VISION. 



805 



will return if the eye is again brought into a dark chamber. The 

 bile acids extract the coloring matter from the retina. The visual 

 purple is a product of melanin or fuscin. 



Kiihne has shown that an image or optogram of an object may 

 be fixed on the retina by plunging it into a 4-per-cent. alum solution 

 immediately after death. 



The visual purple increases in the dark, and is supposed to ren- 

 der the rods more irritable in the dim light of the evening. Hence 

 Van Kries has put forth the theory that the more abundant rods in 

 the peripheral field of the retina are chiefly fitted for vision at night, 

 whilst the cones are chiefly for day vision in strong light. 



Fig. 356. Diagram Illustrating the Decomposition of White Light 

 into the Seven Colors of the Spectrum in Passing Through a Prism. 

 ( BECLARD. ) 



r, Red. o, Orange. ;, Yellow, v, Green. 6, Blue, i, Indigo, vi, Violet. 



Color-vision. White light is composed of rays of different re- 

 frangibility by reason of the different length and duration of the 

 luminous rays. These various rays falling upon the retina determ- 

 ine in the individual different sensations which correspond to the 

 colors. To decompose white light into its different colors, the prism 

 is used. A ray of white light upon issuing from the prism presents 

 the spectrum. That is, there emerge the principal simple colors 

 from the most to the least refrangible. They are violet, indigo, blue, 

 green, yellow, orange, and red. Each primary color cannot be fur- 

 ther decomposed, .but all can be reunited by a biconvex lens so that 

 white light will result again. The ultra-red (thermal) and ultra- 

 violet (chemical) rays do not make any impression upon" the retina. 



