PROPERTIES OF THE RETINA. 347 



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

 differs from the sensitive photographic plate. Comparison of the 

 threshold stimulus for different parts of the retina, in an eye 

 exposed alternately to darkness and to light, has shown that 

 in the dark the sensitiveness in the peripheral field increases 

 greatly during an hour or so, while that of the foveal field is appar- 

 ently unchanged. With such a dark-adapted eye, therefore, 

 there will be a certain dim light which will be seen by the per- 

 ipheral parts of the retina, but perhaps will cause no reaction 

 upon the fovea. For such a degree of light, therefore, the fovea 

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

 Anyone may notice in late twilight, 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 little to the side, so as to bring its image into the periph- 

 eral field. This greater sensitiveness of the dark-adapted eye in 

 the peripheral field where the rods predominate over the cones seems 

 to be associated with the movement of the pigment in the pigment 

 epithelium (see above) and the resulting regeneration 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 light in the rod-region and explain why a similar 

 increase fails to occur in the fovea, where only cones are present. 

 The curve given in Fig. 145 shows that in the dark-adapted eye 

 the threshold of vision in the peripheral field may be lower than in 

 the fovea. In accordance with these facts von Kries* has suggested 

 that the rods, the peripheral field of the retina, are especially 

 adapted for vision in dim lights, night vision, while the cones are 

 especially adapted for vision in strong lights, day vision. This 

 general fact will perhaps accord with the experience of anyone who 

 attempts to estimate the value of his peripheral vision in dim 

 nightlight as compared with daylight. Other interesting differ- 

 ences in the reaction of the light-adapted and the dark-adapted 

 eye are referred to below in connection with color blindness. 



CHARACTERISTICS OF THE VISUAL SENSATIONS. 



In addition to the spatial attributes connected with our visual 

 sensations that is, the perception of form they are characterized 

 by two properties which may be described in general as variations 

 in intensity and in quality. 



Luminosity or Brightness. That characteristic which we 

 describe as the luminosity or brightness of a visual sensation has 



* Von Kries, " Zeitschrift f . Psychologic u. Physiologic d. Sinnesorgane,* 

 9. 81, 1895. 



