COLOR- VISION AND COLOK-BLINDNESS. 701 



he can safely be trusted to distinguish correctly between signals on all 

 occasions. We want, in short, to ascertain the state of his color-vision 

 generally ; and hence to infer his fitness or unfitness to discharge the 

 duties of a particular occupation. 



For the accomplishment of this object, we do not in the least want 

 to know what the examinee calls colors, but only how he sees them, 

 what colors appear to him to be alike and what appear to be unlike; 

 and the only way of attaining this knowledge with certainty is to cause 

 him to make matches between colored objects, to put those together 

 which appear to him to be essentially the same, and to separate those 

 which appear to hiji to be essentially different. This principle of test- 

 ing was first laid down by Seebeck, who required from examinees a 

 complete arrangement of a large number of colored objects; but it has 

 been greatly simplified and Improved by Professor Holmgren, who 

 pointed out that sucli a complete arrangement was superfluous, and 

 that the only thing required was to cause the examinee to make matches 

 to certain test colors, and, for this purpose, to select from a heap which 

 contained not only such matches but also the colors which the color- 

 blind were liable to confuse with them. 



After many trials, Holmgren finally selected skeins of Berlin wool as 

 the material best suitetl for this purpose; and his set of wools com- 

 prises about 150 skeins. The advantages of his method over every 

 other are that the wool is very cheap, very portable, and always to be 

 obtained in everj^ conceivable color and shade. The skeins are not 

 lustrous, so that light reflected from the surfaces does not interfere 

 with the accuracy of the observation, and they are very easily picked 

 up and manipulated, much more easily than colored jiaper or colored 

 glass. The person to be tested is placed before a table in good day- 

 light, the table is covered by a white coth, and the skeins are thrown 

 upon it in a loosely arranged heap. The examiner then selects a skein 

 of pale green, much diluted with white, and throws it down by itself 

 to the left of the heap. The examinee is directed to look at this pat- 

 tern skein and at the heap, and to pick out from the latter and to place 

 beside the pattern as many skeins as he can find which are of the same 

 color. He is not to be particular about lighter or darker shades, and 

 is not to compare narrowly, or to rummage much amongst the heap, 

 but to select by his eyes, and to use his hands chiefly to change the po- 

 sition of the selected material. 



In such circumstances a i)erson with normal color sight will select 

 the greens rapidly and without hesitation, will select nothing else, and 

 will select with a certain readiness and confidence easily recognized by 

 an experienced examiner, and which may even be carried to the extent 

 of neglecting the minute accuracy which a person who distrusts his 

 own color sight will frequently endeavor to display. Some normal 

 sighted people will complete tlieir selctions by taking greens which 

 incline to yellow, and greeiis which incline to blue, while others will 



