The wall of the eyeball is made up of three coats. An outer tough white coat, of connective tissue, is called the _sclerotic coat_. Under the sclerotic coat, in front, the eye bulges outward a little. Here the outer coat is continuous with a transparent tough layer called the _cornea_. A second coat, the _choroid_, is supplied with blood vessels and cells which bear pigments. It is a part of this coat which we see through the cornea as the colored part of the eye (the _iris_). In the center of the iris is a small circular hole (the _pupil_). The iris is under the control of muscles, and may be adjusted to varying amounts of light, the hole becoming larger in dim light, and smaller in bright light. The inmost layer of the eye is called the _retina_. This is, perhaps, the most delicate layer in the entire body. Despite the fact that the retina is less than 1/80 of an inch in thickness, there are several layers of cells in its composition. The optic nerve enters the eye from behind and spreads out to form the surface of the retina. Its finest fibers are ultimately connected with numerous elongated cells which are stimulated by light. The retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. This accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. The retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. The eye, like the camera, has a lens. This lens is formed of transparent, elastic material. It is found directly behind the iris and is attached to the choroid coat by means of delicate ligaments. In front of the lens is a small cavity filled with a watery fluid, the _aqueous humor_, while behind it is the main cavity of the eye, filled with a transparent, almost jellylike, _vitreous humor_. The lens itself is elastic. This circumstance permits of a change of form and, in consequence, a change of focus upon the retina of the lens. By means of this change in form, or _accommodation_, we are able to distinguish between near and distant objects. [Illustration: How far away can you read these letters? Measure the distance. Twenty feet is a test for the normal eye.] Defects in the Eye.--In some eyes, the lens is in focus for near objects, but is not easily focused upon distant objects; such an eye is said to be nearsighted. Other eyes which do not focus clearly on objects near at hand are said to be farsighted. Still another eye defect is astigmatism, which causes images of lines in a certain direction to be indistinct, while images of lines transverse to the former are distinct. Many nervous troubles, especially headaches, may be due to eye strain. We should have our eyes examined from time to time, especially if we are subject to headaches.