PROPERTIES OF THE RETINA. 327 



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. 138 shows that in the dark-adapted eye 

 the acuity of vision in the peripheral field is greater 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 4 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 differences in the reac- 

 tion 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 

 been defined differently by various writers. We may consider it, 

 however, as the expression of the intensity of the acting stimulus. 

 Sensations of the same quality are easily compared as regards their 

 brightness. We can tell as between two whites or two greens which 

 is the brighter of the two, but when two different qualities a red 

 and a green sensation, for instance are compared our subjective 

 determination of the relative brightness is, for most persons, difficult 

 or impossible to make. To a lesser degree the difficulty is similar 

 to that of the comparison of sight and sound. According to the 

 conception adopted here, however, that the brightness is an ex- 

 pression of the intensity of the stimulus, an objective standard of 

 comparison might be obtained by measuring the resulting action cur- 

 rents in the optic nerve fibers. When the spectral colors are ex- 

 amined it is obvious that some of the colors are brighter than others, 

 the extreme red and extreme violet, for instance, possessing little 

 luminosity as compared with the yellow. The relative brightness 

 of the different spectral colors is found to vary with the amount of 

 illumination, as is shown in the curves given in Fig. 139. With a 

 brilliant spectrum the maximum brightness is in the yellow, but 

 with a feeble illumination it shifts to the green. This fact accords 

 with what is known as the "Purkinje phenomenon," namely, the 



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

 9, 81, 1895. 



