1 1 24 VISION. 



mean of that of the two eyes, and the phenomenon has also been 

 referred to the influence of attention, which is supposed to be concen- 

 trated on the open eye when the darkened eye is closed. Fechner 

 supposed that it depended on antagonism between the two retinae. 



Binocular colour mixture. — When a different colour stimulus is 

 applied to each eye, the mixture-colour may be observed during the 

 transitions from one colour to the other which depend on rivalry. With 

 suitable adjustment of the two stimuli in respect of intensity, the 

 rivalry may be very much lessened or abolished, and the mixture-colour 

 persists. The existence of true binocular colour mixture, however, has 

 been denied by Helmholtz 1 and others, but there can be no doubt of its 

 existence, the mixture-colour being in all respects comparable with the 

 colour produced by mixture on the colour wheel or by the double-image 

 prism. It may be seen with simple binocular combination ; with the 

 stereoscope ; and by a method devised by Hering, 2 in which a patch of 

 light is seen in a dark box containing two coloured glasses, arranged in 

 such a way that each eye looks through a different coloured glass. The 

 patch of light is most conveniently produced by covering an aperture 

 in the floor of the box with ground glass, and looking through the 

 box towards a brightly illuminated surface. Binocular colour mixture 

 may also be observed in the combined after-image of the two eyes. 3 



In experiments showing the rivalry of contours, the parts where the 

 images of the two eyes join have a peculiar lustrous appearance. This is 

 also seen in the binocular combination of black and white fields, the 

 combined image looking like graphite. If a red and blue pattern be 

 observed with a red glass before one eye, and a blue glass before the 

 other, the same lustre is very marked. The natural lustre of certain 

 objects has been referred to the same cause. It is supposed that objects 

 with a certain degree of roughness of surface reflect rays in irregular 

 directions, so that corresponding points of the two eyes are stimulated 

 by light of different degrees of intensity. 



Corresponding points. — The physiological basis of single vision 

 with the two eyes is the mechanism of corresponding or identical 

 points. These points may be defined as those parts of the two retinae 

 which receive the two images of the same distant point of light (star). 

 If one retina be placed in front of the other, so that the foveae are in 

 contact, and the temporal half of one retina covers the nasal half of the 

 other, any point in one retina will cover its corresponding point in the 

 other, and such points have therefore also been called covering points. 

 Since the nasal half of each retina has a more extended field than the 

 temporal half, only the central part of the visual field is binocular. 



J. Miiller 4 demonstrated the parts of the two retina? which corre- 

 sponded to one another by means of pressure phosphenes. He found that 

 a phosphene produced by pressure on the outer side of one eye was 

 projected, so that it was localised in the same position in space as 

 one due to pressure on the inner side of the other eye. Hering 5 showed 

 that an after-image of one eye has exactly the same localisation in 

 the visual field of the other eye as the original of the after-image to the 



1 "Handbuchd. phys. Optik," 1867, S. 776. 



2 Hermann's " Handbuch," 1879, Bd. iii. S. 593. 



3 Proc. Oavib. Phil. Soc, 1895, vol. viii. p. 273. 



4 "Zur vergleichenden Physiologie des Gesichtsinnes, " Leipzig, 1826, S. 73. 

 "Beitr. z. Physiologie," Leipzig, 1863, Heft 3, S. 182. 



