Report of the Committee on Colour- Vision. 351 



illustrated in the following way : — Let us take an ordinary 2-iinit 

 colour-blind, and, having given him the set of wools belonging 

 to the Classification Test, ask him to pick out all the reds. On 

 examining the pile of wools selected as red, it will be found that 

 the majority are red, but in addition there will be some browns, 

 and yellow-greens. If he be then told to pick out the whole of 

 the greens, the greater number of those selected will be greens, 

 but there will be also greys, browns, and reds. In each case it 

 will be seen that the majority of wools are of the desired colour. 



If another 2-unit colour-blind be examined in the same way it 

 will be found that, though he may not make exactly the same 

 mistakes, he will in all probability pick out the same greens to 

 put with the reds, and the same reds to put with the greens. 

 The same result will be obtained if the colour-blind persons be 

 asked to name a large number of colours. They will in most 

 cases name the colour correctly. It will be noticed that the 

 greens which were put with the reds when classifying the 

 colours, will be called red in naming them. It is evident that 

 the same idea has guided the colour-blind in each case. 



This shows that, though a person may be red-green blind, he 

 is not absolutely red-green blind in the sense of being totally 

 unable to distinguish between the two colours. This is what we 

 should expect, as the red and green are included in an approxi- 

 mate, not in an absolute psychophysical unit. The fact that they 

 are actually judging by colour may be demonstrated by giving 

 them coloured materials of different kinds, or by asking them to 

 name a large number of coloured objects. To a person with a 

 spectrum of normal length and no neutral band in the blue-green, 

 it is necessary that the colours, to be considered as identical, 

 must be included in an absolute psychophysical unit. One of the 

 most definite signs that persons with a neutral band in the blue- 

 green have a more defective colour-perception than the ordinary 

 2-unit, is that they will put together as identical a red and green 

 which are distinguished by the ordinary 2-unit. In addition to 

 this, they will mistake the reds and greens which have been 

 confused by the ordinary 2-unit. 



It will be seen that if we take a 2-unit and ask him to name a 

 number of red and green wools, in the majority of instances he 

 will name them correctly. But as, almost invariably, the same 

 wools are chosen, for all practical purposes the same result would 

 be obtained by asking a person to name a few of these wools. 

 What more decided and brighter greens could we have than 

 Nos. 76 and 94 of my Pocket Test? yet these are two of the 

 greens which are called reds by the 2-unit. We should have 

 accomplished as much by asking a colour-blind person to name 

 Nos. 76 and 94 as if we had asked him to name a large number 

 of greens. The colours in a test should, therefore, be those which 

 the colour-blind are particularly liable to miscall. At the same 

 time, their nature should be unmistakable to the normal sighted. 



My second principle is that a colour-blind person will name 



