274 HUMAN PHYSIOLOGY 



the dark, e.g. a white object on a dark background appears 

 to be surrounded by a very dark border. 



Irradiation. Light objects upon a dark background appear 

 larger than they really are. This is due either to the fact that the 

 stimulation of a portion of the retina radiates to the neighboring 

 parts or that, because of imperfect accommodation, the image of 

 the light object is enlarged by circles of diffusion. 



B. Quality of the visual sensation. The retina is stim- 

 ulated only by rays whose wave lengths lie between 0.69/4 

 and 0.39^. These visible rays are found in daylight. 



Strictly speaking, not all the visible rays are present in daylight,, 

 for those corresponding to the Fraunhofer lines are not present. 



The sensation produced by daylight is white. A white 

 color of lower intensity is called gray. Objects which send 

 out no rays capable of stimulating the retina appear black. 



The visible rays are the more refractive the shorter their 

 wave length. When the various rays are separated by pass- 

 ing the light through a prism, it is observed that they pro- 

 duce various visual sensations, called color sensations. 



The sensation of 



Red is produced by rays of 0.69 u wave-length. 



Orange " " " " 0.64" 



Yellow " " " " 0.59 " " 



Green " " " " 0.53 " " 



Blue " " " " 0.46" 



Violet " " " " 0.39 " " 



The sensations of color are produced by the simple or 

 homogeneous rays only in medium intensity. All rays which 

 in medium intensity appear colored are colorless in very 

 strong or weak intensity. If the intensity of homogeneous 

 rays is decreased, the relative brightness is changed. The 

 spectral red is brighter in medium intensity and darker in 

 lesser intensity than the blue. This, however, can be 

 observed only by the eye adapted to dim vision. 



Mixed colors are produced by the simultaneous action of 

 rays of various wave lengths upon the retina. Of the mixed 

 colors one, i.e. purple, is not found in the spectrum. It 

 is produced by mixing red and violet. The mixed colors 



