and distant Sight of different Persons. 41 



of two inches with either eye. I do not insist, however, on 

 the present case, because, though there was not any visible 

 opacity in the crystaUine, this sometimes exists in a small de- 

 gree without being perceptible even to an attentive observer ; 

 and it may be doubted whether the amendment in the lady's 

 vision, were not occasioned solely by the retraction of the iris 

 from before a part of the crystalline that was not yet become 

 opaque : it being well known that the outer part of this lens 

 not unfrequently retains its transparency for some time after 

 an opacity has commenced in the part that surrounds its 

 centre. 



It is evident, that near sightedness has no dependence on the 

 greater or smaller degree of convexity possessed by the cor- 

 nea, when this circumstance is considered alone ; since the 

 length of the axis of the eye from the cornea to the retina, 

 and the greater or smaller degree of convexity in the crystal- 

 line humour, must be also regarded, before the distance of 

 accurate vision can be determined. 



It is no less evident, that near sightedness is not necessarily 

 occasioned by a morbid protrusion of the whole eye ; since 

 some persons are born with eyes of this description, and others 

 acquire the peculiarity, when further advanced in life, in con- 

 sequence of a morbid accumulation of adeps at the bottom of 

 the orbit, without either of them being more near sighted 

 than those who are free from this imperfection. 



I have seen many instances in which old persons, who have 

 been long accustomed to use convex glasses of considerable 

 power, have recovered their former sight at the advanced age 

 of eighty or ninety years, and have then had no further need 

 of them. Dr. Porterfield was of opinirii that in such cases 



MDCCCXIII, G 



