Chemistry and Physics. 415 
muihe measure of the axis of the eye we have seen to be only 
1833 in h, according to Treviranus, and ‘855 according 1o Krause; 
epics rays of mean refrangibility (to which Brewster’s measures 
fer) converge to a point no less than *227 inch behind the retina, 
wien the rays fall parallel on the cornea, and -302 when the object 
above, (Dr. Young makes it -91,) does not come up to the requisite 
dimensions; sal Dr. sans with his usual acuteness, ascribes the dif- 
ference to the gradually varying density of the strata or coats of the 
lens,* the dense small ecaada evidently acting as a.lens of compara- 
tively short focus;,and this explanation is probably the correct one, to 
which we may add, that the configuration of the coats of equal density, 
which, near the surface of the lens, are very elliptical, become, 
its centre, gradually nearly spherical. On this account, it is all but 
impossible to predict the exact course of the rays through a structure 
of so much complication. 
Dr. Young had considered the case with his usual attention and pene- 
tration. He investigates the focus of a spherical le ens, or lens with 
surfaces which are segments of spheres, and whose density i is ale 
and the result may be recalled we as one which, per rhaps, has 
been sufficiently. remarked. ‘* On the whole,’ ” he says, oh FP . ‘prpbable 
it to the ratio of 21 10 20 [1- a but that, on account Plow on unequa- 
ble density of the lens, its effec on the eye is SqUINAI OM, to a refraction 
of 14 t0 13 [1-439] for its iss 9g size 
On the whole, these scpiagedovdagen - swell as the considerations into 
which [ entered in a furmer paper, read to the Society in ont 
mechanism of the focal ames @ ar ve left on my faa the convic- 
tion that the optical and mechanical structure of the organ of sight is 
even less understood than it is commonly believed to be. Sim ple as 
are its general arrangements, and comparable, in some (gs mia to 
those of artificial combinations, we perceive surfaces figu red in 
plex manner, and structures of varying refractive e density soca’ | in 
a very complicated manner. Krause’s measures of the curvature of 
1 
versal opinion of the variation of density of the crystalline being in- 
tended to correct the aberration of spherical surfaces, when, in reality, 
no such surfaces exist. We are quite unable to trace the exact course 
by which the rays of light are focalised in the retina, since it depends 
on the internal constitution of the lens that they do not meet very far 
behind it: 3 and it still remains at least doubtful how the adjustment to 
distinct vision of objects at different distances is effected. 
Finally, the question of achromatism of the eye has its own difficul- 
ties. It is not now es hr that the rhe has the power of ene 
* Nat, Phil, vo. ii, p. 580. {Na Pa vl 8 
¢ ransactinn Hal Society of iaburgy vol 0 
