352 HISTOItY OF 



derate distance, by the means of both eyes we see a larger share 

 of it than we ])ossibly could with one, the right eye seeing a 

 greater portion of its right side, and the left eye of its corres- 

 ponding side. Thus both eyes, in some measure, see round the 

 object ; and it is this that gives it, in nature, that bold relievo, 

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

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

 tented 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 behind those very shadings which give 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 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 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 which is in the middle remains totally unseen. Or, in 

 other words when we bring that part of the eye, where the op- 

 tic artery runs, to fall upon the object, it will then become in- 

 visible. This defect, however, in either eye, is always corrected 

 Dy 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 ob- 

 ject we see appears to be within our eyes ; and a child, who has 

 as yet made but little use of the sense of feeling, must suppose 

 that every thing it sees makes a part of itself. Such objects are 

 only seen more or less bulky, as they approach or recede from its 

 eyes ; so that a fly that is near will appear larger than an ox at 

 a distance. It is experience alone that can rectify this mistake ; 



