When you look at an eye you are likely to think that the front of it is blue or brown. The colored part is not the front of the eye. If you look at the eye from the side you will see that there is a curved part in front of the colored part and that the curved part is perfectly clear. This curved clear part of the eye we call the _cornea_. The cornea connects with the white part of the eye, and this white part extends all around the rest of the eye, except at a small point in the back where the _optic nerve_ comes through. This white part we call the _sclera_. [Sidenote: Aqueous humor] The space between the cornea and the colored part of the eye is filled with a clear fluid that is called the _aqueous humor_, which means watery fluid. The space occupied by this fluid is called the _anterior chamber_ of the eye. [Sidenote: The iris] The iris, which is the colored portion of the eye, is a curtain that is hung between the anterior and posterior chambers of the eye. It prevents any light getting into the posterior chamber except that which passes through a round hole in the iris called the _pupil_. The pupil grows larger or smaller according to the amount of light needed by the eye. If you look away from a bright light at something in the dark, the pupil grows larger; if you look back at the light, it grows smaller. You can see this in a hand mirror. [Sidenote: Lens] Behind the pupil is a clear mass shaped like a very strong magnifying glass. This is the _lens_. The lens causes to be formed on the back of the eye a clear picture of whatever you are looking at. When you focus a camera, you move the back towards or away from the lens. When you focus your eye you cannot move the back of the eye, but you can make the lens more or less convex as may be needed to make a clear picture. [Illustration: FIG. 86. A cross-section of the eye.] [Sidenote: Vitreous humor] Behind the iris and the lens, we find the posterior chamber of the eye. This occupies by far the greater portion of the ball and corresponds to the dark chamber of a camera. This chamber is filled with a clear fluid called the _vitreous humor_, which means jelly-like fluid. It is a clear, gelatinous substance. [Sidenote: The retina and the optic nerve] The optic nerve enters the eyeball from the back and spreads its fibers out in a thin membrane called the _retina_, which corresponds to the sensitive plate in a camera. The lens focuses on the retina the image of any object you look at. The impression made on the minute nerve endings in the retina is carried by the optic nerve fibers to the brain. When this impression reaches the brain we see the object. [Sidenote: Movements of the eye]