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(or foci) be at the retina, the other muft be removed from it, and 

 confequently a confufion would enfue in the pidure on tlie 

 retina. 



It is however well known that nature has ufed an admirable 

 precaution againft this imperfedion, by gradually diminifhing the 

 dcnfity (and therefore the refradive power) of the chryftalline 

 from the centre toward the edges ; fo that the excefs of refradion 

 arifing from obliquity of incidence fhould ferve as a compenfation 

 for the defed of it, owing to the diminution of denfity, and thus 

 in ferfe5l vijion produces this efFed, that both the extreme and 

 central rays of the incident pencil (hould be brought to a com- 

 mon focus exadly on the retina. 



Let us now fuppofe a chryftalline humour of fuch a form as 

 it ufually has in a perfed eye, and at the central parts of a juft 

 denfity, but whofe denfity is not fufficiently diminifhed in pro- 

 ceeding from the centre to the edges : From what has been juft 

 obferved on the cafe of uniform denfity in the chryftalline, it 

 is plain that the exterior rays of the incident pencil, and thofe 

 only, will in the prefent cafe fuffer too great a refradion, and 

 produce confufion in the pidure on the retina ; confequently if 

 thefe be intercepted, and the central rays alone tranfmitted to the 

 eye (as is done in the firft experiment by the pin-hole) vifion 

 will be diftind, and no obftacle to its perfedion will remain but 

 what arifes from want of fufiicient light. Vid. Fig. I. 



Fig. II. Even though the denfity of the chryftalline were every 

 where too great as well toward the centre as toward the edges, 



yet 



