1913.] Sensations and the Theory of Forced Vibrations. 497 



sensations owing to the fact that they are not all affected in the same 

 proportion, and also the general effect of the increased extent in the 

 spectrum of the several colour-sensations. " The colour must look pale 

 under feeble illumination owing to the presence of three if not of all the 

 constituents of white." 



If this view is correct it would explain a rather puzzling fact that I have 

 noted in my investigation of cases of colour-blindness.* I found it necessary 

 to specify not only those cases in which a colour-sensation was deficient, but 

 those also in which it was of greater extent than usual. The Swiss girl therein 

 referred to had practically but one colour-sensation, because her green so 

 greatly exceeded the normal in spectral extent, yet by suitable fatigue it 

 could be reduced so as to reveal her possession of the other sensations. We 

 have only to suppose that the supply of sensitive material to the green end- 

 organs was so copious as to cause a quite unusual concentration, with corres- 

 pondingly large coefficient of friction. In such a case the eye would be with 

 respect to green in a condition of dark -adaptation, with this difference, that 

 no ordinary demand could overtake the supply and reduce the store, n, of 

 molecules of visual substance to the normal amount. 



In other words dn/dt — dx/dt was positive for all ordinary values of dx/dt. 



But it is clear that this might arise either from dnjdt being large, or from 

 dx/dt being abnormally small. 



I consider that I have met with cases of both kinds. In one, of which I 

 shall give a full account in my next paper on " Cases of Colour-Blindness," 

 there was monochromatic vision in one eye only. This enabled me to ascertain 

 that the sensation was unmistakeably one of whiteness, with a little colour at 

 the two ends of the spectrum. But the whole intensity of the light-sensation 

 was extremely low although the transparent tissues were perfectly clear. 



In the same way we may explain the fact noted in my paper on " Artificial 

 Temporary Colour-Blindness "f that with some people the " overlaps " of the 

 colour-sensations are very large, and with others almost non-existent, and 

 also that with some the overlaps are large between red and green and small 

 at the other end of the spectrum, so that to them yellow is an important 

 colour, and with others there is little or no overlap between red and green, and 

 large overlaps in the green, blue, and violet region. It seems to indicate that 

 the coefficient of friction is constitutionally large in these people either for all 

 the colours or for those to which the large overlaps belong. And by the 

 expression " constitutionally large," I mean large not because any temporary 

 cessation of the demand dx/dt for the exciting molecules has allowed a large 



* ' Phil. Trans.,' B, vol. 199, pp. 239 and 250. 

 t 'Phil. Trans.,' 1899, B, vol. 191, p. 1. 



