1098 VISION. 



no longer be distinguished (Donders). Exner x found that if a field half 

 white and half black be observed while the eyeball is being pressed, 

 the white half becomes quite dark. If a white square is then placed on 

 the black half, it will be seen. Exner explained this experiment by sup- 

 posing that, while looking at the white half, the store of material in the 

 retina had been used up, and, owing to interference with blood supply, 

 could not be regenerated, while on the black half the retinal material 

 was not used up, and permitted vision of the white square. Kiihne 2 has 

 pointed out that the vision of the white square may be due to its 

 greater distinctness, owing to contrast on the black ground, and found 

 that pressure might produce blindness without the presence of light. 

 Exner also found that pressure on the eyeball influenced the point 

 of fusion of intermittent retinal stimuli; a black and white disc, 

 which still flickers to a normal eye, appearing grey to a pressed eye. 

 He found that it prevented the appearance of after-images and shortened 

 the existence of those already present. 



Binoculah Phenomena. 



Adaptation.— Certain binocular relations are of importance in con- 

 nection with the theories of light and colour sensation. The condition 

 of adaptation of one eye is independent of that of the other eye. It 

 has been clearly established that dark-adaptation of one eye does not 

 increase the sensibility of the other eye. Charpentier 3 at first thought 

 that it did so, but found that the apparent increase of intensity was due 

 to the consensual increase in the size of the pupil admitting more light 

 to the eye, and was absent with a small artificial pupil. 



Binocular contrast. — The influence of stimulation of one retina upon 

 the condition of the other is best shown by producing double images of 

 a white strip on a black ground. If, now, a coloured glass is held before 

 one eye, and a grey glass before the other, the strip seen by the latter 

 will appear in the colour complementary to that of the glass. Another 

 method is to hold before one eye a card with a small hole, which will 

 also be coloured complementarity to the colour of a glass held before 

 the other eye. When the colour stimulus applied to the one eye is 

 removed, the colour of the strip or of the hole will change to the colour 

 of the glass, and this contrast colour is more marked than that which 

 is seen at first, i.e. binocular contrast is more marked during the after- 

 image effect than during the original excitation. 



Another example of binocular colour contrast is known as Fechner's 

 side-window experiment. If one stands so that light falls laterally on 

 one eye, and a white strip of paper be doubled against a dark ground, 

 it will be seen that the strip belonging to the more illuminated eye is 

 darker than that belonging to the other eye, while the former is tinged 

 green and the latter red. These features become more marked the 

 longer the difference between the illumination of the two eyes is 

 allowed to last. The darkness of the strip seen by one eye is explained 

 by the " fatigue " of this eye being greater than that of the other. The 

 different coloration of the two strips is explained partly by the fact that 

 the light which passes through the sclerotic of the more illuminated eye, 

 acquires a red colour (see p. 1057), and partly by binocular contrast. As 



1 Arch.f. d. ges. Physiol., Bonn, 1878, Bd. xvi. S. 407 ; and 1879, Br], xx. S. 614. 



2 Untersuch. a. d. yhysiol. Inst. d. Univ., Heidelberg, 1882, Bd. ii. S. 46. 



3 " La lumiere et les couleurs," Paris, 1888, p. 177. 



