Report of the Committee on Colour- Vision. 379 



and not using the method correctly. The especial purpose of 

 this method must be kept constantly in view, viz., to find 

 the characteristics of the defects in colour-perception of those 

 examined. The characteristic of green-blindness is the confusion 

 of purple with grey or green, or both. This confusion is the point- 

 to be determined : everything else may be neglected. A complete 

 colour-blind, who confuses purple with grey or green (bluish- 

 green), or with both, is green-blind, do what else he may. This is the 

 rule, and the careful and observant examiner who understands 

 the application of the test, will at once distinguish it. It is, 

 indeed, often possible, in marked cases of incomplete colour- 

 blindness, to decide to which class it belongs to by the way 

 the examined acts with his hands. We do not mean b\ 7 this that 

 the diagnosis is always very easy. Practice and knowledge are 

 necessary. As there is a long series of degrees of incomplete 

 colour-blindness between normal vision on the one hand, and 

 complete colour-blindness on the other, there must naturally be 

 a border line where differences of the two kinds of colour- 

 blindness cease to be recognised. 



" The examination may end with this test, and the diagnosis 

 be considered as perfectly settled. It is not even necessary, 

 practical^, to decide whether the colour-blindness is red or 

 green. But to more thoroughly convince railway employes and 

 others, who are not specialists, of the reality of the colour-blind- 

 ness, the examination may be completed by one more test. It is 

 not necessary to the diagnosis, and only serves as a confirmation. 



" Test III. — The red skein is presented to the examinee. It is 

 necessary to have a vivid red colour, like the red flag used as 

 signals on railways. The colour should be that of 116 of the 

 plate, rather towards yellowish-red. 



" Rule. — This test, which is applied only to those completely 

 colour-blind, should be continued until the person examined has 

 placed beside the test skein all the skeins belonging to this hue 

 or the greater part, or else one or more k confusion colours ' 

 (10-13). The red-blind chooses, besides the red, green and 

 shades of brown, which (10-11), to the normal sense, seem 

 darker than red. On the other hand, the green-blind selects 

 shades of these colours, which appear lighter than red (12-13). 



" Remark. — Every case of comparatively complete colour-blind- 

 ness does not always make the precise mistakes we have just 

 mentioned. These exceptions are either instances of persons 

 who are not quite completely colour-blind, or of completely colour- 

 blind persons who have been practised in the colours of signals, 

 and who endeavour not to be discovered. They usually confound 

 at least green and brown ; but even this does not always happen. 



" Mono - chromatic Vision. — The absence of all except one 

 colour sensation, will be recognised by the confusion of every 

 hue having the same intensity of light. 



u Violet-blindness will be recognised by a genuine confusion of 

 purple, red, and orange in the second test. The diagnosis should 



