EYE 



in the ratio of from 1.38 to 1.447 to 1. Dr. Brewoter gives the following- 

 Table, deduced from experiments made on a recent human eye : 



'Water 1.3358 



The Aqueous humour 1.3366 



Vitreous humour 1.3394 



.f "4 outer coat of chrystalline 1.3767 



power of _ middle 13786 



central parts 1.3990 



whole chrystalline 1.3839 



Dr. Brewster also gives the following dimensions : 



Inch. 



Diameter of the chrystalline , 0.378 



cornea 0.400 



Thickness of the chrystalline 0.172 



coinea 0.012 



If the humours of the eye be too convex or too flat, an imperfection 

 in vision is in either case the consequence : a conrave lens will remedy 

 the former defect, and a convex one the latter. The following problems 

 embrace nearly every thing connected with the theory of spectacles. 



1. Given tlfb distance at which a short-sighted person can see distinct- 

 ly, to find the focal length of a concave glass which will enable him to 

 see distinctly at any other given distance. 



Let A" = distance at which he can see distinctly, A a greater distance 

 at which he wishes to view objects, F focal length of the required 

 lens, then (see Refraction jjj, Art. 2.) 



_--- 

 A" ~ F A ' ~ A A"' 



Cor. If A be indefinitely great, F = A". 



2. Given the distance at which a long-sighted person can see distinct- 

 ly, to find the focal length of a convex glass which will enable him to 

 see distinctly at any other given distance. 



Let A" distance at which he can see distinctly, A a shorter dis- 

 tance at which he wishes to view objects, F focal length of the lens, 

 then 



1 1 1 . r A A" 



2F = A* F ;andF== -A^A' 



Cor. If A" be indefinitely great, or the eye require parallel rays, 

 F=A. 



113 



