VII 



EETINAL EXCITATION 



373 



the after-image is that which results from the mixture of the 

 (complementary) colour of the after-image with that of the back- 

 ground on to which it is projected. 



If the after-image is projected on to a background of the 

 inducing colour, the part on which it falls appears dimmed or 



FIG. 176. Gaudenzi's experiment for repetition and interpretation of Brewster-Bocci's "cerebra 

 visual images." OS, uncovered left eye ; OD, covered right eye ; C, central point of perimeter 

 fixated by the two eyes alternately ; ss, position of physiological scotoma (Mariotte's blind 

 spot) in the covered eye ; a, position of a lamp, partially covered by a screen. When the 

 head is fixed in the perimeter, point C is fixated with the right eye, and the limits, ss, of the 

 blind spot of this eye are traced on the arc of the perimeter. Both eyes are then covered and 

 the room darkened, to adapt the retina to darkness ; after a few minutes the left eye is 

 uncovered, and while fixated on C it is exposed for 20-30 seconds to the light of lamp a, so as to 

 provoke a persistent image in a part of the retina corresponding to the optic papilla of the 

 right eye. The left eye is then closed, and the right eye opened and fixated on the centre C ; 

 after a few seconds, Brewster-Bocci's image is seen in the region of Mariotte's blind spot. 



grey ; if, on the contrary, it is projected on to a surface of 

 complementary colour the part it falls on appears brighter and 

 more saturated. Generally speaking, the colours which are 

 complementary or nearest to the complementary become more 



