854 



THE FUNCTION OF THE RETINA IN VISION. 



when the dark figure is produced by pressure on the open eye in con- 

 sequence of which the circulation of the retina is disturbed, external 

 objects are not perceived by the retina. 



When the eye is well rested, it has a diminished sensitiveness for colors in a 

 poor light, and the green portion of the spectrum seems to possess the greatest 

 degree of brightness, v. Kries believes that under such circumstances the rods 

 are active, while with vision in strong illumination the cones functionate. As 

 color-blind individuals perceive the brightness in the spectrum in the same way 

 as does the well-rested eye in a poor light, perhaps in their case only the rods 

 take part in vision. 



The duration of the retinal stimulation need be but brief. The 

 stimulation by light does not reach its full strength at once, but it increases 

 gradually, so that a weak light that persists for some time may appear 



FIG. 299. Perimetric Chart of a Healthy and of a Diseased Eye. 



just as bright as a strong light that persists for but a short time. In 

 general, the larger and the brighter the objects the less the time necessary 

 for their perception. If light and darkness are quickly alternated, the 

 same degree of illumination is produced as if the action of the light 

 were divided uniformly over the entire time of the observation. A 

 light-stimulus having 17 or 1 8 alternations in a second, produces the 

 strongest sensation. In order that two flashes of light shall be per- 

 ceived separately, 0.027 second must elapse between them. Further 

 an increase or decrease of o.oi part of the light-intensity will be recog- 

 nized. A shorter time is required for the perception of yellow than for 

 that of red and violet. Long exposure to darkness, as during the night 

 makes the retina more sensitive to light. When the light-stimulus is 



