[Illustration: FIG. 386.--Cross-section of the eye.] =390. Action of the Eye in Vision.=--When we look at an object, a small, real, inverted image is formed upon the _retina_ at the back of the interior of the eye. The retina is an expansion of the optic nerve and covers the inner surface at the back of the eyeball. Seeing is due to the action of light in forming images upon the retina. Our eyes are so constructed that when they are relaxed the lens is adjusted to form clear images of _distant_ objects upon the retina. If we look from distant to near objects without changing the shape of the eye lens, a sharp image of the latter cannot be formed and we get a blurred impression. It is difficult, however, to look at objects without automatically adjusting the eye lens so that it will make a sharp image. Test this by looking out of a window at a distant object, then without moving the head or eyes look at the glass of the window; you will notice a slight change of some sort _in_ the eye itself as the vision is adjusted. This adjustment is made by muscles that pull or compress the eye lens so as to make it thicker for near objects and thinner for distant ones. The eye ordinarily does not see objects nearer than 10 in. clearly. This means that the greatest possible thickening of lens will not form clear images upon the retina if the object is nearer than 10 in. (25 cm.). [Illustration: FIG. 387.--The visual angle, _AOB_ is greater at _AB_ than at _A´B´_.] =391. The Visual Angle.=--To examine objects carefully we usually bring them as close to the eye as possible, for the nearer to the eye the object is brought, the larger is the visual angle formed by it (see Fig. 387), and the larger is its image upon the retina. _The visual angle of an object is the angle at the eye lens between the rays that have come from the ends of the object._ Consequently the more distant the object, the smaller is its visual angle. Now if we wish to examine small objects with great care, we frequently find that it is necessary to bring them close to the eye so that they have a visual angle of adequate size. If they must be brought closer than 10 in. a double convex lens is placed in front of the eye. This assists the eye lens in converging the light so that a clear image may be formed when the object is close, say an inch or so from the eye. This is the principle of the magnifying glass used by watch-makers and of the _simple microscope_. The action of the latter is illustrated by Fig. 388. The convex lens forms a virtual, enlarged image "_A´-B´_" of the object "_A-B_" which it observed instead of the object itself. [Illustration: FIG. 388.--Action of the simple microscope.] [Illustration: FIG. 389.--"Near sightedness", or myopia. Parallel rays come to a focus at _F_; emerging rays focus at _A_, the far point.]