on the Mechanism of the Eye. 41 
almost always about two lines, or 18 hundredths of an inch. 
The radius of the anterior surface was in the greatest number 
3 lines, but oftener more than less. We will suppose mine 
to be 3^, or nearly ^ of an inch. The radius of the 
posterior surface was most frequently 2 \ lines, or J of an 
inch.* The optical centre will be therefore ( 4 3 ° ~— ) about 
one-tenth of an inch from the anterior surface : hence we have 
22 hundredths, for the distance of the centre from the cornea. 
Now, taking 10 inches as the distance of the radiant point, the 
focus of the cornea will be 1 15 hundredths behind the centre 
of the lens. (Cor. 5. Prop. IV.) But the actual joint focus is 
(91 — 22 =) 69 behind the centre: hence, disregarding the 
thickness of the lens, its principal focal distance is 173 hun- 
dredths. (Cor. 7. Prop. IV.) For its refractive power in the 
eye, we have (by Cor. 7. Prop. IV.) n — 13,5, and m = 14 ,5. 
Calculating upon this refractive power, with the consideration 
of the thickness also, we find that it requires a correction, 
and comes near to the ratio of 14 to 13 for the sines. It 
is well known that the refractive powers of the humours are 
equal to that of water; and, that the thickness of the cornea is 
too equable to produce any effect on the focal distance. 
For determining the refractive power of the crystalline lens 
by a direct experiment, I made use of a method suggested to 
me by Dr. Wollaston. I found the refractive power of the 
centre of the recent human crystalline to that of water, as 2 1 
to 20. The difference of this ratio from the ratio of 14 to 13, 
ascertained from calculation, is probably owing to two circum- 
stances. The first is, that the substance of the lens being in 
some degree soluble in water, a portion of the aqueous fluid 
* Mem. de l’Acad. de Paris, 1730. p. 6. Ed. Amst. 
G 
MDCCCI. 
