Objects distinctly at different DiUanCes, 83 



Doctor's rule is erroneous; for it is well known that large 

 birds of prey can see objects at a distance, far beyond the 

 Jimits of Jisiinct vision of the human eye; and there are 

 smajl birds that c^njiee minute objects at a very great di- 

 stance. • ; : 

 Now, as the eyes pf birds are much smaller than those 

 of men, it is evident ihat the utnjost- distance of distinct 

 vision does not depend upon the length of the axis of the 

 eye. , 



The nearest distance at which a grown person with a 

 good eye can see distinctly, may in general be about six or 

 seven inches; but this distance increases with age, until 

 ,near objects appear indistinct. 



This increase in the focal distance of the eye is gene- 

 rally supposed to be owing to the humours of the eye be- 

 coming too flat through age, so that the rays which fall 

 upon the eye, from an object at a short distance, converge 

 to a point behind the retn)a, and thus cause indistinct vi- 

 sion. JBut this theory is, 1 believe, unsupported by a single 

 clear and decisive experiment. 



That the humours of the eye may waste and the eye grow 

 flatter, when every other part of the human frame is upon 

 the decline, may be supposed with some appearance of rea- 

 son; but that the humours of the eye of a child should de- 

 cay and the eye grow flatter while every other part of its 

 body is advancing towards perfection,- is a theory which 

 cannot be so readily admitted. For, as the eye undergoes 

 a gradual change through life, thiscff^'ci mu3t be produced 

 in every stage of it, by the same cause* tl<jnce,rthis theoi^X 

 requires further investigation. - " r^ " .' - 



II. Those who have written on the properties of the hu- 

 man eye ii>form us, that "if a good eyp vie^vs an object 

 at the least distance it can be seen .distiiictly, and then at 

 twice that distance, and then at an infinite distance, there 

 is about the same alteration made in the figure of the eye 

 between the two last cases, as there^ is between the two 

 first. 



"ForletBCDE (PI. IT ) be tiie axis of the eye infinitely 

 produced ; B C, B D, BE, the three distances of the pb; 

 }ect from the cornea AB; and C A, DA, EA', thret? rays 

 falling upon any given pyji^, of ll^!^^^cpr»qa j ^V^^^eof ^'^ - 

 is parallel to the axis. - ^ , \ .,,: .,,;,».'• .,'/ * ^Jf^: 



••^ Now to produce distinct .vision of the points G, D,.E; 

 it is plain that every one of the rays C A, DA, E A, miist 

 be successively refracted to the same point F, upon the re- 

 tina, where it is cut by the eve\,j||cis, ■ At first let us sup- 

 Kcj F'3'" ■ ' pose , 



