PROPERTIES OF THE RETINA. 343 



The Light-adapted and the Dark-adapted Eye. — The con- 

 dition of the retina changes when after exposure to hght it is sub- 

 mitted to darkness, the change being most marked in the peripheral 

 field. When one passes from daylight into a dark room vision 

 at first is very imperfect, but after some minutes it rapidly im- 

 proves, "as the eye becomes accustomed to the dark." The 

 change is known as an adaptation, and in this respect the retina 

 difi'ers from the sensitive photographic plate. This power of 

 adaptation, the power of automatically adjusting its sensitivity to 

 correspond with the variations in intensity of the illumination, is 

 one of the most interesting and remarkable properties of the retina. 

 In the change from the brightness of a day of full sunhght to the 

 darkness of night the eye must adapt itself to a large range of 

 variations in the intensity of the light. Reckoned in the unit 

 employed by the illuminating engineer this range is estimated to 

 extend, under extreme conditions, between ten lamberts* and 

 one millionth of a millilambert, the latter figure expressing the 

 threshold stimulus for the fully dark-adapted eye, a range of in- 

 intensities therefore from one to approximately ten bilhon. The 

 eye exposed to the light becomes light-adapted, exposed to the 

 dark it becomes dark-adapted. The dark-adapted eye is much 

 more sensitive to light, or, to express the fact in other words, its 

 threshold stimulus is greatly reduced. Comparison of the threshold 

 stimulus for different parts of the retina of the dark-adapted eye 

 shows that the increased sensitivity affects mainly the parts out- 

 side the fovea, the peripheral rather than the central field. With 

 a dark-adapted eye, therefore, there will be a certain dim light 

 which will be seen by the peripheral parts of the retina, but will 

 cause no reaction upon the fovea. For such a degree of light the 

 fovea would be blind. This general fact has, indeed, long been 

 known. Anyone may notice in late twihght, when the stars are 

 beginning to appear, that a very faint star may disappear when 

 looked at — that is, when its image is brought upon the fovea; 

 to see it one must direct his eyes a httle to the side, so as to bring 

 its image into the peripheral field. This greater sensitiveness of 

 the dark-adapted eye in the peripheral field where the rods pre- 

 dominate over the cones seems to be associated with the regenera- 

 tion of the visual purple in the external segments of the rods. 

 The increase in the visual purple in the dark may, indeed, account 

 for the increased sensitiveness to hght in the rod-region and ex- 



* Lambert is the C. G. S. unit of brightness; it is defined as a brightness of 

 one lumen per square centimeter. The lumen is the unit of radiant energy or 

 *flux and is defined as the light emitted by a point source of one candle-power 

 in a unit sohd angle. 



