594 THE NERVOUS SYSTEM. 



been reached. It may be said in conclusion that objections may 

 be advanced to all the theories, and that no one accounts for all 

 the observed facts. 



Holmgren Test. Inasmuch as color-blindness is so common a 

 defect, and inasmuch as it may be a most serious one when the 

 individual who suffers from it is engaged in a pursuit in which 

 the lives of human beings may be jeopardized by his inability to 

 distinguish colors, as a railway engineer or a pilot on a vessel, 

 tests have been devised to determine its presence or absence. 

 The one most commonly used is that of Holmgren, and consists 

 in submitting to the one to be examined a lot of skeins of worsted 

 of various colors, from which he is to select those which will 

 match standard skeins of green and pink. 



Fatigue of Retina. When the retina has been exposed for some 

 time to any color, it ceases to be sensitive to that color. It was 

 this fact which led Helmholtz to the adoption of the theory of 

 color-sensation which bears his name. He found that if the retina 

 was exposed to a red light, it became fatigued for that color, 

 and a yellow light appeared green ; if exposed to a green light, 

 and after it was fatigued for that color it was exposed to blue, 

 the blue would appear violet. It was such experiments as these 

 that led him to the opinion that there were three visual substances 

 in the retina, and that there were three primary colors red, 

 green, and violet. 



In a similar manner, if the retina is exposed to white, it 

 becomes fatigued. Thus, if the eye is directed for a time to 

 a piece of black paper placed on white paper, and then the black 

 paper is removed, so that the eye sees only the white, the spot 

 which was covered by the black will be more intensely white than 

 the rest for the reason that the portion of the retina upon 

 which the light from the white paper falls when the other portion 

 is covered by the black becomes fatigued, while the portion on 

 which the rays from the black fall is not fatigued, and by con- 

 trast the white here appears whiter than does that of the other 

 part of the paper. 



After-images. The white spot which is seen in the above 

 experiment is a negative after-image i. e., it is the opposite of 

 the color which causes the fatigue, white being the opposite 

 of black. Negative after-images are also produced when other 

 colors are used. Thus, if a blue paper is imposed on white and the 

 eye directed to it, and the blue then removed, the spot which had 

 been covered by it appears yellow, blue and yellow being com- 

 plementary colors. A negative after-image is always comple- 

 mentary in color to that which causes the retinal fatigue that 

 produced it. 



If the eye is directed to a brightly illuminated object, as the 



