^§4 •O'' '^^ Mechamfm of the Eye. 



.of a tliin membrane, and the lens approacKes very near to it : we will therefore call this 

 dlftance 12 hundredths. The axis and proportions of the lens mud be eftimated by com- 

 parifon with anatomical obfervations; fince they afFc£l, in a fmall degree, the determina- 

 tion of its focal diftance. M. Petit found the axis almoft always about two lines, or i8 

 hundredths of an inch. The radius of the anterior furface was in the greateft number ^ 

 lines, but oftener more than lefs. We will fuppofe mine to be 3^, or nearly -^^ of an inch. 

 The radius of the pofterior furface was moll frequently 2^ lines, or | of an inch.* The 



optical centre will be therefore ( —= 1 about one-tenth of an inch from the an- 



terior furface : hence we have 22 hundredths, for the diftance of the centre from the cornea. 

 Now, taking 10 inches as the diftance of the radiant point, the focus of the cornea will be 

 < 15 hundredths behind the centre of the lens. (Cor. 5. Prop. IV.) But the adual joint 

 focus is (91 — 22 =) 69 behind the centre : hence, difregarding the thicknefs of the lens, 

 its principal focal diftance is 173 hundredths. (Cor. 7. Prop. IV.) For its refraftive power 

 in the eye, we have (by Cor. 7. Prop. IV.) « — 13,5, and m — 14,5. Calculating upon this 

 *efra£live power, with the confideration of the thicknefs alfo, we find that it requires a 

 corre£lion, and comes near to the ratio of 14 to 13 for the fines. It is well known that the 

 refra£live powers of the humours are equal to that of water ; and, that the thicknefs of the 

 cornea is too equable ro produce any efFedl on the focal diftance. 



For determining the refradive power of the cryftalline lens by a dired experiment, I 

 -made ufe of a method fuggeftcd to me by Dr. Wollafton. I found the refraiSlive power of 

 ■the centre of the recent human cryftalline to that of water, as 21 to 20. The difference of 

 this r3t*io from the ratio of 14 to 13, afcertained from calculation, is probably owing to two 

 citcumftances. The firft is, that the fubftance of the lens being in fome degree foluble in 

 water, a portion of the aqueous fluid within its capfule penetrates after death, fo as fome- 

 what to Jeflen the denfity. When dry, the refradtive power is little inferior to that of crown 

 ^lafs. The fecond circumftance is, the unequal denfity of the lens. The ratio of 14 to 1 3 

 is founded on the fuppofition of an equable denfity : but, the central part being the moft 

 denfe, the whole a<5ls as a lens of fmaller dimenfions ; and it may be found by Prop. VII. 

 that if the cen-tral portion of a fphere be fuppofed of uniform derifity, refra£ling as 2 1 to :^o, 

 to the diftance of one-half of the radius, and the denfity of the external parts to decreafe 

 gradually, and at the furface to become equal to that of the furrounding medium, the 

 iphere thus conftituted, will be equal in focal length to a uniform fphere of the fame fize, 

 with a refraftion of 16 to 15 nearly. And the efFeft will be nearly the fame, if the central 

 ■portion be fuppofed to be fmaller than this, but the denfity to be fomewhat greater at the 

 furface than that of the furrounding medium, or to vary more rapid externally than inter- 

 nally. On the whole, it is probable that the refradive power of the centre of the human 



* Mera.de I'Acad. de Paris, 1730. p. 6. Ed. Amft. 



cryftalline, 



