360 PHYSIOLOGY CHAP. 



tion of the eye to darkness (scotopia), caused by the gradual 

 recuperation and consequent increase of light sensibility in the 

 retina. 



The opposite effect is also an everyday experience. If after 

 remaining a long time in the dark, or in semi-darkness, one passes 

 suddenly into full light, the illumination owing to the great 

 sensitiveness of the retina is at first so dazzling that it is im- 

 possible to see any object distinctly. Little by little, however, the 

 retinal sensibility decreases from fatigue, and within a few minutes 

 adaptation to light (photopia) is established. 



Aubert, and more recently Piper, determined the curve of 

 retinal adaptation to light in different people. Piper's results 

 (1903) show that sensibility increases very slowly during the first 

 10 minutes in darkness ; from 10 to 30-40 minutes the progressive 

 increase is very rapid ; from 30-40 up to 60-70 minutes the increase 

 is again very slow, till the maximum of adaptation, i.e. of retinal 

 sensitiveness, is reached. 



k The general character of the curve differs little with the indi- 

 vidual; the degrees of sensitiveness, on the contrary, vary widely. 

 This agrees with the common fact that the capacity for seeing 

 with very weak illumination differs greatly in different individuals, 

 even within physiological limits, quite apart from the pathological 

 conditions known as hemeralopia or night blindness. 



The increased functional capacity of the eye when adapted to 

 darkness is always very considerable. According to Piper retinal 

 sensitiveness to light may increase 1400-8000 times in the dark ; 

 but these values depend to a great extent on the size of the object 

 and the quality of the light with which the threshold of sensi- 

 bility is determined. 



No special experiments have been made to discover the curve 

 of progressive decline in retinal sensibility during light adapta- 

 tion, but it is a priori obvious that it must vary to a large extent 

 with the strength of the luminous stimulus. It is easy to verify 

 the fact that light adaptation occurs much more rapidly than dark 

 adaptation even for lights of moderate intensity, so that the eye 

 soon attains its minimal sensitiveness. 



Another fact that can be easily verified in the dark-adapted 

 eye is that the increase of sensibility in the centre of the retina is 

 much less than in its more eccentric parts. A delicate observation 

 by Arago illustrates this fact. He noted that certain small stars, 

 which are visible from the periphery of the retina by indirect 

 fixation, become invisible as soon as they are viewed directly, so 

 that their image falls on the fovea. A more direct proof of this 

 was given by v. Kries. When, with the eye well adapted to 

 darkness, we gaze in a room dimly illuminated by diffuse light at 

 a black velvet field in which a number of small discs of white or 

 blue paper are fastened, we can by looking aside (indirect vision) 



