66 On the Adjustment of the Eye, &c. 



cliision drawn by Dr. Porterfield seems to be erroneous 

 (although it has been used, not only by himseU, but by 

 other writers, as an unanswerable ar(.^,unM nt to prt»ve that 

 the eye is adjusted by means of the crystiiUmp lens,) ; 

 for if the iris of the person's eye mcntioix^i Uv Dr. Porter- 

 field received any injury from the operaiiotjoF' couching, 

 the adjustment of that eye would be imperf .:t, and rcipure 

 glasses of different degrees of con. cxity for vie«A'irig objects 

 at diflferent distances ^ as it i^ well known, that persons who 

 read with spectacles of very cowvfex len«es require glas-^es 

 -of a less degree of convexity to view remote objects ;' stdl 

 the less conv(^x, the more remote the object. ,-'■■■■ • • '• 



But if the operation mentioned by Dr. Youitg'wds' fter- 

 formed without doing any injury to the iris, iiiat oerson 

 would still be able to see objects distinctly at diifercnt 'ii- 

 stances, by the assistance of the same convex jca^s ar)plied 

 to that eye ; because the iris is'the only organ by whiih the 

 »eye is adjusted to distinct, vi^sion. ft is a physical truth, 

 which any one may convince hmiself of by trial] that lio 

 mental exertion can change the adjustnient of the eye, 

 when every part of the iris is covered. This may be tried 

 by viewing objects through a hole in a card, made rather 

 less than the pupil in its most contracted state: ^ v ,ri 



V. — Sir Isaac Newton was of opinion, that the hufn<!>t5t^ 

 of the eye decav or shrink by old age, which causes the eye 

 of the short-sighted to grow flatter till it comes to a due 

 figure. For short-sighted men see remote objects best in 

 old age*. And this opinion has been adopted by some of 

 our best writers on the theory of vision. But let opinions 

 give place to facts. a .m. 



Mr. Adams says, *^ It is generally supposed,' thai 'the 

 short-sighted become less so as they advance in years, as 

 the natural shrinking and decay in the humours of the eye 

 lessen its convexity, and thus adapt it bettcrfor viewing 

 distant objects : but among the great number of short- 

 sighted that I have accommodated with glasses, I have ever 

 found the reverse of this theory to be trae, and the eyes of 

 ■the myopes never required glasses less concave, but gene- 

 rally more concave, as they grew older, to enable them to 

 see at the same distance f." 



Hence it is evident, that the humours of the eyes of the 

 short-sighted undergo no change as they grow older, and 

 that visi'on generally becomes less perfect. For, as the iris 

 )oses some part of its. contracting power w^th age, the pu- 



*' Sec Newton's Opti'cs, p.'lS. 

 I Adams on Vision, p. 126.'" 



— pil 



