140 COLOR 



these three colors in proper proportions, any color of the 

 spectrum, such as yellow or indigo or orange, may be ob- 

 tained. The three spectral colors, red, green, and blue, are 

 called primary or essential hues, because all known tints of 

 color may be produced by the careful blending of blue, green, 

 and red in the proper proportions ; for example, purple is 

 obtained by the blending of red and blue, and orange by the 

 blending of red and yellow. 



135- Color Blindness. The nerve fibers of the eye which 

 carry the sensation of color to the brain are particularly sensi- 

 tive to the primary colors red, green, blue. Indeed, all 

 color sensations are produced by the stimulation of three sets 

 of nerves which are sensitive to the primary colors. If one 

 sees purple, it is because the optic nerves sensitive to red 

 and blue (purple equals red plus blue) have carried their sep- 

 arate messages to the brain, and the blending of the two dis- 

 tinct messages in the brain has given the sensation of purple. 

 If a red rose is seen, it is because the optic nerves sensitive 

 to red have been stimulated and have carried the message to 

 the brain. 



A snowy field stimulates equally all three sets of optic 

 nerves the red, the green, and the blue. Lavender, which 

 is one part blue and three parts white, would stimulate all 

 three sets of nerves, but with a minimum of stimulation for 

 the blue. Equal stimulation of the three sets would give the 

 impression of white. 



A color-blind person has some defect in one or more of the 

 three sets of nerves which carry the color message to the brain. 

 Suppose the nerve fibers responsible for carrying the red are 

 totally defective. If the person views a yellow flower, he will 

 see it as a green flower. Yellow is made up of red and green, 

 and hence both the red and green nerve fibers should be 

 stimulated, but the red nerve fibers are defective and do not 



