408 



PHYSIOLOGY 



CHAP. 



sensations of light and dark are continuously oscillating at each 

 point of the two retinae, as the action now of one eye and now of 

 the other predominates. For the same reason the two photographic 

 images taken for the stereoscope reproduce the natural lustre of 

 objects. In fact, if the two photographs are closely examined, it 

 will be seen that the light parts of the one correspond with the 

 shaded parts of the other, and vice versa. Lustre, therefore, results 

 from the rivalry of the sensations of light and dark in both eyes. 



PIG. 192. Binocular rivalry of contours. (Hering.) 



The struggle in the visual fields of the two eyes is even 

 plainer when instead of two surfaces differently coloured, or one 

 white and one black, each eye is confronted with a white and 

 black or bi-coloured surface with sharp outlines, which are not 

 superposed. On looking with a mirror stereoscope at the two 

 lateral, half- white, half- black squares (L, R) of Fig. 192 they are 



FIG. 193. Binocular rivalry. (Hering.) 



superposed in vision, and the 'central square C appears subdivided 

 into four, one black, one white, and two grey squares of varying 

 luminosity, separated by brighter and darker outlines according 

 as these bound a black or a white square (conflict of outlines). 



On superposing the two lateral squares (L, R, Fig. 193), on the 

 black ground of which oblique and parallel white lines are drawn 

 in opposite directions, the rivalry between the two eyes is 

 evident in gazing at the central square C, in which the image 

 alternates rapidly in different parts. The predominance 



