ON COLOUR VISION. 307 



Colour Vision. Report of Committee (Sir Charles Sherrington, Chair- 

 man ; Prof. H. E. Roaf, Secretary ; Dr. Mary Collins, Dr. F. W. 

 Edridge-Green) appointed to report upon Colour Vision, with 

 particular reference to the classification of Colour-blindness. 



1. Defects of colour vision usually consist of a decrease in the perception of 

 colours, therefore hypochromatism or hypochromatopia is a suitable term for defects in 

 colour vision. 



2. In order to classify satisfactorily the defects in colour vision, one must know 

 the actual differences in colour discrimination between normal and hypochromatic 

 individuals. 



3. Defects in colour discrimination are not always accompanied by decrease in 

 sensitivity to light. Localised decrease in sensitivity to long wave lengths is not 

 infrequent, a similar decrease in sensitivity limited to short wave lengths is also 

 found, but the statement that there are cases with a localised decrease in sensitivity to 

 intermediate wave lengths is not proved. 



4. The terms red blindness, violet blindness, &c, are meaningless, except on the 

 basis of a decrease in sensitivity limited to the corresponding portion of the spectrum. 



5. Decrease in sensitivity to light such as is mentioned in paragraph 4 will cause 

 change in colour value of all colours which include those wave lengths to which the 

 individual is less sensitive than the normal, but when there is no decreased sensitivity 

 to light the term confusion should be used. Thus red-yellow-green confusion would 

 describe the usual form of hypochromatism in which there is decreased ability for 

 distinguishing colour differences at the longer wave lengths of the spectrum. It 

 would be preferable to indicate the actual wave lengths at which discrimination is 

 defective, as that would indicate the extent of the defect and not merely the qualitative 

 region of the spectrum involved. 



6. The nomenclature at present in use is unsatisfactory, and it is recommended 

 that it be discarded. The descriptions of cases are very defective, as one cannot be 

 certain whether they are based on theory or on examination of the cases. By the loss 

 of one system of the Young- Helmholtz hypothesis dichromatic vision would 

 occur, there being only two colours recognisable in the spectrum. Such a loss is 

 implied in the following terms, which should be abolished. 



(a) Protanopia, or scoterythrous vision, has been used to imply a failure to 

 distinguish between red, yellow and green, with a shortening of the long wave 

 length end of the spectrum. 



(b) Deuteranopia, or photerythrous vision, has been used to imply a failure to 

 distinguish between red, yellow and green without any shortening of the 

 spectrum. 



(c) Tritanopia has been used to indicate a failure to distinguish between blue and 

 green, not necessarily accompanied by shortening of the short wave length 

 end of the spectrum. 



7. The following method of classification is recommended, as it avoids al? 

 theoretical views and allows for variation in degree of the defect. 



(a) Any marked decrease in sensitivity to a special region of the spectrum should 

 be specified as blindness ; thus red blindness indicates decreased sensitivity to long 

 wave lengths, and if one can state the wave lengths involved the description will be 

 more accurate. 



Note to 7 (a). — The length of the spectrum for a normal individual will vary 

 with the intensity and quality of the light used, as well as with the characteristics of 

 the optical system. Therefore the best method of showing the defect is to present 

 a curve showing the increase in threshold to light of the region involved. Any 

 spectral apparatus with a shutter for isolating a narrow region of the spectrum can 

 be used to determine the rise in threshold, if the intensity of the light is varied by an 

 episcotister or a photometric wedge. Any shift in the region of maximum intensity 

 would be shown by such a method. 



(6) Any failure to discriminate colours should be described as confusion, prefixing 

 the colours confused, e.g. yellow-green confusion. The number of distinct colours 

 which are recognised in the spectrum is a rough measure of the degree of colour 

 discrimination ; thus Dr. Edridge-Green classifies those with defective colour 

 discrimination as penta-, tetra-, tri-, di-, and mono- or a-chromats. As in 7 (a), it is 

 preferable to have some quantitative statement of the degree of the defect. 



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