fficient to
collect the rays into a focus, and form the picture of an object upon
the retina. By the experiments of the accurate Dr. Robertson, it
appears that there is less difference in the density, as well as in
the refracting power of the humours, than has been generally thought:
by weighing them in a hydrostatic balance, he found that the specific
gravities of the aqueous and vitreous humours were very nearly equal,
each being nearly equal to that of water: and that the specific
gravity of the crystalline did not exceed the specific gravity of the
other humours in a greater proportion than that of about 11 to 10.
Hence it would seem to follow, that the crystalline is not of such
great use in bringing the rays together, and thereby forming the
pictures of objects on the retina, as has been commonly thought by
optical writers; for though in shape it resembles a double convex
lens, and is, on that account, fitted to make the rays converge; yet,
be cause it is situated between two humours nearly of the same
refractive power with itself, it will alter the direction of the
light but a little. From this, the reason is evident why the sight
continues after the operation for the cataract, in which the
crystalline is depressed, or extracted, and why a glass of small
convexity is sufficient to supply the little refraction wanting,
occasioned by the loss of this humour. But without doubt, several
important purposes are effected by this construction of the eye;
which could not have been attained if it had been composed of one
humour only. Some of those purposes seem sufficiently evident to us;
for instance, by placing the aqueous humor before the crystalline,
and partly before the pupil, and making the cornea convex, a greater
quantity of light is made to enter the eye than could otherwise have
done without enlarging the size of the pupil; the light will also
enter in a less diverging state than it could have done if the pupil
had been enlarged, and consequently be more accurately collected to a
focus on the retina; for a perfect eye can only collect such rays to
a focus on that membrane, as pass through the pupil nearly in a state
of parallelism.

Another, and perhaps a principal advantage derived from the different
humours in the eye, is, probably, to prevent that confusion arising
from colour, which is the consequence of the different degrees of
refrangibility of the rays of light. From the experiments of Mr.
Dollond, it a