78. As the image upon the retina is inverted, how does the mind perceive the object in its true, erect position? Many explanations have been advanced, but the simplest and most satisfactory appears to be found in the fact that {212} the retina observes no difference, so to speak, between the right and left or the upper and lower positions of objects. In fact, the mind is never conscious of the formation of a retinal image, and until instructed, has no knowledge that it exists. Consequently, our knowledge of the relative location of external objects must be obtained from some other source than the retina. The probable source of this knowledge is the habitual comparison of those objects with the position of our own bodies: thus, to see an elevated object, we know we must raise the head and eyes; and to see one at our right hand, we must turn the head and eyes to the right. [Illustration: FIG. 51.--THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE. N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye. S, Parallel Rays from the Sun.] [Sidenote: 79. The uniform perfection of the eye? Examples? The most common imperfection?] 79. LONG-SIGHT OR HYPEROPIA, AND SHORT-SIGHT OR MYOPIA.--The eye is not in all cases perfectly formed. For example, persons may from birth have the cornea too prominent or too flat, or the lens may be too thick or too thin. In either of these conditions sight will be more or less defective from the first, and the defect will not tend to disappear as life advances. The most common imperfection, however, is in the shape of the globe; which may be short (Fig. 51, H), as compared with the natural eye, N, or it may be too long, M. [Sidenote: 80. How is "long-sight" explained? "Short-sight?"] 80. When the globe is short, objects can only be clearly {213} seen that are at a distance, and the condition of the vision is known as "long-sight," or hyperopia. It will be observed, by reference to Fig. 51, that the focus of the rays of light would fall behind the retina of this eye. When the globe is too long, objects can only be clearly seen that are very near to the eye; and the condition resulting from this defect is termed "short-sight," or myopia. The focus of the rays of light is, in this case, formed in the interior of the eye in front of the retina. [Sidenote: 81. Long-sight, how common? With what must it not be confounded? Kind of glasses for short-sight? Why? Squint?]