In the eye the clear part that projects between the lids, and is called the _cornea_, is the important lens. Just behind is the arrangement that corresponds to the diaphragm of the camera; this is the colored part with a round hole in the center; it is called the _iris_ and the round hole is the _pupil_. Behind the iris, and resting right against it, is the secondary lens of the eye, known as the _crystalline lens_. The eye as a whole is a globe just under an inch in diameter; at the back of it, straight behind the lenses and pupil, is the sensitive surface upon which images are formed. This is the _retina_; it extends pretty well around to the sides, but the part we use most in seeing is the small portion straight in line with the pupil. The cornea by itself is a lens whose focus is [Illustration: VERTICAL SECTION OF THE RIGHT EYE AND ITS LIDS _c_, cornea; _l_, crystalline lens, its margin shielded by _i_, iris; _p_, pupil; _r_, retina; _m_, muscles that move the eyeball; _o. n._, optic nerve. (From “Human Physiology,” Stiles.)] longer than the length of the eyeball, and the crystalline lens by itself also has a long focus, but the two in combination give a focus that just corresponds with the length of the eyeball, so that the images of all objects at a distance of eighteen feet or more fall sharply on the retina. Since near objects focus farther away from the lens than far objects, the effect of this is to make objects nearer than eighteen feet out of focus. For them a longer eyeball would be needed, and it would have to become longer and longer the nearer the object was brought to the eye. We all know that when we look at near objects we make an adjustment in the eyes. This is known as accommodation; for a long time it was supposed that _accommodation_ was actually secured by lengthening or shortening the eyeball to bring the focus right, but we now know that the eyeball does not change in shape when we accommodate. The same result is secured by another means, namely, by letting the crystalline lens bulge out and become thicker. It was stated incidentally a few pages back that thick lenses have shorter focuses than thin. So when the crystalline lens thickens it shortens the focus of the eye, and throws the image forward. This will locate the image of near objects on the retina, instead of behind it, as in the unaccommodated eye. The crystalline lens is not stiff like glass, but rather like a thick jelly; it is in a transparent bag, or capsule, which is fastened to the inside of the eyeball all around, and the pull on the capsule stretches it out pretty flat, making the lens thin. There are tiny muscles inside the eyeball, known as the _ciliary_ muscles. These are so fastened that when they contract they pull the eyeball forward and inward, loosening the tension on the capsule of the lens. This then bulges out, taking up all the slack. When we learn, as babies, to accommodate for near objects we