THE EYE AND THE PHYSIOLOGY OF VISION. 575 



blue produce purple. Red and white a sensation in which 

 the red nerves are more stimulated than in the proportion 

 of white, produces pink. Violet, although usually indi- 

 cated in elementary books as one of the fundamental color 

 sensations, is in reality not one, but is a mixture of the blue 

 and red, for it will be remembered when the number of 

 vibrations of the various colors was given that it was pointed 

 out that the violet was just an octave above the red, and 

 that the red seemed to appear again there mixed with the 

 blue. It was pointed out in the case of sound that a string 

 set in vibration by a certain sound would be readily affected 

 by its octave. So a nerve in the retina that is affected by 

 red seems also affected by an octave of red, and for that 

 reason blue will gradually "shade into the violet as the 

 octave of the red is there approached. As the sensation of 

 blue was attributed to the rods, yellow and green sensa- 

 tions and the colors into which they enter are attributed to 

 the cones. 



R Y Gr Bl V 



Fig. 180. GRAPHIC REPRESENTATION OF THE COMPOSITION OF COLOR SENSATIONS IN 



TERMS OF THK YOUNG-HELMHOLTZ THEORY. THE CURVES FOR RED. GREEN AND 

 BLUE ARE HERE SUPERIMPOSED. 



THE BERING THEORY. 



According to the theory of Hering there are in the retina 

 three different color substances which are either in process 

 of being made, assimilation, or in process of destruction, 

 dissimilation. One of these color substances he calls his 

 white-black substance. When this substance is being pro- 

 duced in the eye the sensation of black results; when it is 

 being destroyed in the eye the sensation of white results. 

 Hence, objects appear white when the light falling into 

 the eye causes this white-black substance to disintegrate. 

 Things appear black when on that point of the retina this 

 white-black substance is being formed. 



