1. Light must pass from the object into the eye. Objects cannot be seen where there is no light or where, for some reason, it is kept from entering the eye. 2. The light from the object must be focused (made to form an image) on the retina. In forming the image, an area of the retina is stimulated which corresponds to _the form of the object_. 3. Impulses must pass from the retina to the brain, stimulating it to produce the sensations. 4. The sensations must be so interpreted by the mind as to give an impression of the object. *Focusing Power of the Eyeball.*—The eyeball is essentially a device for focusing light. All of its transparent portions are directly concerned in this work, and the portions that are not transparent serve to protect and operate these parts and hold them in place. Of chief importance are the crystalline lens and the cornea. Both of these are lenses. The cornea with its inclosed liquid is a plano-convex lens, while the crystalline lens is double convex.(123) Because of the great difference in density between the air on the outside and the aqueous humor within, the cornea is the more powerful of the two. The crystalline lens, however, performs a special work in focusing which is of great importance. The iris also aids in focusing since it, through the pupil, regulates the amount of light entering the back chamber of the eyeball and causes it to fall in the center of the crystalline lens, the part which focuses most accurately. [Fig. 161] Fig. 161—*Diagram showing changes in shape of crystalline lens* to adapt it to near and distant vision. *Accommodation.*—A difficulty in focusing arises from the fact that the degree of divergence of the light waves entering the eye from different objects, varies according to their distance. Since the waves from any given point on an object pass out in straight lines in all directions, the waves that enter the eye from distant objects are at a different angle from those that enter from near objects. In reality waves from distant objects are practically parallel, while those from very near objects diverge to a considerable degree. To adjust the eye to different distances requires some change in the focusing parts that corresponds to the differences in the divergence of the light. This change, called _accommodation_, occurs in the crystalline lens.(124) In the process of accommodation, changes occur in the shape of the crystalline lens, as follows: 1. In looking from a distant to a near object, the lens becomes more convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary because the greater divergence of the light from the near objects requires a greater converging power on the part of the lens.(125)