Report of the Committee on Colour-Vision. 303 



signal-men on railways, evidently require sight unaffected by 

 defects in colour or form, and there may be other positions, 

 both in the marine service and in that of railways, which should 

 also be included. Some central authority should make a schedule 

 of such positions, and should take measures to enforce the exclusion 

 of colour-blind persons from them. 



Note («). 



The cause of the different sensations which are conveyed to Cases of 

 the brain is a matter which is still in doubt. It is difficult to abnormal 

 conceive that matter which is so comparaTively gross as the rods colour- 

 and cones which are situated on the retina can be affected by the khndness. 

 merely mechanical action of the vibrations of light. 



The little we know about the actual nature of sensations leads us 

 rather to believe that the nervous processes which are the founda- 

 tion of sensations are, like other nervous processes, the outcome 

 of chemical changes in nervous substances. And it has been 

 suggested that vision originates in the chemical changes of a 

 certain substance (or substances) in the retina, that the chemical 

 condition of this substance, which has been called visual sub- 

 stance, is especially affected by the incidence of light, and that 

 the changes so induced determine the beginnings of visual 

 impulses and thus of visual sensations. We know that light 

 can decompose a substance by acting- on its molecules, and 

 thus induce a chemical change in it. 



In photographic processes, for instance, we know that the 

 molecules of the sensitive substance are split up by white light, 

 and further, that when these comparatively simple substances 

 are exposed to the spectrum, although it is found that a con- 

 siderable extent of it produces chemical changes, there is one 

 particular part which acts more strongly than the rest of it. 

 The curve of sensitiveness exhibits the same characteristics 

 as those of the colour sensations in the Young-Helmholtz theory. 

 If it be conceded that the retinal substance acted upon by light 

 is a mixture of three analogous compounds, each having a 

 maximum sensitiveness at a different point of the spectrum, we 

 can account for the three fundamental sensation curves shown 

 in the diagram at page 286. 



Note (b). 



Any complete theory of colour- vision must account not only fur 

 normal vision and congenital colour-blindness, but also for those 

 cases of defective colour-sense which are due to disease or 

 injury, and which differ so widely in character from each other. 



It is somewhat difficult to see how the Young-Helmholtz Difficulties of 

 theory accounts for the last species of colour-blindness. Accord- accounting 



