Spectacles, contrivances worn to assist sight or to protect the eyes from injury. 1. Spectacles to assist Sight. These may operate in two general ways: first, by correction of some optical defects to which the eyes are liable; and secondly, by compensation for functional insufficiency on the part of certain muscles concerned in the exercise of sight. The eye is a camera, where a system of lenses throws an image upon a screen, represented by the retina. For perfect sharpness of this image, the curves of the lenses must be symmetrical, and the refractive power of the system exactly adjusted to the distance of the retina. In the normal or "emmetropic" eye these conditions obtain, the adjustment being such that when the eye is at rest the rays from distant objects come to an exact focus upon the retina. But every possible deviation from these conditions is found. First, there may be a disproportion between the refractive power of the eye and the distance of the retina. If the refractive power is proportionately too great, the rays from distant objects will come to a focus a certain distance in front of the retina.
This constitutes the condition called 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 generally the result of a disease of the tunics of the eye at their back part, whereby being weakened, they bulge out backward. However 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 concave 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 condition 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 retina, in which case vision of objects both far and near is indistinct.
This constitutes the condition known as hypermetropic!,, and, as in myopia, the deviation from the normal condition may be either in the refractive power or in the depth of the eye. Thus a tolerably common congenital malformation is an undue shallowness of the eyeball. Such an eye is necessarily 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 Cataract, and Eye), a high degree of hypermetro-pia necessarily results. The fault being that the refractive power of the eye is disproportionately 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 congenital hypermetropia from deficient depth of the eyeball, so many other considerations than the mere optical one affect the matter of correcting the defect by glasses, that perfect neutralization 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 Visiox, section on accommodation of the eye.) Hence the organ can itself compensate for a certain amount of hypermetropia, and may thus be able to do without glasses, or with weaker ones than those required to neutralize the defect completely. The third optical 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 malformation. 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., whether convex or concave, will depend on whether the refractive power of the meridian to be corrected requires to be strengthened or lessened.
As it is obvious that this irregularity of corneal curvature may coexist with a general myopia or hypermetropia, compound 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 compensating 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 crystalline 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 consequence 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 simple. 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 increased. As soon as presbyopia begins to show itself, the proper weak glass should be promptly 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 occur at any age from inherent weakness or paralysis through disease of the muscle concerned, and in such case, as in true presbyopia, a convex glass will he needed for near work. It was probably to compensate for presbyopia by convex glasses that spectacles were first invented. Roger Bacon first pointed out the benefit to old men and " to those that have weak eyes " of viewing letters through a planoconvex lens. Alessandro di Spina, a monk of Pisa who died in 1313, is generally accredited with having made public the use of spectacles, which were apparently invented some time between 1280 and 1311. - Another form of muscular insufficiency that can be compensated by optical means is where some of the muscles moving the eyeball in its socket are unduly weak.
In such case the holding of the two eyes fixed upon the same point is attended by a feeling of straining or actual pain, and upon prolonged effort the overtaxed muscle may suddenly relax, producing immediately a temporary confusion of sight. Here, if the insufficiency be but slight, the wearing of a weak plain prism, properly adjusted, compensates for the defect; for even while the eyes are allowed to keep the faulty relative position enforced by the muscular weakness, the rays coming from the object desired to be seen can, by means of refraction through a prism, be made to enter both eyes in the same direction, the only condition necessary for binocular single vision. But this mode of compensation will only do in slight degrees of muscular insufficiency; in the higher grades a radical cure by a surgical operation is necessary. The strength and position of the prisms will of course be determined by the degree and seat of the muscular weakness; and if, as is often the case, the affection in question is associated with myopia, hypermc-tropia, or astigmatism, a compound glass may be needed, where one or both faces of the prism bear the necessary curves to correct the optical defect. - The designation of the strength of glasses is nowadays by the fraction expressing the refractive power of the lens in terms of inches, the words "positive" and "negative" or the signs + and - indicating respectively a convex or a concave glass; thus " - 1/10" means a concave lens of 10 in. focal length.
Glasses are commonly ground with an equal curve on both faces, but a meniscus for a positive and a concavo-convex for a negative lens may also be used (see Optics), in which case the spectacles are called periscopic. The advantage of this form is, that there is less distortion of objects seen through the edges of the lens; but the disadvantages are, that the glasses are heavier than those of the ordinary style, and give more reflection from their back surface. The material for spectacles is commonly glass, but a variety of rock crystal called "Brazilian pebble" is also used. The latter substance is less apt to scratch or to become dimmed by deposit of moisture on being brought from a cold to a warm temperature, but it is heavier and far more expensive than glass. The claims for its "preserving the sight " are fanciful, and many of the spectacles sold as pehbles are not such at all. It is always important that the lenses should he of first class, the substance without flaw, and the grinding accurate. A convenient test is to hold the glass some distance from the eye, and then, moving it from side to side and to and fro, note if there be any apparent flickering or distortion of objects seen through it. If there be, the glass is worthless.
In style of frames, as is well known, there is great variety. In general the word "spectacles" is now used to designate a frame held in place by bows reaching behind the ears, and " eye glasses" one held in the hand or made to clasp the nose. The spectacle frame is the best where the glass has to be continuously worn, as in myopia, as the lenses can be more accurately centred and made to set perpendicular to the line of sight. For temporary use, as for reading glasses in presbyopia, good eye glasses, selected so as to be well centred to suit the distance of the eyes apart, are convenient and unobjectionable. The material for the frames is various; silver was formerly in general use, but has been superseded by steel and gold. Tortoise shell is light, but easily broken; it is only used in eyeglass frames. The frame, whether spectacle or eye glass, should be selected to suit the individual conformation of face and the purpose for which the glass is wanted, so that the line of sight shall be through the centre of the glass and perpendicular to its surface. Hence glasses for distant vision, as in myopia, should be set high and vertical, while for near work only, as in presbyopia, they should be lower and inclined.
A style of spectacles was invented by Franklin for special cases where a different glass is needed for far and near vision respectively, in which the glass is bisected horizontally, the two segments being of the different curvatures required, the upper for the far and the lower for the near. 2. Spectacles/or Protection. To shield sensitive eyes from excess of light, colored glasses, either with plane surfaces or of a watch-glass form, are used. The latter give most protection, as they cut off the side light more perfectly. Still better are goggles with wings at the sides. Shades of blue and "London smoke" neutral tint are the best colors. For protection against the glare of snow or white sand, an opaque disk pierced with a narrow horizontal slit is very efficient. A spectacle frame set with wire gauze or plain glass is sometimes worn by workmen as a protection against bits of flying stone or steel.