In emmetropia and hyperopia the visual line cuts the cornea to the inside of its major axis, and the angle gamma is therefore positive. Owing to the shortness of the eyeball in hyperopia, the effect of which is to increase the distance between the fovea and the optic axis, the angle gamma is very much greater than in emmetropia. This may give to the eyes the appearance of an apparent divergent strabismus, as the axes of the corneae seem to diverge though the fixation is correct.
In myopia the length of the eyeball is too great, so the visual line cuts the cornea nearer the major axis, or they may coincide, or it may cut it to the outer side making the angle gamma negative. In the latter case the effect will be to give the eyes the appearance of an apparent convergent strabismus.
Abnormalities of Refraction and Accommodation.-As has been explained in the preceding pages, a normal or emmetropic eye is one whose static refraction is sufficient to bring parallel rays to a focus on the retina; or, one whose retina is situated at the focus of its dioptric system. Its far point is always at infinity. Any departure from emmetropia is known as ametropia of which three different forms are recognized: 1. Hypermetropia, in which the retina is situated in front of the focus of parallel rays. 2. Myopia, in which the retina is situated behind the focus or parallel rays. 3. Astigmatism, in which the refraction of the different meridians is different.
Hypermetropia or Hyperopia.-In hyperopia the static refraction is not sufficient to bring parallel rays to a focus on the retina. Such rays if not intercepted by the retina would come to a focus behind it. As they are intercepted by the retina they do not form there a distinct image of the object looked at but a circle of diffusion. In order to bring parallel rays to a focus on the retina, it is necessary either to place an appropriate convex lens before the eye which causes them to converge or to call the accommodation into play.
Fig.12 shows how the parallel rays a b converge toward a point c, behind the retina, after passing through the dioptric system, and how the diffusion circle d e is formed upon the retina.
As the retina in hyperopia is nearer the dioptric system than its principal focus, rays passing out from any point upon it such as R (Fig.13) will leave the eye divergent and will appear to come from a point R’ situated behind the eye.
The point R’ the virtual conjugate focus of R, is the far point of the eye, or the point towards which the rays must converge before entering in order to be brought to a focus on the retina. Being behind the eye it is negative. In order that parallel rays may be brought to a focus on the retina, it is necessary that the refractive power of the eye be increased by such a lens as will render them convergent towards the point R’. This is shown in Fig. 14 where the lens L renders the parallel rays convergent towards R’, and which the dioptric system render still more convergent so that they come to a focus at R on the retina.
The greater the hyperopia the nearer the far point is to the eye, the more convergent the rays must be in order to come to a focus on the retina, and the stronger must be the lens which renders them so. But the power of accommodation is also sufficient to increase the static refraction sufficiently to bring parallel rays to a focus on the retina if the degree of hyperopia is not too great. In fact, it ordinarily does so in such cases so that the vision may be normal for distant objects, which has given rise to the misleading term of farsightedness. A beginner might fall into the error of considering such an eye emmetropic; but it can be proven to be hyperopic by successively placing stronger and stronger convex glasses before it, which, as the accommodation relaxes, do not interfere with the distinctness of the object until the hyperopia is overcorrected, or an artificial myopia is produced. Hence, it is necessary to find the strongest convex glass through which the hyperopic eye can see distant objects most distinctly in order to find the measure of the error. Generally the ciliary muscle, through force of habit, does not relax to its fullest extent, so that the strongest convex glass simply represents the amount of manifest hyperopia (Hm). The balance of it, the latent (H1), can only be made manifest by instilling a solution of some cycloplegic like atropine which suspends the accommodation.
The sum of the latent and manifest hyperopia gives the total (Ht). Theoretically, that glass placed in contact with the eye whose focal distance is equal to the distance of the far point behind the eye, or which renders parallel rays convergent towards the far point, is the measure of the hyperopia. In practice, however, the glass is placed about 15 mm. in front of the eye, and it is regarded as the measure, though in reality it is not as great.
CAUSES.-The eyeball is either abnormally short, constituting axial hyperopia, or its refractive power may be deficient, curvature hyperopia. Hyperopia is nearly always congenital. Most children are so at birth, but as they grow older the refraction increases and they become less hyperopic, or emmetropic, or myopic. Senile changes in the lens, flattening, give rise to it; and its removal, as for cataract, produces a high degree. The latter condition, however, is termed aphakia.
SYMPTOMS.-The constant effort of the accommodation necessary in order to see distinctly gives rise to many symptoms. As the ciliary muscle tires, vision blurs, and it is necessary to stop work and rub the eyes. The respite obtained in this way is only temporary, as the muscle soon tires again and the performance must be repeated again and again until finally the work must be discontinued. Such people often seek a good light because the contraction of the pupil renders the vision clearer. Frequently too they hold the object near the face to secure larger retinal images and contract the lids to shut off the more divergent rays. This gives the semblance of myopia, and many children are erroneously given concave glasses which aggravate the trouble.
When left uncorrected, hyperopia frequently gives rise to conjunctivitis, blepharitis, nictitation of the lids, and congestion of the retina, choroid and optic nerve. Headaches and various reflex neuroses are very common.
Strabismus convergens is frequently associated with hyperopia the discussion of which will be found in the chapter upon that subject.
Hyperopia is often complicated with spasm of the ciliary muscle, the effect of which is to bring nearly or wholly the entire accommodation into play. This reduces the amount of manifest hyperopia when it is of high degree, and in some instance may even convert the case into one of false myopia. The vision in the latter instance will be improved by concave glasses, though it would be a serious error to prescribe them. Such a mistake is prevented by detecting the real nature of the refractive error by means of the ophthalmoscope, as described in the chapter on dipotometry.
When spasm of the accommodation is present, it is imperative that a cycloplegic be instilled to temporarily paralyze the ciliary muscle and so suspend the accommodation.
Manifest hyperopia is divided into facultative, relative, and absolute.
Facultative hyperopia may be overcome by using the accommodation without squinting.
Relative hyperopia represents a greater degree, and can only be overcome by the accommodation when the patient squints inward.
Absolute hyperopia is the highest degree, and cannot be overcome by using the entire accommodation.
The determination of hyperopia will be described in the chapter on dioptometry.
CORRECTION OF HYPEROPIA.-If the patient has normal acuteness of vision and no asthenopic symptoms glasses need not be prescribed for him.
When distant vision is imperfect, and asthenopic symptoms are present, it is necessary to prescribe glasses which represent the amount of manifest trouble, either for constant use or for near work. In some instances, it may be necessary to correct the manifest and part of the latent if the latter exists. As a rule, if hyperopia is associated with exophoria it is best to prescribe as weak a convex glass as possible, whereas if esophoria is present, the strongest. In spasm of the accommodation it is advisable to put on nearly the entire correction while the eye is under the influence of the cycloplegic, and later glasses which correct all the manifest and as much of the latent as is tolerated.
Many cases of convergent strabismus in children are cured by prescribing appropriate glasses. The degree of hyperopia can be determined by the direct examination with the ophthalmoscope or skiascopy if the child is too young to know its letters.
Myopia.-In this form of ametropia parallel rays of light are brought to a focus in front of the retina, therefore the latter is situated beyond the principal focus.
The focus of the rays a b (Fig.15) is at f where they cross each other, and on arriving at the retina from the diffusion circle c d.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.