82 On the Adjustment of the Eye to see 



cock so far within the ball is to prevent the liquid in the 

 ball from passing into the cupping glass when (he stop- 

 cock is opened. 



November 14, 1809. 



XI IT. Oil that Power of the Eye^ by which it is adjusted to 

 see Objects distinctly at different Distances, By Ez, 

 Walker, Esq, 



[Continued from vol. xxix. p. 340.] 



I. X* ROM observations made on the human eye at different 

 ages, it appears to undergo a gradual change from infancy 

 to old age. For when a child of three or four years old at- 

 tentively views a very small object it is held close to the 

 eye, not further off than two or three inches. The reason 

 is this : in the early part of life the iris, being flexible, has 

 the power of contracting the pupil to a smaller dimension 

 than at any future perriod ; consequently a child can see a 

 small object at a less distance than a grown person : but by 

 degrees this power of the iris decreases, as the rest of the 

 body becomes less flexible ; and the eye, if a good one, 

 gradually becomes longer-sighted. 



It is generally supposed, however, that the pupil of a 

 child's eye is larger than that of a grown person's : but this 

 is true only when the iris is in a state of relaxation ; for it 

 is well known, that the pupil always contracts in viewing 

 a near object. To suppose that a child can see a small ob- 

 ject at two or three inches distance with a large pupil is 

 contrary to the laws of dioptrics, because the rays of light 

 would then enter the eye in a diverging state, and conse- 

 quently form an indistinct picture of the object upon the 

 retina. 



Dr. Jurin observes, that *' children read much nearer 

 than grown persons, for their eyes are smaller, and the 

 least distance any person can see distinctly at, is propor- 

 tional to the length of the eye*." 



. But to show that this rule is- erroneous, let us suppose 

 that the nearest distance of distinct vision of an eye is 18 

 inches ; then this eye, according to the Doctor's rule, must 

 be six times as large as the eye of a child that can see di- 

 stinctly at the distance of three inches; which is contrary 

 'to experience. It also appears from the eyes of birds that the 



^ £e«ayoadi»tiact andiudiitinct Yisioji, p. 147. 



• Doctor** 



