THE PERCEPTION OF SIGHT. 257 



upon an alternation between the perceptions of the two eyes, 

 on what is called the ' rivalry of the retinae,' is proved by illu- 

 minating stereoscopic pictures for an instant with the electric 

 spark. The same effect is perfectly produced. 



In the third place, it can be proved, not only that the 

 images received by,. the two eyes do not coalesce in our sensa- 

 tion, but that the two sensations which we receive from the 

 two eyes are not exactly similar ; that they can, on the contrary, 

 be readily distinguished. For if the sensation given by the 

 light eye were indistinguishably the same as that given by the 

 ^left, it would follow that, at least in the case of the electric 

 spark (when no movements of the eye can help ns in distin- 

 guishing the two images;, it would make no difference whether 

 we saw the right-hand stereoscopic picture with the right eye, 

 and the left with the left, or put the two pictures into the 

 stereoscope reversed, so as to see that intended for the right eye 

 with the left, and that intended for the left eye with the right. 

 But practically we find that it makes all the difference, for if 

 we make the two pictures change places, the relief appears to 

 be inverted : what should be further off seems nearer, what 

 should stand out seems to fall back. Now since, when we look 

 at objects by the momentary light of the electric spark, they 

 always appear in their true relief and never reversed, it follows 

 that the impression produced on the right eye is not indistin- 

 guishable from that on the left. 



Lastly, there are some very curious and interesting pheno- 

 mena seen when two pictures are ptit before the two eyes at 

 the same time which cannot be combined so as to present the 

 appearance of a single object. If, for example, we look with 

 one eye at a page of print, and with the other at an engraving, 1 

 there follows what is called the ' rivalry ' of the two fields of 

 vision. The two images are not then seen at the same time, 

 one covering the other; but at some points one prevails, and at 

 others the other. If they are equally distinct, the places where 



1 The practised obferver is able to do this without any apparatus, but most 

 persons will find it necessary to put the two objects in a stereoscope or, at least, 

 to hold a book, or a sheet of paper, or the hand in front of the face, to serve for 

 the partition in the stereoscope. TK. 



I. S l 



