I 35 ] 



and that they may try the efFed of the concavo-convex pofTibly 

 with advantage. 



And if any chromatic errors ftould accompany ' thefe errors 

 of fphericity, as probably there would, they will be diminished 

 or removed by the difperfion that attends the refradion of the 

 glafs. 



But where the application of a pin-hole to the eye does not 

 render vifion more diftind, the defed probably does not arife 

 from fuch imperfed ftrudure*of the eye as has been fuppofed, 

 but from fome other caufe, as turbid humours, callofity of the 

 retina, or reflexion of the oblique rays from the fides of the eye, 

 by which the pidures on the back part of the retina would be 

 confufed. 



Dr. Porterfield and others account for the diftindnefs afforded 

 to fhort-fighted perfons by looking through a pin-hole, by the 

 dimin-ution of the breadth of the pencil incident upon the retina, 

 fuppofing ftill that all the rays mierjed? in one point within the eye ; 

 but befide that the breadth is not fo much leflened on that hypo- 

 thefis as on the hypothefis here given, and therefore the phseno- 

 menon not fo adequately accounted for, it alfo feems a very 

 extraordinary and improbable ruppofition, that while all the nume- 



* It is poffible that the ftrudure of the eye might be fuch as that the foci of 

 the exterior and central rays would fall at different fides of the retina, which defeft 

 would not be eafily remedied ; but even this I conceive might be accompliflied by 

 combinations of different lenfes, like the compound objea.glaffes of DoUand. 



F 2 rous 



