402 Why the Banks of a River feem contiguous. [Book IX. 



faint or confufed. 6thly, We are affifted in judging 

 of the diftance of any particular objects, by the other 

 objects which are interpofed. On this account, dif- 

 tances upon uneven ground do not appear fo great 

 as upon a plain ; for the valleys, rivers, and other 

 objects that lie low, are many of them loft to the 

 fight. This too is the rcafon why the banks of a river 

 appear contiguous when the river lies low and is not 

 feen *. 



In children the pupil is ufually more dilated than 

 in grown perfons. The rea-fon of this appears to be, 

 that in childhood the cornea is more flexible, fo as to 

 be very eafily bent into any curvature necefiary for 

 diftinct vifion, and confequently the pupil has lefs 

 occafion to contract. In grown perfons the cornea is 

 {lifter, they have therefore more neceflity to contract 

 the pupil. In elderly perfons the cornea grows ftill 

 more rigid ; for this reafon they are obliged fometimes 

 to hold the candle between the eye and the paper on 

 which they read ; and their doing fo is a direct indi- 

 cation that they begin to want fpectacles f. 



Children read much nearer than grown perfons, 

 both becaufe their eyes are fmaller, and becaufe their 

 cornea is more flexible. That elderly perfons fee 

 better at a great diftance than younger perfons is gene- 

 rally allowed. 



It is a certain and very important fact, that long- 

 fightednefs may be acquired, for countrymen, failors, 

 and thofe that are habituated to look at remote ob- 

 jects, are generally long-fighted, want fpectacles 

 fooneft, and ufe the deepeft magnifiers ; on the other 

 hand, the far greater part of the ihort fighted are to 



* EfTay on Vifion, quoted by Prieftley. 

 f Porterfield on tl.e Eye, quoted by JPneit. Op. Per. 6. f. 12. 



be 



