cerebral ai 



EETINAL EXCITATION 371 



>*ebral after-images occur in the sense assumed by Bocci (1896) 

 and other observers. 



The fact from which Bocci assumed the existence of cerebral 

 after-images is only the confirmation by improved methods of an 

 experiment which Brewster made sixty years earlier. 



If, in diffuse daylight, we look with the right eye for some 

 time at a black figure in the middle of a white card, while the left 

 eye is closed, the eye that is fatigued is apt to produce an after- 

 image. But if we then close the right (fatigued) eye, and open 

 the left (rested) eye, no after-image appears. When, however, the 

 card is vividly illuminated by bright sunlight, on opening the 

 rested eye the image of the figure reappears upon a more or less 

 obscure field and is bright red on a green background ; this after- 

 image exhibits the variations of the chromatic phases, with 

 rhythmical waxing and waning. This is the image claimed by 

 Bocci as " cerebral " because, in his opinion, it arises in the visual 

 centres of the cortex, and is projected externally. 



The same result may be obtained from a simpler form of this 

 experiment. Thus, Ovio gazed for some moments at the sun with 

 one eye, through a red glass. He then closed that eye and looked 

 with the other at the white wall of a shaded room, and found it 

 slightly tinged with red. This is the positive after-image, which 

 seems to arise in the retina of the eye at rest, although the 

 stimulus acts only on the other eye. 



Baquis was the first who confirmed Bocci's experiment and 

 at the same time disproved his explanation of it. He maintained 

 that the after-image which Bocci referred to the rested eye really 

 originated in the fatigued eye, and was projected, as is always the 

 case, into the binocular field of vision. 



Baquis convinced himself of the accuracy of this view by the 

 following experiment. On fixating the sun-lighted card, by Bocci's 

 method, with the right eye alone, and then covering both eyes, 

 the image of the figure is seen in glowing colours (usually red and 

 green), and appears and disappears rhythmically. If the rested 

 eye is suddenly opened while the image is in the field, a quite low 

 degree of objective light will cause it to fade immediately. If both 

 eyes are then again covered, and, after making sure that the after- 

 image is still present, the rested eye is quickly opened again during 

 the phase of disappearance, no image is visible. So that the 

 rested eye sees nothing, either in the phase of return or in the 

 fading of the after-image ; and Bocci's after-image can conse- 

 quently be referred only to the active eye, though it is projected 

 into the binocular field. 



Baquis further found that if, after stimulating the right eye by 

 light in the usual way, both eyes were covered in a dim room, and 

 the stimulated eye was reopened during a phase of appearance or 

 disappearance, the image remained visible, or reappeared in the 



