M HISTORY OF 



totally blind. This is that part of the optic nerve 

 where its vein and artery run ; which being in- 

 sensible, 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 distance 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 be- 

 hold 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 optic artery runs, to 

 fall upon the object, it will then become invisi- 

 ble. This defect, however, in either eye, is al- 

 ways corrected by both, since the part of the 

 object that is unseen by one, will be very dis- 

 tinctly 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 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 ; and a long acquaint- 

 ance with the real size of every object, quickly 

 assures us of the distance at which it is seen. 



