878 Report of the Committee on Colour-Vision. 



" Diagnosis. — An examinee who places with the test-skein 

 4 confusion colours ' (1-5)— that is to say, finds that it resembles 

 the 'test-colour' — -is colour-blind, whilst if he evinces a manifest 

 disposition to do so, though he does not absolutely do so, he 

 has a feeble chromatic sense. 



" Remark. — We might have taken more than five colours for 

 4 confusion ; ' but we must remember that we are not taking- into 

 consideration every kind of defective colour-sense, but only those 

 which are important in connection with railways. 



"As to No. 1, which represents a grey, we would remark 

 that too much stress must not be laid on its luminosity, or 

 on any slight difference in its hue from the grey skeins which 

 the examinee puts with the sample. 



*' If it is only required to determine whether a person is colour- 

 blind or not, i.o further test is necessary, but if we want to know 

 the kind and degree of his colour-blindness, then we must proceed 

 with the next test. 



" Test II. — A purple skein is shown to the examinee. The 

 colour should be midway between the lightest and darkest. It 

 will only approach that given in II of Plate II, as the colour of 

 the wool is much more brilliant and saturated, and bluer. 



" Rule. — The trial must be continued until the examinee has 

 placed all or the greater part of the skeins of the same shade 

 near the sample, or else, simultaneously or separately, one or more 

 skeins of 'the coi i fusion-colours ' (6-9). If he confuses the 

 colours he will select either the light or deep shades of blue and 

 violet, especially the deep (6 and 7), or the light or deep shades 

 of one kind of green or grey inclining to blue (8 and '.)). 



" Diagnosis. — 1. A person who is proved colour-blind by the 

 first test, and who, in the second test, selects only purple skeins, 

 is incompletely colour-blind. 



" 2. If, in the second test, he selects with the purples blue and 

 violet, or one of them, he is completely red-blind. 



" 3. If, in the second test, he selects with purple only green 

 and grey, or one of them, he is completely green blind. 



''■Remark. — The red-blind never selects the colours taken by 

 the green-blind, and vice versa. The green-blind will often place 

 a violet or blue skein by the side of the green, but it will then 

 only be the brightest shades of these colours. This does not 

 affect the diagnosis. 



" The fact that, in this test, many green-blind select, besides 

 grey and green or one of these colours, also bright blue, has led 

 to misunderstanding. Some have concluded from this that red and 

 green blindness may exist together in the same individual; others 

 have thought that these two kinds of colour-blindness are not 

 readily distinguished by this method. The first conclusion is not 

 correct. The two kinds of colour-blindness have great similarity, 

 but differ in innumerable slight variations. They are to be 

 considered as two sharply defined classes. 



"The second conclusion can only arise from not understanding 



