THE PERCEPTION OF SIGHT. 283 



impossible for any picture to represent actual objects was 

 the great master of painting, Leonardo da Vinci, 1 who 

 was almost as distinguished in natural philosophy as in 

 art. He pointed out in his Trattato della Pittura^ that 

 the views of the outer world presented by each of our 

 eyes are not precisely the same. Each eye sees in its 

 retinal image a perspective view of the objects which 

 lie before it ; but, inasmuch as it occupies a somewhat 

 different position in space from the other, its point 

 of view and so its whole perspective image is dif- 

 ferent. If I hold up my finger and look at it first 

 with the right and then with the left eye, it covers, 

 in the picture seen by the latter, a part of the opposite 

 wall of the room which is more to the right than in 

 the picture seen by the right eye. If I hold up my right 

 hand with the thumb towards me, I see with the right 

 eye more of the back of the hand, with the left more 

 of the palm ; and the same effect is produced whenever 

 we look at bodies of which the several parts are at 

 different distances from our eyes. But when I look at a 

 hand represented in the same position in a painting, the 

 right eye will see exactly the same figure as the left, and 

 just as much of either the palm or the back of it. Thus 

 we see that actual solid objects present different pictures 

 to the two eyes, while a painting shows only the same. 

 Hence follows a difference in the impression made upon 

 the sight which the utmost perfection in a representation 

 on a flat surface cannot supply. 



The clearest proof that seeing with two eyes, and the 

 difference of the pictures presented by each, constitute 



1 Born at Vinci, near Florence, 1452 ; died at Cloux, near Amboise, 1519. 

 Mr. Hallam says of his scientific writings, that they are ' more like revela- 

 tions of physical truths vouchsafed to a single mind, than the super- 

 structure of its reasoning upon any established basis. ... He first laid 

 down the grand principle of Bacon, that experiment and observation must 

 be the guides to just theory in the investigation of nature.' Ta. 



