s simply a hole in the iris, or colored part.
Now this iris is formed of two sets of muscles: one set of elastic
rings, which, when left to themselves, contract the opening; and another
set at right angles to them, like the spokes of a wheel, pulling the
inner edge of the iris in all directions to the outside. In fact it is
not so much a sunblind, as a self-acting window, opening and closing the
aperture according to our need of light, and doing this so
instantaneously that we are not sensible of the process.

It is self-evident that the Maker of such an eye was acquainted with the
properties of light, and the alternations of night and day, as well as
with the mechanical contrivances for adjusting the eye to these variable
circumstances. He has given us an eye capable of seeking knowledge among
partial darkness, and of availing itself for this purpose of imperfect
light; an apt symbol of our mental constitution and moral situation in a
world where good and evil, light and darkness, mix and alternate.

Perhaps some one is ready to ask, What is the use of so many lenses in
the eye? It seems as if the crystalline lens and the optic nerve were
sufficient for the purpose of sight, with the cornea simply to protect
them. What is the use of the aqueous humor and the vitreous humor?

Light, when refracted through the lens, becomes separated into its
component colors--red, yellow, green, blue, and violet; and the greater
the magnifying power of the lens, and the brighter the object viewed,
the greater the dispersion of the rays. So that if the crystalline lens
of the eye alone were used, we should see every white object bluish in
the middle, and yellowish and reddish at the edges; or, in vulgar
language, we should see starlight.

This difficulty perplexed Sir Isaac Newton all his life, and he never
discovered the mode of making a refracting telescope which would obviate
it. But M. Dolland, an optician, reflecting that the very same
difficulty must have presented itself to the Maker of the eye,
determined to ascertain how he had obviated it. He found that the Maker
of the eye had a knowledge of the fact that different substances have
different powers of refracting or bending the rays of light which pass
through them, and that liquids have generally a different power of
refraction from solids. For instance, if you put a straight stick in
water, the part under water will seem bent at a considerable angle,
while if you put the st