1911.] 



A Confusion- Test for Colour- Blindness. 



83 



IV. For a fourth card we made a full-toned lilac with mauve, magenta 

 and zinc white. The colour by which this was matched was a fairly pure 

 blue quite free from any tint of purple, but showing in the spectroscope a 

 good deal of green. On the lilac half I painted P.K.W. in blue, and on the 

 blue half the letters X and I in a slightly paler blue, made by mixing a very 

 little more zinc white with the ground colour. The difference is so slight 

 that very few people can trace these letters through the perforated zinc, 

 although they see the letters P.K.W. immediately. 



Lieut.-Col. Scott, on the other hand, could not detect P.K.W. and the. 

 whole card appeared of one uniform bright blue tint to him, but he instantly 

 perceived the letters X and I. I have noticed this same sensitiveness to 

 depth of colour in other cases of red-blindness. 



Complete or nearly complete red-blindness is immediately detected by 

 these cards. Partial cases can sometimes read the letters, but do so with 

 difficulty. Green-blind people easily read the first three, but are stopped by 

 the fourth, being unable to distinguish between lilac and the greenish blue. 



The apparatus has been in use in Oxford for colour-testing ever since we 

 made it, and answers admirably. We found that the X and I on No. IV 

 were easily visible to all the cases of red-blindness, but not to the green-blind 

 nor the normal. 



With regard to the permanence of the colours, we found in 1908, i.e. after 

 five years, some slight changes had taken place in all but No. I, which had 

 not altered after being oiled. In No. II the geranium had lost something of 

 its blue and violet components, and in No. Ill the yellow mixed with the 

 emerald green had faded, so that Lieut.-Col. Scott, who knew what to look for, 

 could just make out the lettering. But other colour-blind people were still 

 deceived and No. IV was as effective as ever with both the red- and the 

 green-blind. 



All test-cards, especially such as Nagel's and Stilling's, are liable to 

 deteriorate, but the use of the perforated zinc enables fresh designs to be 

 prepared with so little trouble that this objection is easily remedied in our 

 test-cards. It is much to be desired that some one who is green-blind, or has 

 otherwise abnormal colour-sensation, would prepare a set of cards with as 

 much care and accuracy as Lieut.-Col. Scott did these for red-blindness. 



Most of the testing was done in the Physiological Laboratory, Oxford, some 

 in University College, Pleading, and some were cases from the Oxford Eye 

 Hospital. The expenses, which were trifling, were defrayed out of the 

 Government Grant Fund. 



