248 RECENT PROGRESS OF THE THEORY OF VISION. 



surfaces ; and this is sufficient to supply what the drawing does 

 not directly show. Moreover, in the case of figures of men 

 or animals, our knowledge that the two sides are symmetrical 

 further assists the impression conveyed. 



But objects of unknown and irregular shape, as rocks or 

 masses of ice, baffle the skill of the most consummate artist ; 

 and even their representation in the most complete and perfect 

 manner possible, by means of photography, often shows nothing 

 but a confused mass of biack and white. Yet, when we have 

 these objects in reality before our eyes, a single glance is enough 

 for us to recognise their form. 



The first who clearly showed in what points it is 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 

 Jetta 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 be- 

 fore it; but, inasmuch as it occupies a somewhat different position 

 in space from the other, its point of view, and so its whole per- 

 spective image, is different. 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 when- 

 ever we look at bodies of which the several parts are at different 

 distances from our eyes. But when I look at a hand repre- 

 sented 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 



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

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

 of physical truths vouchsafed to a single mind, than the superstructure 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.' TB. 



