_The Formation of the Image._—The eye is a camera, consisting of a series of lenses and media arranged in a dark chamber, the iris serving as a curtain. The object of the apparatus is to form on the retina a distinct image of external objects. _The Mechanism of Accommodation._—In the passive condition of the eye, when it is adjusted for far objects, the anterior surface of the lens is somewhat flattened. Accommodation for near objects consists in a contraction of the circular ciliary muscle and an increase in the convexity of the anterior surface of the crystalline lens. The light enters the eyeball through the pupil, falls upon the retina, which has often been compared to the sensitive plate of a camera, is received and transmitted by the optic nerve to the visual centers of the brain. The eyeball does not see. It is only a sensitive end-organ, which receives and transmits the impressions to the higher centers of sight. The act of vision is performed in the brain. The focusing power of the eye is the property of bending nearly parallel rays of light from distant and divergent rays or from close range so that they meet exactly on the sensitive retina; this is called refraction. In the normal eye these rays are focused exactly on the retina. =Optical Defects and their Correction.=—In the normal eye the near limit of accommodation is from 4 to 5 inches, and the far limit may be put at an infinite distance. _Myopia or Near-sightedness._—This is one of the most common refractive defects of the eye. In this condition, because of the greater length of the eyeball or increased refractive changes of the media, rays of light from a distance are focused in front of the retina, producing an indistinct image. The near-point is brought much nearer—from 2 to 2½ inches—and the far limit is at a very short distance. In reading, the myope is obliged to hold her book very close to the eyes in order to see. In doing so, she strains her muscles of convergence, producing ocular congestion and compression of the eyeball. The predisposing causes of myopia are heredity; it is said that half of myopics are descended from near-sighted parents; uncorrected astigmatism, and the effort to read very fine print or figures, entails severe strain on the eyes, which may result in myopia. Myopia is corrected by a concave lens, which diverges the rays of light, prolonging the focal distance, so that the rays of light are focused exactly on the retina. Not only are myopic eyes not injured by wearing suitable glasses, but, on the contrary, are often preserved from injurious pressure on the globe, due to the indulgence of the habit to nearly close the lids in order to see better, as is commonly done when glasses are not worn.