THE PERCEPTION OF SIGHT. 293 



white and black would give grey. The fact, therefore, 

 that when they are actually combined in the stereoscope 

 they produce the effect of lustre, that is to say, an 

 effect which cannot be produced by any kind of uniform 

 grey surface, proves that the impressions on the two 

 retinae are not combined into one sensation. 



That, again, this effect of stereoscopic lustre does not 

 depend upon an alternation between the perceptions 

 of the two eyes, on what is called the ' rivalry of the 

 retinae,' is proved by illuminating 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 

 sensation, 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 sen- 

 sation given by the right 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 us in distinguishing 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 stereo- 

 scope 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 

 indistinguishable from that on the left. 



