A self-acting window, which shall expand with the opening of light in the mornings and evenings, and close up of its own accord as the light increases toward noon, has never been manufactured by man.

But the Former of the eye took note of the necessities and conveniences of the case, and besides giving a pair of shutters to close up when we go to sleep, he has given the most admirable sunblinds ever invented.

The nerve of the eye at the back of its chamber can not see without light, and its light comes through the little round window called the pupil, or black of the eye--which is 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.