372 PHYSIOLOGY CHAP. 



field. It is obvious that no impulse originates in the rested eye, 

 either in the phase of return or of fading, but that the image is 

 present in the stimulated eye alone, and is only perceived there. 



Gaudenzi instituted other experiments to confirm Baquis' con- 

 clusions, and showed that when, in persons who are wholly or 

 partially blind in one eye owing to any kind of retinal lesion, an 

 after-image is evoked in the sound eye, the disease of the other 

 does not exclude or affect the appearance of Brewster-Bocci's 

 after-image in the field of vision. 



The most striking experiment in proof of the impossibility of 

 explaining this phenomenon by any functional alteration induced 

 in the rested eye is as follows : If in an individual in whom both 

 eyes are normal an after-image is evoked from that part of the 

 retina of the left eye which coincides in the right eye with the 

 optic papilla (Fig. 176), then, on closing the stimulated eye and 

 opening the other, the after-image appears in that part of the 

 visual field of the rested eye which corresponds to Mariotte's blind 

 spot i.e. to a part where there is no retina, and to which, accord- 

 ingly, no excitation can be transmitted eccentrically from the 

 central nervous organs. 



It is an interesting point that tin's crucial experiment w;is 

 made almost at the same time, but independently, by Gaudenzi to 

 refute Bocci's theory, and by Bocci himself to establish a fresh 

 argument in support of his view. As the two observers came 

 to identical results, the conclusion should agree also. Gaudenzi 

 concluded logically that the after-image in question "could 

 only originate from the physiological processes which affect the 

 retina of the exposed eye in consequence of the primary excita- 

 tion." Nothing more is wanted to explain the genesis of this 

 phenomenon. 



The effects known as colour-contrast are closely allied to those 

 of the after-image. By " contrast " we mean the altered impression 

 reciprocally produced by two different colours, when these are not 

 superposed or mixed, but are presented to the eye successively or 

 simultaneously in two distinct and adjacent fields. Chevreul 

 (1839), who first studied this subject systematically, drew a 

 distinction between successive and simultaneous contrast. Briicke 

 gave the name of " induced colour " to that which is altered, or 

 which appears on a colourless surface, of " inducing colour " to that 

 which brings about the change. 



Successive contrast depends largely upon negative after-images 

 and their projection upon a dissimilar background. If after gazing 

 for some time upon a small red square on a black ground the eyes 

 are turned upon a white field, an after-image appears in the colour 

 complementary to red (blue-green) ; if the gaze is now directed to 

 a violet field, the after-image becomes blue; if to an orange field, 

 yellow. Speaking generally, the colour of the region occupied by 



