242 Short and long Sight. [Book III. 



by our Creator, that we fhould have the power of 

 adapting the lens to the diftance of the objeft, fo that 

 its image fliould.be always upon the retina. This is 

 the cafe with the generality of mankind j but all per- 

 fons have not this power j for the eyes of fome are fo 

 conftrufted, that the rays of light either converge too 

 foon or too late j that is, the image is made in fome 

 place between the retina and the pupil, or it would 

 be made behind the retina, if the retina was removed ; 

 hence thefe perfons cannot {ee objefts diftinctly, and 

 to remedy thefe defects glafies are ufed, which in the 

 one cafe make the rays diverge, and in the other cafe 

 make them converge. Thus for Ihort-fighted perfons 

 as they are called, double concave glafies ; for long- 

 fighted perfons, double convex glaffes are ufed. By 

 means of the concave glaffes, the rays incident on the 

 eye are made to diverge more ; and confequently the 

 eye, which before made them- converge too foon, will 

 now be able to form the image on the retina. By the 

 convex glafles the rays are made to converge, and con- 

 fequently the image, which would othcrwife have been 

 behind the retina, is now formed in its proper place 

 on the retina. 



It is a long time, probably, before the child is able 

 to ufe all the mufcles by which the pupil is contracted 

 or expanded, fo that the image fhould fall exactly up- 

 on the retina, and then the ideas formed by the fight 

 muft be exceedingly inaccurate. At firfl all objects 

 will appear equally to touch the eye, and the apparent 

 magnitude of the objedl: will depend on the part of the 

 retina covered by die image. By degrees thefe ideas 

 are corrected, and the hands inftrucl:, after fome diffi- 

 cult experiments, the eye. - The child difcovers that 

 objects are at a diftance from its body, and that fych 

 as make the fame angle at the pupil are not always of 



the 



