The range of sight at which we read or write is, in a normal condition, about 12 to 14. inches; this point in myopia is much nearer, and in presbyopia much farther off; but for the latter, distinct vision does not go beyond 28 to 32 inches, that is, about double the distance considered normal; in myopia, on the contrary, this distance may diminish to within an inch. This condition of the sight is the result of modifications of the media of the eye. In myopia the cornea or the crystalline is more, and in presbyopia less, convex than in the normal condition. In myopia, therefore, the focus is in front of the retina for objects which, not being very near the eye, send to it rays which diverge but slightly; in presbyopia, on the contrary, the slight refraction caused by the flattening of the cornea, or the crystalline, tends to place the focus behind the retina, the point of convergence of rays coming from objects near at hand. The faculty of accommodation is rather limited in myopia, as well as in presbyopia, and is necessarily almost entirely wanting in the very near-sighted.
To remedy these modifications of the eye, the shortsighted person requires double concave glasses, which increase the divergence of the rays in proportion to the refraction by the media of the eye; the long-sighted person requires double convex glasses, which produce the opposite effect.
It is not uncommon to find persons who can only read and write at a very short distance, but who can notwithstanding see objects at a distance perfectly well In this case only one eye is myopic, the other is normal. A slight inequality in the eyes is very common, and often unper-ceived. This is undoubtedly the reason that many persons use but one eye even when looking with both, without being conscious of it, and this inequality, which is either the cause or effect of this exclusive action, can only be augmented by it.
Short-sightedness, even when slight, is an infirmity from which its subjects suffer all their lives, and it may be aggravated by the use of too strong glasses, and, as we have already stated, by the use of the microscope. Long-sightedness, on the contrary, does not make itself felt much before the age of forty, and then only in persons whose sight is good. It is, as the name presbyopia indicates, a mark of age, and a little philosophy enables us to resign ourselves to it, and wear the glasses which were useless in youth.