460 Dr. F. W. Edridge-Greeu. The Relation of [June 3, 



at first surprising when we consider how insensi])ly one part of the spectrum 

 seems to shade into the next when the whole of the spectrum is looked at. 

 The number and position of the patches present, however, great uniformity 

 from one case to another. 



I propose to deal with a certain number of cases of defective colour vision 

 as investigated by this instrument, and to show how we may differentiate 

 those defects due to failure of light perception from those due to failure of 

 colour or hue perception. 



1. Defective Ligltt Pcreeption with Normal Colour Perception. 



As an illustration of this class I may mention a case I have recently 

 examined in which there was shortening of the red end of the spectrum with 

 absolutely normal hue perception. A boy wishing to enter the Navy had 

 been examined with the Holmgren test and certified to have normal colour 

 perception. He was rejected, however, with the lantern test. He was then 

 sent to me. I found his hue perception quite normal. He matched and 

 named colours with ease and accuracy. When tested with the spectrometer, 

 however, I found that his spectrum for bright light was shortened to X 700, 

 the limit for normal individuals being about \ 760 to A, 780. He had also 

 defective perception for the red rays adjacent to the shortened portion. 

 When examined with my lantern* he failed altogether to see the standard 

 red light No. 2 at about 20 feet distance, though he could clearly see the 

 aperture through which the light came. The bright red light consisting 

 almost exclusively of rays from \G25 to A, 731 was not even visible to him 

 as light. 



In the latest reports of the ]3oard of Trade there are sevei al cases of men 

 who have passed the green test of Holmgren, but have failed with the rose 

 test and have therefore been designated completely red-blind. These cases 

 are probably similar to the boy wliose condition 1 liave just described. 

 Such cases are undoubtedly red-blind for the rays which are not seen, but 

 n(jt in tlu! sense of the Young-Helmholtz or Hering theories. This l)oy was 

 in a similar position to a person who is unable to hear very low notes on an 

 organ. Such persons may have absolutely normal perception of tone 

 <liirerences above a certain number of vibrations ])er second, and in the same 

 way these cases of so-called red blindness may luivi; a normal appreciation 

 of colour difTei'ciiicos for all the spectrum except the least refrangible rays, 

 which do not infhience their retinas at all. In the same way, just as certain 

 individuals may be deaf to the liigliest notes of music, so there is a class of 



* TliiH in (](!w:r)l»'(l in my hook on 'Colour ItliiidncHH luul ('oloiir I'circptioii ' (Inter- 

 nutioiial Si iciitific Sci icH), 190!), j). 2G4. 



