80 THE VISUAL PROCESS 



pulse, the frequency of discharge thus bearing a relation to the amount 

 of stimulation. 



The attached ganglion cells now behave similarly and conduct in 

 synchrony with the activity in the bipolars. The electrical aspect of their 

 discharges can be picked up in the optic nerve as action currents 

 with proper amplifying and recording devices. In the brain, a sensation 

 of light is now aroused whose strength depends upon the resultant of 

 the number of active nerve fibers and their frequency of discharge. As 

 dark-adaptation proceeds further, the number of rods per unit area of 

 the retina whose activity actually registers in consciousness steadily in- 

 creases, due to the activation of more and more bipolars having smaller 

 and smaller numbers of associated rods. As the mosaic of functional 

 receptor units becomes more and more dense, visual acuity rises hand in 

 hand with the rise in the strength of the brightness sensation. When 

 dark-adaptation is complete, both visual acuity and brightness are max- 

 imal for the intensity being supplied, and any further increase in either 

 will depend upon an increase of illumination above the threshold of the 

 cones, thus bringing the latter into play. The destruction of rhodopsin 

 may then increase to such an extent that the brightness would decrease 

 in the face of increasing objective intensity — in other words, light adap- 

 tation would have commenced. Incidentally, rising intensities above the 

 cone thresholds naturally bring into action more and more cone bipolars 

 and associated ganglion cells, so that visual acuity continues to rise until 

 all elements are functioning. Beyond this point, further increase of in- 

 tensity brings no additional visual acuity — though of course brightness 

 can increase until all involved optic nerve fibers are discharging into 

 the central nervous system at their maximum rates. 



If, with the retina thoroughly dark-adapted, it is now subjected to 

 bright light, rhodopsin is immediately broken down in large amounts in 

 all of the rods which are receiving stimulation, and all of their associated 

 nerve fibers begin to conduct at high frequency. As the rhodopsin fades, 

 however, the rod thresholds rise and the frequency falls off. As the rod 

 thresholds approach those of the cones, a comfortable brightness is 

 attained with the pupil now reopened, and with the rods perhaps still 

 all in action, contributing all that they ever can to the resolving power 

 of the retina — considering that they are of course still summated. In 

 comfortable illuminations above the cone threshold, however, the cones 

 are contributing only a part of their potential resolving power, which 

 becomes maximal only at intensities above 100 lux. 



