804 



PHYSIOLOGY. 



in breadth to two of the height of the letters are distinctly legible 

 to a normal eye under an angle of five minutes. These letters are 

 numbered, the numbers expressing in meters the distance at which 

 the letter can be seen under an angle of five minutes. The eye is 

 tested with letters smaller and smaller at the same distance from 

 the eye, 6 meters. Suppose No. 6 type is thus seen ; then 6 / 6 = 1, 



Fig. 354. The Visual Angle. 



If No. 8 is only seen at 6 

 or three-fourths of that of 



the acuteness of vision of a normal eye. 

 meters, then the acuteness of vision is 6 

 the normal eye. 



DURATION OF BETINAL STIMULATION. Light impresses the re- 

 tina, but the excitation of it does not cease immediately with the 

 disappearance of the luminous vibrations. Indeed, they persist for 

 a certain time, about one-eighth of a second: that is, proportional 

 to the intensity of the excitation. Upon a disc black and white sec- 



Fig. 355. Optogram on the Rabbit's Retina of a Window Four 

 Meters Distant. (KtJHNE.) (From Tigerstedt's "Human Physiology," 

 copyright, 1906, by D. Appleton and Company.) 



6, &, White streak of nerve-fibers, and yellow spot in its center. 



tions are alternately painted. When the disc is made to rotate 

 rapidly the disc appears neither black nor white, but gray. 



VISUAL PURPLE, OR KHODOPSIN. The outer part of the rods 

 contains a reddish coloring matter which is called visual purple. 

 The cones do not have any. This coloring matter must be kept in 

 the dark, for it bleaches the moment light strikes it. But the color 



