he is quite 

 satisfied tliat all the senses are allied, by analogy. 



