THE SENSE OF VISION 



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Colour Vision. Numerous theories have been advanced to explain 

 the phenomena of colour vision. The two classical views are those of 

 Young-Helmholtz and of Her ing. 



The Young-Helmholtz Theory. This theory assumes that there 

 -are three separate substances in the retina stimulated by the 

 wave-lengths of the three fundamental colours red, green, and 

 violet. Simultaneous excitation of all three gives the sensation of 

 white, while absence of stimuli gives that of black. The colour 

 sensations depend upon different degrees of stimulation of these 

 "three substances (see Fig. 355). From the diagram, it will be seen 

 that orange is due to a large stimulation of the red, and lesser stimu- 

 lation of the green and violet substances. In the sensation of 

 yellow, the red and green elements are almost equally excited, and the 

 violet but little, and so on according to the manner indicated in the 

 diagram. 



Green 



Blue. 



Violet. 



Orange Yellow. 



FIG. 355. SCHEMA TO ILLUSTRATE THE YOUNG-HELMHOLTZ THEORY OF COLOUR 



VISION. (Helmholtz.) 



The curves represent the intensity of stimulation of the three colour substances: 

 \, The red-perceiving substance ; 2, the green-perceiving ; 3, the violet-perceiving. 

 Vertical lines drawn at any point of the spectrum indicate the relative amount 

 of stimulation of the three substances* for that wave length of the spectrum. 



The Hering Theory. According to this view, the fundamental 

 colour sensations are red, yellow, green, and blue. These are grouped 

 together in pairs red-green, blue-yellow, and in addition black 

 and white (black-white). In the retina are three corresponding 

 substances the green-red, blue-yellow, and black-white. These 

 substances may be stimulated either in an anabolic (a building-up) 

 direction or in a katabolic (a breaking-down) direction. Thus, if one 

 or other of the substances be broken down, the receptors of the retina 

 are stimulated in such a manner that red, yellow, and white, are 

 respectively appreciated by the brain. If the changes be in an 

 anabolic direction, then the conscious sensation is respectively green, 

 blue, or black. 



A nabolism. Katabol is in. 



Green. Red. 



Blue. Yellow. 



Black. White. 



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