THE PERCEPTION OF SIGHT. 261 



whole without any attempt at analysis, or whether the observer 

 is able by praciice to recognise the parts of which it is composed, 

 and to separate them from one another. The former is our 

 usual (though not constant) habit when looking with one eye, 

 while we are more inclined to the latter when using both. But 

 inasmuch as this inclination must chiefly depend upon practice 

 in observing distinctions, gained by preceding observation, it is 

 easy to understand how great individual peculiarities may arise. 

 If we carefully observe the rivalry which ensues when we 

 try to combine two stereoscopic drawings, one of which is in 

 black lines on a white ground and the other in white lines on 

 black, we sha.ll see that the white and black lines which affect 

 nearly corresponding points of each retina al\v ays remain visible 

 side by side an effect which of course implies that the white 

 and black grounds are also visible. By this means the brilliant 

 surface, which seems to shine like black lead, makes a much 

 more stable impression than that produced under the operation 

 of retinal rivalry by entirely different drawings. If we cover 

 the lower half of the white figure on a black ground with a 

 sheet of printed paper, the upper half of the combined stereo- 

 scopic image shows the phenomenon of Lustre, while in the 

 lower we see Retinal Rivalry between the black lines of the 

 figure and the black marks of the type. As long as the observer 

 attends to the solid form of the object represented, the black 

 and white outlines of the two stereoscopic drawings cany on in 

 common the point of exact vision as it moves along them, and 

 the effect can only be kept up by continuing to follow both. 

 He must steadily keep his attention upon both drawings, and 

 then the impression of each will be equally combined. There 

 is no better way of preserving the combined effect of two stereo- 

 scopic pictures than this. Indeed it is possible to combine (at 

 least partially and for a short time) two entirely different draw- 

 ings when put into the stereoscope, by fixing the attention upon 

 the way in which they cover each other, watching, for instance, 

 the angles at which their lines cross. But as soon as the 

 attention turns from the angle to follow one of the lines which 

 makes it, the picture to which the other line belongs vanishes- 



