PROPERTIES OF THE RETINA. 



339 



are sensitive only to light and darkness, and by virtue of their power 

 of adaptation in the dark through the regeneration of their visual 

 purple they form also the special apparatus for vision in dim 

 lights (night vision). Color blindness, therefore, whether total or 

 partial, may be regarded as an affection or lack of normal develop- 

 ment of the cones. On the other hand, those interesting cases in 



Fig. 145. Schema of the structure of the human retina (Greeff): I, Pigment layer; 



layer; 6, fiber of Miille'r. 



which the vision, while good in daylight, is faulty or lacking in dim 

 lights (night blindness, hemeralopia) may be referred to a defective 

 functional activity of the rods, probably from lack of formation of 

 visual purple. 



