160 



A HISTORY OF 



corrected by the touch ; and although, in re- 

 ality, every object we see appears inverted ! 

 and double, yet the judgment and habit have ' 

 so often corrected the sense, that we no lon- 

 ger submit to its imposition, but see every ' 

 object in its just position, the very instant it ' 

 appears. Were we, therefore, deprived of 

 feeling, our eyes would not only misrepresent 

 the situation, but also the number of all things ; 

 around us. 



To convince us that we see objects invert- 

 ed, we have only to observe the manner in ! 

 which images are represented, coming through 

 a small hole, in a darkened room. If such a | 

 small hole be made in a dark room, so that | 

 no light can come in, but through it, all the \ 

 objects without will be painted on the wall 

 behind, but in an inverted position, their 

 heads downwards. For as all the rays which 

 pass from the different parts of the object 

 without, cannot enter the hole in the same ex- 

 tent which they had in leaving the object; 

 since, if so, they would require the aperture 

 to be as large as the object; and, as each 

 part, and every point of the object, sends forth 

 the image of itself on every side, and the rays, 

 which form these images, pass from all points 

 of the object as from so many centres, so such 

 only can pass through the small aperture as 

 come in opposite directions. Thus the little 

 aperture becomes a centre for the entire ob- j 

 ject ; through which the rays from the up- 

 per parts, as well as from the lower parts of 

 it, pass in converging directions; and, conse- 

 quently, they must cross each other, in the 

 central point, and thus paint the objects be- 

 hind, upon the wall, in an inverted position. 



It is, in like manner, easy to conceive, that 

 we see all objects double, whatever our pre- 

 sent sensations may seem to tell us to the con- 

 trary. For to convince us of this, we have 

 only to compare the situation of any one ob- 

 ject on shutting one eye, and then compare 

 the same situation by shutting the other. If, 

 for instance, we hold up a finger, and shut the 

 right eye, we shall find it hide a certain part 

 of the room ; if again reshutting the other eye, 

 we shall find that part of the room visible, i 

 and the finger seeming to cover a part of the 

 room that had been visible before. If we | 

 open both eyes, however, the part covered j 

 -wiH appear to lie between the two extremes. 



But the truth is, we see the object our finger 

 had covered, one image of it to the right, and 

 the other to the left ; but, from habit, suppose 

 that we see but one image placed between 

 both; our sense of feeling having corrected 

 the errors of sight. And thus, also, if instead 

 of two eyes we had two hundred, we should, 

 at first, fancy the objects increased in pro- 

 portion, until one sense had corrected the 

 errors of another. 



44 The having two eyes might thus be said 

 to be rather an inconvenience than a benefit; 

 since one eye would answer the purposes of 

 sight as well, and be less liable to illusion. 

 But it is otherwise ; two eyes greatly contri- 

 bute, if not to distinct, at least to extensive 

 vision.* When an object is placed at a mo- 

 derate distance, by the means of both eyes 

 we see a larger share of it than we possibly 

 could with one ; the right eye seeing a grea- 

 ter portion of its right side, and the left eye of 

 its corresponding side. Thus both eyes, in 

 some measure, see round the object; and it 

 is this that gives it, in nature, that bold re- 

 lievo, or swelling, with which they appear ; 

 and which no painting, how exquisite soever, 

 can attain to. The painter must be content- 

 ed with shading on aflat surface; but the 

 eyes, in observing nature, do not behold the 

 shading only, but a part of the figure also, 

 that lies behind (hose very shadings, which 

 gives it that swelling, which painters so ar- 

 dently desire, but can never fully imitate. 



" There is another defect, which either of 

 the eyes, taken singly, would have, but which 

 is corrected, by having the organ double. 

 In either eye there is a point, which ha^ no 

 vision whatsoever ; so that if one of them only 

 is employed in seeing, there is a part of the 

 object to which it is always totally blind. 

 This is that part of the optic, nerve where its 

 vein and artery run; which being insensible, 

 that point of the object that is painted there 

 must continue unseen. To be Convinced of 

 this we have only to try a very easy experi- 

 ment. If we take three black patches, and 

 stick them upon a white wall, about a foot 

 distant from each other, each about as high 

 as the eye that is to observe them ; then re- 

 tiring six or seven feet back, and shutting one 



" Leonardo da Vinci. 



