Page:Popular Science Monthly Volume 67.djvu/744

738 it is too small or too short and hence effort and muscular exertion are necessary to bring the image forward, in proper definition upon the retina. Without this exertion the image would be behind the retina. Hence a similar blurring or badly defined picture as in myopia, but from the reversed cause. In this kind of eye the picture is naturally blurred even of the most distant object (making the term far-sightedness a misnomer), and a slight increase of focussingfocusing [sic] power is required on the part of the 'accommodation' mechanism in each eye, in order to shorten the focus of the image-forming rays of light. With every lessening of the distance of the object, still greater converging power is demanded, as one may understand by tests with different-power glass lenses. When the object is brought within ten or fifteen inches of the eye the greatest effort is required to make the image clear. The degree of this effort will depend upon how much too short the eyeball is, or upon the amount of hyperopia. It will also depend upon the kind and amount of ocular labor, especially the continuance of 'near-work,' the reading, writing, etc., required of the eyes. The farmer, ranchman or sailor will obviously be able to overcome or be indifferent to higher degrees of far-sightedness than those living in cities, while the literary man, bookkeeper or seamstress will experience eyestrain with still lower degrees of hyperopia.

One must carry in the mind another modifying condition—the difference almost always existing between the far-sightedness of one eye and that of its fellow. The two are under more stringent orders than in myopia to work together and in harmony. When the hyperopia differs in the two the chances of strain are obviously increased many times.

These chances already multiplied now become inevitables if astigmatism is a complication of the hyperopia. Alas, also, it is very certain to complicate. I have measured something like ten thousand pairs of eyes, and not one of these pairs was without some imperfection of shape, size or curvature, either in one or both of the eyes. An absolutely perfect pair of eyes does not probably exist.

'And do tell me, what is astigmatism?' is the puzzling question put to the oculist every day. It is one, fortunately, very simply and easily answered. The front part of the eye, that behind which lies the pupil, is called the cornea. It should be round or equally curved like the central portion of the end of an egg. It is usually not uniform in its curvature, but is more curved in one meridian than in the reverse meridian—that is, it is, approximately, of the shape of the side of the egg. When symmetrically curved it will naturally refract, or help to focus, correctly, the entering cone of light which is to form the picture on the retina. If it is unsymmetrically curved, like the side of the egg, it will produce a misshapen and unsymmetrical picture which does not accurately represent the object. The condition of safety,