238 SPECTACLES occur at any ago from inherent weakness or paralysis through disease of the muscle con- cerned, and in such case, as in true presbyopia, a convex glass will be 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 plano- convex lens. Alessandro di Spina, a monk of Pisa who died in 1318, is generally accredited with having made public the use of spectacles, which were apparently invented some time be- tween 1280 and 1811. Another form of mus- cular 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 tem- porary confusion of sight. Here, if the insuf- ficiency 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 con- dition 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 oper- ation 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, hyperme- 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 ex- pressing 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 " -iV' 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 dis- tortion 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 com- monly 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 SPECTRUM heavier and far more expensive than glass. The claims for its " preserving the sight " are fanciful, and many of the spectacles sold as pebbles are not such at all. It is always im- portant that the lenses should be 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 dis- tortion 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 gen- eral the word "spectacles" is now used to designate a frame held in place by bows reach- ing 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 su- perseded by steel and gold. Tortoise shell is light, but easily broken ; it is only used in eye- glass frames. The frame, whether spectacle or eye glass, should be selected to suit the in- dividual 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 in- vented 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. Spec- tacles/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. SPECTRUM (Lat., an image), the name given to the image or colored band formed by the decomposition of a beam of light into its ele- mentary colors. Thus, when a beam of sun- light enters a dark room through a narrow slit, passes through a triangular glass prism, and then falls upon a screen, we may observe
