THE HUMAN COLOUR SENSE. 167 



infallibly give rise to minute interruption in any simple wash 

 of colour, in which nothing of the kind is perceptible to the 

 normal eye, no matter what the hue or tint may be. 



Hehnholtz's independent theory is practically the same as 

 that of Young, but he has simply carried inventive histology 

 one step further, and thus given additional force to the 

 foregoing argument and the legitimate deduction drawn 

 from it. As to the support which certain forms of colour 

 blindness are supposed to yield to this theory, the matter 

 will not bear close investigation, and certain other con- 

 siderations may be adduced to meet any apparent difficulty 

 in the case. We are thus brought back to the simple but 

 sound exposition of the physiology of the colom- sense given 

 by Newton himself, and have gained nothing by setting it 

 aside. He held that all the nerve filaments of the retina 

 were equally sensitive to the different prismatic hues Avhose 

 specific vibrations were thus in effect conveyed to the 

 centre of perception. Dr. Rutherford, F.R.S., in his excellent 

 address delivered before the British Association in Edin- 

 burgh. 1892, contends very naturally that " we should refer 

 our diff"erent colour sensations to differences in the nerve 

 vibrations transmitted from the optic terminals rather than 

 to specifically different activities of cells in the vision centre." 



In reference to the pseudo-anatomical refinement of the 

 Young- Helmholtz theory, Dr. Rutherford or any other 

 anatomist must fail to see a tripartite constitution in 

 the retina which such a view would necessitate. " There 

 are indeed," he remarks, "two anatomical elements, namely, 

 the rods and the cones so called, that would require special 

 notice. The rods are very much more numerous than the 

 cones, and though both are found in the general field of the 

 retina, the cones alone occur in the yellow centre where the 

 visuyl sensitiveness is most perfect. Here then we perceive 

 that the colour sense is not impaired by the absence of the 

 rods." But to return to a subject already referred to, 

 though the cones are necessarily very small they would 

 appear to be a sufficient distance apart, with nnmo'ous 

 intervening rods in surrounding zones to render it improbable, 

 if all the colour sense be attributed to them, that a con- 

 tinuous wash of any colour could be perceived witnout 

 minute speckling or a finely broken ground, but as we know 

 that no such obstruction occurs normally, we can scarcely 

 withhold colour perception from the rods. Further, as 

 Professor Rutherford remarks, " the image of a coloured star, 



