1466 



VISION. 



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 headaeh ; 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 witli 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 failed, 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 

 versa, 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 efforts 

 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- 



* See Practical Remarks on Near Sight, &c. 

 pp. 90 95, 



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. Stoopmg 

 over work should be avoided as one great 

 cause of congestion of the eyes; for which 

 reason a high desk is useful. Whenever the 

 e} es feel latigued, a few minutes' rest and 

 bathing with cold water will be refreshing 

 and beneficial. There are two descriptions of 

 lenses jin common use for spectacles, the dou- 

 ble concave for short sight, the double convex 

 for long or aged sight ; plano-convexes and 

 piano-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, but 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 

 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. 



Frames, of whatever material they may be 

 formed (and blue or bronze steel is the best), 

 should possess the following qualifications. 

 The rims should accurately n't the form of 

 the lenses, and be sufficiently strong to re- 

 tain them ;. the connecting arch or bridge 



