253 



vature of tho first surface of the lens is disproportionately great in 

 the last column. This arises from the circumstance, that it is de- 

 rived by calculation, for the curvature of an ellipse at tho lesser axis, 

 the two axes of which are alone given by Krause. Now, it is evi- 

 dent, that if wo regard the lens as a whole, or even any considerable 

 breadth of it, its mean radius of curvature will be sensibly smaller. 

 In fact, Krause finds that it may be tolerably represented by a cir- 

 cular curvature, having a radius of "329 inches. It occurred to me, 

 however, that by taking the greatest density of the lens, as given by 

 Brewster, and the curvature of the middle part, both anterior and 

 posterior, as given by Krause, I ought to arrive at a close approxi- 

 mation to the course of the axial pencil. 



" I have adopted for the refractive indices of the parts of the eye, 

 those given by Sir D. Brewster in his original paper in tho Edin- 

 burgh Philosophical Journal, vol. i., page 44, with the exception of 

 that of tho densest part of the lens, which is almost certainly mis- 

 prmted. They are as follow : — 



Aqueous humour, 

 Crystalline, outer coats, 



middle coats, 



central coats, 



the whole, 



1-3366 =/*. 

 1-3767 

 1 3786 

 1-3990*= fi, 

 1-3839 

 1-3394 =^3 



Vitreous humour, 



" Calculating from the preceding data, with Sir D. Brewster's in- 

 dices of refraction, the author finds the positions of the foci, towards 

 which the rays converge, after refraction at the successive surfaces, 

 to bo the following (reckoning from the interior surface of the cornea, 

 the thickness of which has been neglected) — 



For rays fading For rays diverging 

 parallel on the from a point 10 inches 

 cornea. distunt. 



Inches. Inches. 



After first refraction at the aqueous I _ -i.oqo 1-541 



humour, . . . i 



After second refraction at first sur- 1 _ i.ocq 1-377 



face of the lens, . . i 



After third refraction into vitreous 1 _ i.qqq 1135 



humour, . . . i 



* In the Baiuburgh Phil. .Journ., we find 1-3999. But I take this to be a 

 misprint, .is in Sir P. Rrowster's own subsequent writings, we always find 

 1 399i>. 



