The Special Senses 



261 



the defect is accidentally revealed. The common form of 

 defective color vision is the inability to distinguish between 

 red and green. This abnormal defect of vision may be a 

 matter of great practical importance to those employed on 

 steam and electric railways, on shipboard, and elsewhere. 



Experiment 120. To test color-blindness. On no account is the 

 person being tested to be asked to name a color. In a large class of 

 students one is pretty sure to find some who are more or less color- 

 blind. The common defects are red and green. 



Place worsteds on a white background in a good light. Select, as 

 a test color, a skein of light green color, such as would be obtained 

 by mixing a pure green 

 with white. Ask the 

 examinee to select and 

 pick out from the heap all 

 those skeins which appear 

 to him to be of the same 

 color, whether of lighter 

 or darker shades. A color- 

 blind person will select 

 amongst others some of 

 the confusion colors, e.g., 

 pink, yellow. A colored 

 plate showing these should FIG. 137. Showing the Relative Position of 

 be hung up in the room. the Lachrymal Apparatus, the Eyeball, and 

 Any one who selects all the Eyelids. 



the greens and no con- A, lachrymal canals, with the minute orifices repre- 

 fusion colors has normal sented as two black dots (puncta lacrymalia) to 



the right ; , tendon of the orbicularis palpe- 

 brarum muscle ; apparently under B is seen the 

 lachrymal sac. The minute openings of the 

 Meibomian glands are seen on the free margins 

 of the eyelids. Below A is seen a small conical 

 elevation, with black dots (the lachrymal papilla 

 or caruncle). 



color vision. If, however, 

 one or more confusion 

 colors be selected, proceed 

 as follows : select as a 

 test color a skein of pale 



rose. If the person be red 

 blind, he will choose blue and violet ; if green blind, gray and green. 

 Select a bright red skein. The red blind will select green and 

 brown ; the green blind will choose red or lighter brown. 



