The first indications of' preshyopia are always perceived in the evening, and it is im portant that they should be distinctly borne in mind. During the day, persons past the meridian of life, accustomed to t ead, write, or work can do so without inconvenience, but as the shades of evening draw in and lights are introduced, they find that they cannot fix their eyes upon their work as before, without fatigue; if they persist, this fatigue increases, and after a time amounts to headach ; but if they put on convex glasses of a low power, all discomfort vanishes, and their sight is at once restored, and they can pursue their occupa tions with perfect ease. If however they persist in abstaining from glasses, and by in creasing the light endeavour to improve their sight, the retina will be over stimulated, and in addition to presbyopia, they will acquire dull and imperfect vision of all objects, far as well as near.
There are persons whose sight, never par ticularly good, but who from easy circum stances have used it little, become alarmed at finding, about the age of fifty, that their eyes appear to have suddenly tilled, and they think they are becoming blind. Many such cases have fallen under our observation. The facts are simply these : presbyopia has advanced so slowly and gently, and the eyes have been used so little, that until the defect has become confirmed, the parties are uncon scious of its existence. Some accidental cir cumstance, as an attempt to read a news paper in the evening, reveals the imperfection : proper glasses are alone required to restore the vision.
Belladonna and atropine produce temporary presbyopia by suspending the power of ad justment, the eye being fixed at the focus for distant objects. Mydriasis also renders the individual more or less presbyopic. Presby opia may be converted into myopia, and vice verses, of which cases are related by various writers.* Dr. Sichel has justly pointed out that neu ralgia of the eye-ball is by no means infre quent in connection with presbyopia. We have seen several instances, and generally traced it to overstraining the eyes in effbrts to read or work without glasses. The pain is at first transient, but if the exciting cause be continued, it becomes more severe and per sistent, extending from the eye to the neigh bouring parts and not readily yielding to treatment. In such a case, rest to the eyes is all important in the first instance, and suitable glasses are indispensable.
The chief remedial measures for presbyopia are comprised in the early and judicious use of suitable glasses, but the means of prolonging natural sight and of staying the progress of presbyopia are as follows : Persons habitually engaged in minute work should ascertain by experiment the greatest distance at which they can clearly and with out effort see their work, and always endea vour to maintain that distance ; they should raise their eyes from time to time, and direct them to some object at the opposite end of the room to alter the focus: but when en gaged in lighter pursuits they may if agree able read at less than their working distance.
In reading or writing just that amount and quality of light is proper which thoroughly illuminates the object, and yet feels grateful and pleasant to the eyes. It is injurious to face the light ; the best position when reading is with the light rather behind and on one side; the eyes are thus protected from all heat and glare, while the object is fully illu minated. When this arrangement is incon venient, a screen or shade may be used with advantage. Reading by twilight and firelight is highly injurious to feeble eyes. Stooping over work should be avoided as one great cause of congestion of the eyes; for which reason a high desk is useful. 'Whenever the ey es feel fatigued, a few minutes' rest and bathing with cold water will be refreshing and beneficial. There are two descriptions of lenses fin common use for spectacles, the dou ble concave for short sight, the double convex for long or aged sight ; plano-convexes and plano-concaves are scarcely ever employed. It sometimes happens however that the cur vatures of the surfaces are unequal; for in stance, a ten-inch lens may be required, but the optician may not have what is technically called the "tool" of the proper curve, and therefore selects two tools, the numbers of which combined make ten—as one of seven and the other of three inches—and the lens produced by these, though with surfaces of very unequal curvature, answers the purpose perfectly. Periscopic glasses were invented by Dr. Wollaston, but are seldom employed, and there are others with attractive names but not deserving of particular notice.
A common prejudice exists in favour of pebbles, hut it is erroneous, for whilst on the one hand their chief merit consists in extreme hardness so that they are not easily scratched, that very hardness renders them difficult to cut and grind, many being broken in the process. They are expensive from this cir cumstance, and the difficulty of meeting with crystal of sufficent size which is pure and without flaw. Crown glass is now made of such excellent quality and so colourless that . lenses made, from it possess every qualifica tion that can be desired. Among the many singular things characteristic of the Chinese is their spectacles. Without knowing any thing of the theory of the convergence and divergence of light by means of lenses, both convex and concave lenses are used all over the empire, and of such a singular size and shape that there can be little doubt of their I being original inventions. They are made of rock crystal ground with the powder of corundum and are of immense size, being re tained in their position on the face by means of silken cords with weights attached, which are slung over the ears.