THE PERCEPTION OP SIGHT. 299 



time) two entirely different drawings 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 



Let us now put together the results to which our 

 inquiry into binocular vision has led us. 



I. The excitement of corresponding points of the two 

 retinae is not indistinguishably combined into a single 

 impression ; for, if it were, it would be impossible to see 

 Stereoscopic Lustre. And we have found reason to believe 

 that this effect is not a consequence of Eetinal Kivalry, 

 even if we admit the latter phenomenon to belong to 

 sensation at all, and not rather to the degree of attention. 

 On the contrary the appearance of lustre is associated 

 with the restriction of this rivalry. 



II. The sensations which are produced by the excita- 

 tion of corresponding points of each retina are not in- 

 distinguishably the same ; for otherwise we should not 

 be able to distinguish the true from the inverted or 

 ' pseudoscopic ' relief, when two stereoscopic pictures are 

 illuminated by the electric spark. 



III. The combination of the two different sensations 

 received from corresponding retinal points is not pro- 

 duced by one of them being suppressed for a time ; 

 for, in the first place, the perception of solidity given by 

 the two eyes depends upon our being at the same time 

 conscious of the two different images, and, in the second, 

 this perception of solidity is independent of any move- 

 ment of the retinal images, since it is possible under 

 momentary illumination. 



We therefore learn that two distinct sensations are trans- 

 U 



