XCVI 



INTRODUCTION TO OPTICS. 

 Fig. 33, 



dered distinct and strong. If you conceive (mjig. 32) every point of the 

 tree to send forth a pencil of rays similar to those, A, B, every part of the 

 tree will be as accurately represented on the retina as the points a, b. You 

 may perhaps inquire why, since the eye requires refracting humours in 

 order to form a distinct representation on the retina, the same refractions 

 are not necessary for the image formed in the camera obscura ? It is 

 because the aperture through which we receive the rays into the camera 

 obscura is so extremely small, that but very few of the rays diverging from 

 a point gain admittance ; but if the aperture [be enlarged, and furnished 

 with a lens, the landscape will be more perfectly represented. 



That imperfection of sight which arises from the eyes being too promi- 

 nent, is owing to the crystalline humour, D (fig. 34) being too convex ; 

 in consequence of which it refracts the rays too much, and collects a 

 pencil, proceeding from the object A B, into a focus, F, before they reach 

 the retina. From this focus, the rays proceed diverging, and conse- 



Fig. 34. 



quently form a very confused image on the retina, at a b. This is the 

 defect of short-sighted people ; and it is remedied by bringing the object 

 nearer to the eye ; for the nearer an object is brought to the eye the more 



Fig.35. 



divergent the rays fall upon the 

 crystalline humour, and conse- 

 quently do not so soon converge 

 to a focus : this focus, therefore, 

 either falls upon the retina, or at 

 least approaches nearer to it, and 

 the object is proportionally dis- 

 tinct, as in Jig. 35. The nearer, 

 therefore, an object is brought to 

 the crystalline or to a lens, the 

 further the image recedes behind it. 



another resource for objects which they cannot approach to their eyes: 

 this is Jo place a concave lens, C D a (^. 36), before the eye, in order to 



But short-sighted persons have 



