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.
A SCHOOL EYE-TEST
A normal eye should be able to read the smaller type easily at a distance of twenty feet.
We have just seen how the eye deals with rays of light coming from a distance, which are practically parallel. When, however, books or other objects are brought near the eye, the rays of light coming from them do not remain parallel, but begin to spread apart, or diverge; and a stronger lens is required to bring them to a focus upon the retina. To provide for this, there is in the middle of the eyeball a firm, elastic, little globular body about the size and shape of a lemon-drop, called the crystalline lens. Around this is a ring of muscle, which is so arranged that when it contracts it causes the lens to change its shape and become more bulging, or thicker in the middle. This makes the eyeball a "stronger" lens so that the rays of light can be brought to a focus upon the retina.
This action is known as accommodation, or adjustment; and you can sometimes feel it going on in your own eye, as when you pick up a book or a piece of sewing and bring it up quickly, close to the eye, in order to see clearly.
If this little muscle is worked too hard, as when we try to read in a bad light, it becomes tired and we get what is called "eye-strain"; and if the strain be kept up too long, it will give us headache and may even make us sick at the stomach. The commonest cases of eye-strain are in eyes that are too flat (hyperopic) where this little muscle has to "bulge" the lens enough to make good the defect and bring the rays to a focus. This, however, of course keeps it on a constant strain; and the eye is continually giving out, and its owner suffering from headache, neuralgia, dyspepsia, sleeplessness, and other forms of nervous trouble, until the proper lens or spectacle is fitted.[30]
A surface as delicate and sensitive to light as the retina, would, of course, be damaged by too bright a glare; so in the front of the eye, just behind the cornea, a curtain has grown up, with an opening or "peep-hole" in its centre, which can be enlarged or made smaller by little muscles. This opening is the pupil; the curtain, which is colored so as to shut out the rays of light, is known as the iris, for the quaint, but rather picturesque, reason that Iris in Greek means "rainbow," and this part of the eye may be any one of its colors.
