There is another reason for the rounded shape of the eye--that it may act as a lens in condensing the rays of light. In order that we may see things clearly, the rays of light must be brought to a focus upon or close to the retina, at the back of the eye; and our eyes are so shaped that they form a lens of proper thickness, or strength, to do this. You can see how this is done with an ordinary magnifying glass, or burning-glass. The little sharply lighted and heated point to which the light-rays can be brought is the focus of the lens, and the distance it lies behind the lens is called the focal distance. The thicker the lens, or burning-glass, is in the middle, the shorter its focal distance, and the more strongly it will magnify. A healthy, or normal, eye is of just such shape and "bulge" that rays of light entering the eye are brought to a focus on, or close to, the retina at the back of the eyeball. Some people, however, are unfortunately born with eyes that are too small and flat, or do not "bulge" enough; and then the rays of light are focused behind the retina instead of upon it, and the image is blurred. This is known as "long sight" (_hyperopia_), and can be corrected by putting in front of the eyes lenses of glass, called spectacles, which bulge sufficiently to bring the rays to focus on the retina. An eye that is too large and round and bulging brings the rays to a focus in front of the retina, and this also blurs the image. This form of poor sight is called "short sight" (_myopia_), and can be relieved by putting in front of the eye a glass that is concave, or thinnest in the middle and thickest at the edges, in the right proportions to focus the image where it belongs, right on the retina. This kind of glass is sometimes called a "minifying" glass, from the fact that it makes objects seen through it look smaller. It is also called a "minus" glass, while the magnifying glass is called a "plus" glass. The shape of the glasses or spectacles prescribed for an eye is just the opposite of that of the eye. If the eye is too flat (_long-sighted_), you put on a bulging, or convex, glass; and if the eye is too bulging (_short-sighted_), a hollow, or concave, glass. Other eyes are irregularly shaped in front and bulge more in one direction than another, like an orange. This defect is called _astigmatism_ and is very troublesome, making it hard to fit the eye with glasses, as the glasses have to be ground irregular in shape. [Illustration: A SCHOOL EYE-TEST A normal eye should be able to read the smaller type easily at a distance of twenty feet.]