the defect is revealed. In the normal eye distant objects are focused on the retina without the use of the ciliary muscle, which is only employed when looking at near objects; but the hyperope has to use this muscle all his waking hours for both near and distant vision, so that his eyes are never at rest. Fortunately he has some compensation for this extra work, for in most hyperopes the ciliary muscle becomes more or less hypertrophied; but even so, if near work is at all excessive, or if the defect is associated with astigmatism or anisometropia, symptoms of eye-strain will sooner or later show themselves (see Eye-strain, below).
Fig. 29.—Showing Parallel Rays focused on the Retina of a Hyperopic Eye by means of a Convex Lens.
In older people a very common symptom is blurring of the type while reading; the book has to be put down and the eyes rested for some minutes before reading can be resumed. This is due to the fatigued ciliary muscle giving way and becoming unable to focus.
As we advance in years we lose accommodation power (see Presbyopia, below), so that the time comes to every hyperope, if he live long enough, when he not only has to use glasses for reading (at an earlier period than the normal person), but he also finds that he is gradually losing his distant vision. This is very alarming to many, until it is explained that all that has happened is the loss of power to correct the defect, which defect, of course, has always existed, and which in future will have to be corrected by suitable glasses. The higher the hyperopia the sooner will these symptoms manifest themselves.
In quite young children, sometimes the earliest sign of the presence of hyperopia is a convergent strabismus (internal squint). As a rule, this squint is nothing more than an over-convergence brought about by over-accommodation in those who cannot dissociate their convergence and accommodation; if we remove the necessity for over-accommodation by correcting the defect with suitable glasses, the over-convergence disappears and the squint is cured.
The total hyperopia of the eye is divided into manifest hyperopia and latent hyperopia. Manifest hyperopia is expressed in amount by the strongest convex glass that allows clear distant vision when the eye is not under atropine. Latent hyperopia is the additional hyperopia which is revealed under atropine. With advancing years the latent hyperopia becomes more and more manifest, and between the ages of 45 and 50 the total hyperopia is entirely manifest.
In addition to the symptoms already described, a very common one among young hyperopes is spasm of the ciliary muscle. This cramp of the muscle causes distant objects to be very indistinct, improvement only taking place with a concave glass, and near work has to be approached very close to the eyes, thus giving a wrong idea that the child is suffering from myopia; by paralysing the ciliary muscle with atropine the spasm disappears and the true nature of the defect is revealed.
The treatment essentially consists in ascertaining the total hyperopia of the eye, and this can only be done satisfactorily, when latent hyperopia is present, by paralysing the accommodation, using atropine for those under 25, and homatropine for those between the ages of 25 and 35 or 40. Over 40 (and when the hyperopia is high, even at an earlier age) no cycloplegic is necessary—in fact it is in many cases dangerous, as an attack of glaucoma may be induced. (See Eye: diseases.)
Having found the total hyperopia, we learn the amount of the latent hyperopia, and, roughly speaking, the convex glass required is equal to the whole of the manifest hyperopia added to, from one-third to a half, of the latent; but the treatment varies with the age of the individual and the amount of the hyperopia, and is too complicated to be detailed here.
Myopia (M.) (Short-sight).—Typical myopia is due to an elongation of the antero-posterior diameter of the eye, so that the retina is situated behind the principal focus, and only divergent rays of light from a near point (fig. 30), or parallel rays made divergent by a concave glass (fig. 31), can come to a focus on the retina. In other words, the far point of a myope is at a short distance in front of the eye, the distance being the measure of the myopia.
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Fig. 30.
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Fig. 31.
A myope can see distinctly at a distance when the eye is at rest (i.e. when accommodation is not being used), with that concave glass whose focal length is equal to the distance of the far point from the eye, and the converse is true; the measurement of myopia is that concave glass with which the myopic eye sees distinctly objects at a distance, and its focal length is equal to the distance of the myope's far point from the eye.
The Causes of Myopia.—Although myopia is hereditary, it is, with few exceptions, not congenital. We have seen that almost all eyes are hyperopic at birth. The savage is rarely myopic: it is civilization that is responsible for it; the necessity for constantly adapting the eye for near objects means undue convergence. We find that myopia generally first shows itself at the age of 8 to 10, when school work begins in earnest—that is, when convergence is first used in excess—and there is no doubt that it is excessive convergence that is mostly responsible for the development of myopia. The over-used internal recti constantly pulling at the sclerotic tend to lengthen the antero-posterior diameter of the eye, and as this lengthening of the antero-posterior axis necessitates greater convergence still, a vicious circle is produced, and the myopia gradually increases. The hereditary character of myopia is explained by the existence in such eyes of an “anatomical predisposition” to myopia. The sclera is unusually thin, and consequently less able to resist the pull of the internal recti, and the relative position of the recti and the position of the optic nerve, both of which may be hereditary, may be factors in the production of this defect. Anything which causes young subjects to approach their work too near the eyes may be the starting-point. Bad illumination, or the light coming from the wrong direction (for instance, in front), or defective vision produced by corneal nebulae, or lamellar cataract, &c., all necessitate over-convergence in order to obtain clearer images, and myopia may be produced.
