Page:The American Cyclopædia (1879) Volume XV.djvu/249

 SPECTACLES 237 myopia or near-sightedness, and may arise either from excessive convexity of the lens system of the eye, or from an undue depth of the organ from before backward. The latter origin is by far the more common, and is gen- erally the result of a disease of the tunics of the eye at their back part, whereby being weakened, they bulge out backward. How- ever produced, the correction of myopia is the same. The difficulty being that the refractive power of the eye is too great for the distance of the retina, the obvious remedy is to weaken the former, and this is done by wearing a con- cave glass. (See OPTICS.) But there are many physiological reasons why full correction of the defect is often improper or useless, which cannot be discussed here. In any but very moderate degrees of myopia glasses should be worn only under competent advice; and in any case great injury may be produced by the use of too strong glasses. The opposite con- dition to myopia is also very common, that is, where the refractive power of the eye lenses is disproportionately weak, so that the rays from distant objects come to a focus behind the ret- ina, in which case vision of objects both far and near is indistinct. This constitutes the condition known as hypermetropia^ and, as in myopia, the deviation from the normal condi- tion may be either in the refractive power or in the depth of the eye. Thus a tolerably com- mon congenital malformation is an undue shal- lowness of the eyeball. Such an eye is neces- sarily hypermetropic. A normal eye may also become hypermetropic in old age, and in all cases where the crystalline lens of the eye is wanting, as after removal for cataract (see CAT- AEAOT, and EYE), a high degree of hypermetro- pia necessarily results. The fault being that the refractive power of the eye is dispropor- tionately weak to suit the distance off of the retina, the necessary additional power can be supplied by a convex glass worn before the eye. But in the case of the more common con- genital hypermetropia from deficient depth of the eyeball, so many other considerations than the mere optical one affect the matter of cor- recting the defect by glasses, that perfect neu- tralization is often unadvisable or unnecessary. For the eye has itself the power of increasing the refraction of its lens within a certain range, to provide for the focalizing upon near objects. (See VISION, section on accommodation of the eye.) Hence the organ can itself compen- sate for a certain amount of hypermetropia, and may thus be able to do without glasses, or with weaker ones than those required to neu- tralize the defect completely. The third opti- cal error remediable by glasses is a certain want of symmetry in the curve of the cornea, where there are two opposite meridians of unequal curvature. This condition is called astigmatism, and is generally a congenital mal- formation. The consequence of it is that the retinal image, whether of far or near objects, is never sharp. For the correction of this defect a glass is worn having a cylindrical curve equal to the difference in curvature between the two dissimilar meridians, the axis of the cylindrical surface being carefully adjusted so as to be at right angles to the direction of the meridian to be corrected. The nature of the curve, i. e. t whether convex or concave, will depend on whether the refractive power of the meridian to be corrected requires to be strength- ened or lessened. As it is obvious that this irregularity of corneal curvature may coexist with a general myopia or hypermetropia, com- pound glasses are often required, having on one face a cylindrical curve to neutralize the astigmatism, and on the other the proper spherical curve required for the other defect. The second general way in which glasses operate to assist sight is, as already said, by compensa- ting for failure of certain muscles concerned in the use of the eyes to fulfil their function. The most common of these troubles is want of power to focalize the eye upon near objects. This faculty resides in a little muscle within the eye, by the action of which the convexity, and thus the refractive power, of the crystal- line lens is temporarily increased. But the substance of the crystalline lens steadily grows harder, and thus less and less compressible, so that the same amount of muscular action comes to produce less and less effect. The con- sequence is that during adult life the focalizing power upon near objects steadily diminishes, and hence the nearest point of distinct vision gets further and further from the eye, until at about the age of 47 it has receded beyond the distance for convenient use of the hands. Reading, writing, sewing, or any manual work requiring sharp vision of small objects at the customary distance, then become impossible without artificial compensation for the failure of focalizing power. This condition, which is natural to all eyes, is called presbyopia or old- sightedness, and the compensation is very sim- ple. The difficulty being an inability on the part of the eye itself to increase temporarily its refractive power, the needed addition is artificially supplied by a convex glass, which is worn of course only when near objects are to be viewed. As the focalizing power keeps on diminishing until in old age it is wholly lost, the strength of the glasses must be steadily in- creased. As soon as presbyopia begins to show itself, the proper weak glass should be prompt- ly assumed, as only injury to the eyes, or at least useless inconvenience, can result from a fruitless struggle to do without this aid. In all cases the weakest glass with which ordinary type can be clearly and comfortably seen at the usual distance is the proper one to wear. With normal eyes, individuals of the same age take very nearly the same strength of glass ; but, for obvious reasons, in myopes the glass will be weaker in proportion to the degree of the optical defect, while in hypermetropes it will be correspondingly stronger. This same inability to focalize upon near objects may also