216 A HISTORY OF 



any one object on shutting one eye, and then compare the same situa- 

 tion 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, 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 

 will 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 fancy the objects increased in proportion, until 

 our sense had corrected the errors of another. 



" The having two eyes might thus be said to be rather an inconve- 

 nience 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 contribute, if not to distinct, at least extensive vision.* 

 When an object is placed at a moderate distance, by the means of 

 both eyes we see a larger share of it than we possibly could with one ; 

 tne right eye seeing a greater portion of its right side, and the left eye 

 of its correspondent side. Thus both eyes, in some measure, see 

 round the object ; and it is this that gives it, in nature, that bold re- 

 lieve, or swelling, with which they appear, and which no painting, 

 how exquisite soever, can attain to. The painter must be contented 

 with shading on a flat surface ; but the eyes, in observing nature, do 

 not behold the shading only, but a part of the figure also, that lies be- 

 hind these very shadings, which gives it that swelling which painters 

 so ardently 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 has no vision whatsoever ; so that 

 if one oT them only is employed in seeing, there is a part of the ob- 

 ject 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 experiment. 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 retiring six or seven feet back, and shutting 

 one eye, by trying for some time, we shall find, that while we distinctly 

 behold the black spots that are to the right and left, that vvhich .'s in 

 the middle remains totally unseen. Or, in other words, when we 

 bring that part of the eye, where the optic artery runs, to fall upon 

 the object, it will then become invisible. This defect, however, in 

 either eye, is always corrected uy both, since the part of the object 

 that is unseen by one, will be very distinctly perceived by the other." 



Beside the former defects, we can have no idea of distances from 

 the sight, without the help of touch. Naturally, every object ve see 

 Hppears to be within our eyes ; and a child, who has as yt nwdo bin 



* Leonardo da Vinci. 



