^o Mr. Ware's Observations, ^c, 



adopted, increasing years have either removed or lessened this 

 imperfection. 



Secondly ; though the usual effect of time on perfect eyes 

 be that of inducing a necessity to make use of concave glasses, 

 in order to see near objects distinctly, yet sometimes, even 

 after the age of fifty, and after convex glasses have been used 

 many years for this purpose, the eyes have not only ceased 

 to derive benefit from them, when looking at near objects, 

 but they have required concave glasses to enable them to dis- 

 tinguish, with precision, objects at a distance. 



Thirdly; though the cause of this change be not always 

 known, yet sometimes it has been induced by the use of eva- 

 cuating remedies, particularly of leaches applied to the temples; 

 and sometimes by looking through a haicroscope, for a con- 

 tinued length of time, in several successive days. 



Fourthly ; instances are not uncommon, in which persons, 

 far advanced in life, (viz. between eighty and ninety,) whose 

 eyes have been accustomed for a long time to the use of 

 deeply convex glasses, when they have read or written, have 

 ceased to derive benefit from these glasses, and they have 

 become able, without any assistance, to see both near and 

 distant objects almost as well as when they were young. 

 Although it be not easy to ascertain the cause of this amended 

 vision, it seems not improbable that it is occasioned by an 

 absorption of part of the vitreous humour ; in consequence of 

 which the sides of the eye collapse, and its axis, from the 

 cornea to the retina, is lengthened ; by which alteration the 

 length of this axis is brought into the same proportion to the 

 flattened state of the cornea or crystalline, or both, which it 

 had to these parts before the alteration took place. 



