39 



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

 not behold the shading only, but a part of ihe figure 

 also, that lies behind those very shadings, which gives 

 it that swelling, which painters can never fully imi- 

 tate. 



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, 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 ; that 

 point of the object that is painted there, must conti- 

 nue 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 lofty that which 

 is in the middle remains totally unseen. When we 

 bring that part of the eye, where the optic artery 

 runs, to fall upon the object, it will become invisible. 

 This defect, in either eye, is corrected by both, since 

 the part of the object that is unseen by one, will be 

 distinctly perceived by the other. 



Trie forai of the eye is the most commodious which 

 can be imagined. It is fittest both to contain the hu~ 

 mours within, and to receive the images of objects 

 from without. VV 7 as it square, or of any multangular 

 form, some of its parts would lie too far off, and some 

 too nigh those lenticular humours, which by their 

 refractions cause vision. But by means of this form, 

 the humours are fitly placed to perform their office of 

 refraction, and the little darkened cell neatly adapted 

 to receive the image of the object. <-> 



Again. As it is necessary for the eye to move 

 various ways, in order to adjust itself to various ob. 

 jects ; so by this %urc it is well prepared for such 



