462 Dr. F. W. Edridge-Green. The Relation of [June 3, 



if we subtract the rays occupying the missing portion from the colour of 

 confusion. For instance, if we take a blue and a pink which have been put 

 together as identical by a person with a shortened red end of the spectrum, 

 and look at them through a glass whicli is opaque to the red but transparent 

 to the remaining rays of the spectrum, both will appear alike in hue and 

 shade. A person with considerable shortening of the red end of the spectrum 

 will look at a red light (wliich is so dazzlingly bright to a normal-sighted 

 person as to make his eyes ache after looking at it closely for a few seconds), 

 at a distance of a few inches, and remark that there is nothing visible, and 

 that the whole is absolutely black. It is obvious that the light must consist 

 only of rays occupying the missing portion of the spectrum. 



The same remarks which I have made for a shortened spectrum apply to 

 cases in which there is defect of light perception through absorption or any 

 other cause. Tlie person having the defect Is placed in a similar position to 

 a normal-sighted person with those particular rays removed or reduced to 

 the same intensity. 



Another effect of shortening of the spectrum when it is sufficient to 

 interfere with the difference perception which appears to be inherent in the 

 central nervous system is that the colours appear to be moved in the 

 direction of the unshortened portion. I'or instance, we find the neutral 

 point of the dichromic with shortening of the red end of the spectrum 

 further towards the violet end of the spectrum in comparison with a case in 

 which the spectrum is of normal length. In the same way a trichromic with 

 a shortened red end of the spectrum has the junction of the red and green 

 nearer the violet end than in a case where there is no shortening. 



The point that I specially wish to emphasise is that, though every case in 

 which there is defective light perception can be explained by a defective 

 sensibility to light of certain wave-length, not a single case of the very 

 large numlter of persons tliat I have examined can be explained on the 

 older theories, tliat is, the defect of light perception cannot be explained 

 on tlie assumption that theio is a defect in a light-perceiving substance 

 whicli is sensitive to rays of liglit from a considerable range of the 

 si)ectrum. 



'1. Ca.s'-.-i III if/ilc/i tkcrc is Defcdive Colour Perception. 

 Even in cases where there is actual colour ])lindness or deficiency of hue 

 porception it is e(|uaHy iin])ortant to diffenintiate betwcnm ihe perception of 

 light and the percu'ptioii of hue. Let us take as example a case which at 

 first sight appears to bi; in supy)ort of the older theories, a case of so-caJh;d 

 redMiiiibicss, in whicJi is (li(;ln()mi(; vision with considerable shortening 



• if t he (ukI of (lie spectrum. It can . be easily shown that the defect 



