248 Caution in the Ufe of Spettaclcs. [Book III. 



\ve muft take into confideration the diftance of the ob- 

 jedl, and the diftance of diftinct vifion, from which it 

 will be eafy to find a lens of fuch a focal length as will 

 make the object appear at the diftance required *. 



By want of attention in the choice of glaffes, a de- 

 fect in fight may be considerably increaied, for the eye 

 may be ftrrined to an accommodation with the glafs. 

 Short-fighted perfons, as they advance in years, are 

 often found to improve in fight; long-fighted perfons, 

 on the contrary, find their fight impaired by age. For 

 the convexity in the pupil of both diminifhcs with 

 years -j- ; in one cafe it was too great, and confequently 

 the dintmution was beneficial ; in the other cafe it was 

 already too fnnall, and the diminution muft be confe- 

 quently prejudicial. 



* For in Plate XXI. Fig. 41, 42, the triangles Q_A 7, E g $ , 

 are fimilar ; therefore, 



QA : Q^? :: Eg : Ey, or 



QE : CL ? :: E/ : E r , 



That is, the focal length of the glafs is equal to the reftangle un- 

 der thediftances of ciiitindl vil.on,and the given object divided by 

 the difference ''of thefe djftances. 



If in the cafe of (hort-fighted perfons, the object is at fuch a dif- 

 tance that the rays coming from it to the eye may be confidered 

 as parallel, QJi and Q^ may be confulered as equal without any 

 material error, and the focal length of the glafs will then be equal 

 to the diltance of difuixft vifion. 



{ This' is the genera'ly received opinion, but from fome ob- 

 fervations lately communicated to the world in the Philofophical 

 Transactions, by Dr. Hofack, there is reafon to .believe, that the 

 convexity and concavity are not changed, as was generally ima- 

 gined, but that the mufcles of the eye grow weaker, like other 

 mufcies, by age, and confequently are nov able, as in eaily life, to 

 vary the difiance bet'.vccn the retina and anterior furface of the eye, 

 fo as to make it coirefpond to the diilance cf the object. See 

 Book IX. Chap. 41. 



With 



