The eyeball is spherical in form, having the segment of a smaller and much more prominent sphere ingrafted on its anterior part. The segment of the larger sphere, which forms about five-sixths of the globe, is opaque, and forms the sclerotic coat, the so-called “white” of the eye. The smaller sphere, which forms the remaining sixth, is transparent, and is formed by the cornea.
The iris is a circular contracting membrane, suspended from the edges of the cornea, in front of the eye like a curtain. The iris gives color to the eye, and when we say that an eye is blue or brown, we mean that is the color of the iris. The iris is freely movable, and, according as to whether it dilates or contracts, there is an alteration in the size of the central aperture, called the pupil.
The chief function of the iris is to regulate the quantity of light admitted to the interior of the eye. In a very strong light the pupil quickly contracts, shutting out the excessive light, while in a subdued light the pupil dilates, thus allowing more light to enter. When looking at a distance or when looking languidly into space, the pupil dilates.
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.