42 
Dr. Young’s Lecture 
within its capsule penetrates after death, so as somewhat to 
lessen the density. When dry, the refractive power is little infe- 
rior to that of crown glass. The second circumstance is, the 
unequal density of the lens. The ratio of 14 to 13 is founded 
on the supposition of an equable density : but, the central part 
being the most dense, the whole acts as a lens of smaller dimen- 
sions; and it may be found by Prop. VII. that if the central 
portion of a sphere be supposed of uniform density, refrac- 
ting as 21 to 20, to the distance of one half of the radius, and 
the density of the external parts to decrease gradually, and at 
the surface to become equal to that of the surrounding medium, 
the sphere thus constituted, will be equal in focal length to a 
uniform sphere of the same size, with a refraction of 16 to 15 
nearly. And the effect will be nearly the same, if the central 
portion be supposed to be smaller than this, but the density 
to be somewhat greater at the surface than that of the sur- 
rounding medium, or to vary more rapidly externally than in- 
ternally. On the whole, it is probable that the refractive 
power of the centre of the human crystalline, in its living state, 
is to that of water nearly as 18 to 17; that the water imbibed 
after death, reduces it to the ratio of 21 to 20; but that, on 
account of the unequable density of the lens, its effect in the 
eye is equivalent to a refraction of 14 to 13 for its whole size. 
Dr. Wollaston has ascertained the refraction out of air, into 
the centre of the recent crystalline of oxen and sheep, to be 
nearly as 143 to 100; into the centre of the crystalline of fish, 
and into the dried crystalline of sheep, as 152 to 100. Hence, 
the refraction of the crystalline of oxen in water, should be 
as 15 to 14 : but the human crystalline, when recent, is de- 
cidedly less refractive. 
