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An ESSAY to mprove the THEORY o/^ DEFECTIVE 

 SIGHT. By the Rev. JOHN STACK, F.T.C.D. and 

 M. R. I. A. 



X H E common theory of defedive fight attributes the confu- Read Jan. 

 fion in imperfed vifion to the interfedion of the rays of each '- 



pencil that enters the eye, in points either before or behind the 

 retina. It ftates the former of thefe defeds as proceeding from 

 too great a convexity of the cornea, or of the chryftalhne 

 humour, or of both ; and the latter from the contrary ftrudure 

 of thofe parts of the eye. Bcfide thefe fcarce any other caufes 

 are affigned, or any farther explanation of the defeds given. 



This, however, feems inadequate to the folution of feveral 

 phaenomena in cafes of defedive fight that frequently occur ; fuch 

 as the following : 



I. I HAVE frequently caufed fiiort-fighted perfons to place a 

 printed book a little beyond their limits of diflind vifion ; then 



E 2 applying 



