and distant Sight of different Persons, 4g 



Although old persons lose the power of distinguishing cor- 

 rectly near objects, and require for this purpose the aid of convex 

 glasses, they usually retain the sight of those that are distant 

 as well as when they were young. Instances, however, are 

 not wanting of persons advanced in life, who require the aid. 

 of convex glasses to enable them to see near, as well as dis- 

 tant, objects. Dr. Wells is one of these. He informs us, in 

 the paper to which I have more than once adverted", that 

 when twenty years younger, he was able, with his left eye, 

 to bring to a focus on the retina, pencils of rays which flowed 

 from every distance greater than sqsqvi inches from the cor- 

 nea ; but at the age of fifty-live, he required not only a convex 

 glass of six inches focus, to enable him to bring to a point on 

 the retina rays proceeding from an object seven inches from 

 the eye, but likewise a convex glass of thirty-six inches focus, 

 to enable him to bring to a point parallel rays. — ^There are 

 also instances of young persons, who have so disproportionate 

 a convexity of the cornea or crystalline, or of both, to the 

 distance of these parts from the retina, that a glass of consi- 

 derable convexity is required to enable them to see distinctly, 

 .not only near objects, but also those that are distant; and it 

 is remarkable, that the same glass will enable many such per- 

 sons to see both near and distant objects ; thus proving that 

 the defect in their sight is occasioned solely by too small a 

 convexity in one of the parts abovementioned, and that it 

 does not influence the power by which their eyes are adapted 

 to see at distances variously remote. In this respect such 

 persons differ from those who have had the crystalline humour 

 removed by an operation ; since the latter always require a 

 glass to enable them to discern distant objects, different from 



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