806 



PHYSIOLOGY. 



The former do not pass through the media of the eye, since by vibra- 

 tion-rates beneath 435,000,000,000 per second the retina is not stim- 

 ulated; the latter color produces no sensation, since to vibration- 

 rates above 764,000,000,000 per second the retina is insensible. 



SENSATIONS OF COLOR. In the production of the sensations of 

 color there are three chief factors: tone, saturation, and intensity. 

 The tone of the color depends upon the number of vibrations of the 

 ether. A color is said to be saturated when it does not contain any 

 white light. The simple colors of the spectrum are saturated. The 

 intensity of color depends upon the amplitude of the vibrations. 



x -y >> 



Ilia 



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Fig. 357. Wools for the Detection of Color Blindness. (OLIVER.) 



Loss OF COLOR-VISION, OR DALTONISM. Young stated, as the 

 explanation of color-vision, that all the colors were referable to three 

 fundamental sensations: those of red, green, and violet. Corre- 

 sponding to the three sensations excited by these three colors were 

 three kinds of retinal fibers, stimulation of which gives rise to sen- 

 sations of red, green, and violet. It is also supposed that white light 

 stimulates these fibers with different degrees of activity according to 

 the length of the wave. The longest wave acts most on the fibers 

 which respond to the red color, the medium wave on the fibers which 

 respond to green, and the shortest wave on the violet. Helmholtz 

 adopted the theory of Young. It is also supported by the facts of 

 color-blindness, in which there is an inability to distinguish one or 

 more of the fundamental colors. The commonest form of color- 

 blindness i"s that in which red is the invisible color, and in the com- 





