THE SIGHT, AND HOW TO PRESERVE IT. SOME KNOWLEDGE OF THE EYE NECESSARY FOR ITS PROPER CARE. To care O take proper care of our eyes is to do what we can to avoid such diseases or weakness of the eyes as are avoidable. But to do this efficiently we must know something about the eye and its diseases and weaknesses. We must know what certain feelings about the eyes mean, what pain and fatigue mean, and why the eye is so often subject to fatigue and pain. We cannot afford to give up the entire use of our eyes because they ache, nor can we afford wholly to disregard ominous symptoms lest we run the risk of impairing sight or losing it. We wish to use our eyes, to preserve our sight to the last, and to avoid all avoidable disease. THE SYMPTOMS OF WEAK SIGHT (ASTHENOPIA). The first symptom is fatigue of the eyes during or after reading, writing, sewing, and similar work in the evening. Very soon the same fatigue is noticed after like occupations during the day, when the light is good. After a time, fatigue of the eyes comes on immediately on attempting to read or sew, and, if disregarded, pain and confusion of vision follow. Letters appear to run together, lines are blurred and indistinct, and continued use of the eyes in reading becomes impossible. If we then look up, across the room, or in the distance, quick relief comes, and vision is as clear and distinct as possible; but an attempt to read again brings a return of pain and confusion of sight. HOW WE SEE. In order that we may comprehend exactly the nature of weak sight, let us consider for a moment the structure of the eyes and how we see. The figure numbered A, on the next page, gives an idea of the anatomy of the eye, sufficiently full for our purpose. The rays of light from an object enter the eye through the pupil (P), and, passing back to the inner coat of the eye, the retina (R), form an image of the object upon it very much as an image is formed on the plate in the camera Fig. A gives a profile view of the eye; that is, the eye-ball is divided into two equal parts, just as one might divide an apple or an orange, and the flat, cut surface held up to view. W points out the white of the eye, a strong protective coat extending quite around the eye-ball except in front. It is called the sclerotic coat. C, the cornea, or transparent front of the eye; and, behind it, I, the iris, or coloured part of the eye, extending in a circle around, P, the pupil, which is a circular hole in the iris, that admits the light into the eye, and out of which the eye sees. L is the crystalline lens directly behind the pupil. In health, the lens is transparent like the front of the eye, and offers no obstruction to the passage of rays of light; but if it becomes opaque, it is called cataract, and the eye is more or less blind from the obstruction of the passage of light. MA is a little muscle, called the muscle of accommodation, that adjusts the focus of the lens and the eye for near objects. M C is a muscle attached to the white of the eye on the outside, that converges or turns the eyes inward, and is the muscle of convergence. R points to the inner lining of the eye, the retina, upon which images of objects are pictured; and ON is the optic nerve that conveys these pictures to the brain. H is the body of the interior of the eye, filled by a transparent humour. of the photographer. Indeed, quite lately, experiments have been made that show a very close relation between the nature of photography and the nature of the process that goes on in the act of seeing. Rabbits have been held a few seconds before a window, and then, being immediately killed, the picture of the window has been found distinctly photographed on the retina of their eyes. After the reception of the image upon the retina, the act of seeing is completed by the transmission of the impression to the brain through the optic A, the rays of light entering the eye. R, the retina, where they should be focussed. L, the lens by which the rays of light are chiefly focussed. nerve (ON in the figure). But in order that the image of an object on the retina shall be clear and distinct, it is necessary that the rays of light passing into the eye through the pupil should be properly focussed. See Fig. B. |