Of Sight, [Book IX. 



it is appointed by the all- wife author of our exiftence, 

 that by the force of the refraction which they fuffer in 

 the eye, they fliould form another cone oppofed to the 

 fcrft at its bafe, and the apex of which is at a, and thus 

 an imprefiion fufficiently forcible to produce diftinct 

 vifion is made on the retina. 



In the preceding inftance, the" luminous body A 

 vas confidered as a point, and what has been faid of it 

 will apply to every point of a vifible object, which is 

 capable of tranfmittiug or reflecting to the eye a pen- 

 cil or collection of rays. Thus we may eafily fup- 

 pofe that from every point of the arrow A, D, B, 

 (Fig. 7.) pencils of light may be tranfmitted ; thefe, 

 like all pencils or collections of rays, coming from a 

 j$oint, will diverge, and will fall upon the eye in the 

 form of cones or pyramids, fuch as A, M, C, from the, 

 point A ; D, <?, i from the point D -, and B, G, N, from 

 the point B. If the eye, therefore, is in a proper ftate, 

 the divergent rays proceeding from the point D will 

 be united together into one pencil or mafs, fuch as 

 they were when they firft proceeded from the object, 

 at the point d, upon the retina ; the divergent rays, 

 which fall more obliquely from the point A, will be 

 united on the retina, at the point a ; and thofe which 

 proceeded from B will, by 'the fame rule, be converged 

 and meet at b. Hence it is evident, that by means of 

 this refraction there are certain points at which the 

 rays of light, after paffing the pupil, crofs each other, 

 and the image which is formed on the retina is confe-r 

 quently inverted. 



If the humours of the eye, through age or weaknefs, 

 have fhrunk or decayed, the cornea will then be too 

 flat, and the rays not being fufficiently bent or refracted, 

 arrive at the retina before they are united in a focus, 

 and would meet, if not intercepted in fome place be- 



hjncj 



