540 



VISION 



that point, so that when light from the uniformly illuminated surface now 

 strikes the retina, that particular point is incapable of being excited so strongly, 

 as the remaining relatively unfatigued parts; hence, the corresponding point 

 of the field of vision appears dark in comparison with the other parts. 



In fact the sensitiveness of the retina is all the time changing whether it 

 is being acted upon by the light or is protected from the light in the one 

 case becoming progressively less and in the other progressively greater. These 

 changes taken together are described by Aubert as the adaptation of the retina. 



For example, when we pass from a light room into one very feebly lighted 

 at first we are unable to see anything; gradually, however, the sensitiveness of 

 the retina becomes greater, until the feeble light produces a plainly perceptible 



40 58 104 



A 

 A 

 A 



I 



IA 



8 3749 81 127 359 650 



FIG. 235. Excitation of the retina as a function of the time exposed, after Exner. The abscissae 

 represent the time in thousandths of a second, the ordinates the strength of the sensations. 



impression; in fact after remaining longer in the dark room the light can be 

 greatly reduced in intensity without passing below the threshold. 



According to exact measurements on the adaptation of eyes to the dark the 

 rate is about the same for different individuals, but the absolute increase in 

 sensitivity varies all the way from 1,400 to 8,000 fold. With both eyes the 

 increase in sensitivity is 1.6-1.7 times as great as with only one eye. These 

 facts apply only to the peripheral parts of the retina; for the fovea centralis 

 the adaptation is very much less complete, the increase in sensitivity being only 

 twenty to thirty times the sensitivity of the eye adapted for light. 



When we pass back after complete adaptation to the dark into a brightly 

 illuminated room, the strong light at first has a blinding effect upon the retina, 

 which has meantime become extremely sensitive. But, after a short time, its 

 sensitiveness has so far decreased that there is no excessive stimulation. In 

 other words, the eye adapted to the dark has now become adapted to a high 

 degree of illumination. Another point of evidence that the condition of the 

 retina is adapted to the strength of the light, is the fact that the size of the 

 pupil remains the same for a rather wide range of intensity, changing only at 

 the moment the intensity changes. 



