THE EYE AND THE PHYSIOLOGY OF VISION. 571 



It is said that about one female in twenty-five is affected 

 with color blindness of one variety or another, and about 

 one male in every eight or ten. Red-blindness is the more 

 frequent defect. It is hard to explain the relatively higher 

 per cent, of color-blindness in men, unless it is due to the 

 fact that they pay relatively less attention to colors and that 

 this neglect in their visual education has contributed to this 

 defect. 



What we call green-blindness and red-blindness are, after 

 all, varieties of the same defect, for red-blind and green- 

 blind alike see blue and yellow only. In the green-blind 

 people our red seems a most intense yellow, while in red- 

 blind people our green seems most intense. As it is ex- 

 ceedingly necessary in many professions, especially that of 

 navigation and railroading, that employes shall be able to 

 readily distinguish between a green signal and a red danger 

 signal, such corporations now very generally subject their 

 employes to a very careful color test in order to preclude 

 as much as possible an accident from such a lack of dis- 

 crimination. 



With a general description of these three varieties of 

 color-blindness we are enabled to understand possibly a 

 little more readily each of the two attempts made to explain 

 them, as well as the color sensations of the normal eye. 



THE YOUNG-HELMHOLTZ THEORY. 



According to this theory there are found in the retina 

 three kinds of nerve fibers, physiologically considered. To 

 these three different kinds of fibers are attributed respect- 

 ively sensations of red, green and blue. The sensation of 

 blue is attributed to the rods. It will be remembered that 

 the rods are scattered over the entire retina but are wanting 

 in the yellow spot, for which reason, if this theory be true, 

 the yellow spot should be blind to sensations of blue. And 

 such seems really to be the case. If with proper apparatus 

 one arranges a blue point in such a way that it shall fall 

 right in the fovea centralis, it seems to disappear, while red 



