428 LONG AND SHORT SIGHT: SPECTACLES. 



distinctly at the same time ; but that, on whichever of the 

 two objects we fix our eyes, we shall see it clearly, whilst the 

 other will become indistinct. Recent observations have con- 

 clusively shown that this adjustment is brought about by an 

 alteration in the curvature of the crystalline lens ; its con- 

 vexity being increased when a near object is looked at, so as 

 to act more powerfully in bringing its diverging rays into 

 convergence ; and being diminished when the gaze is turned 

 towards a distant object. 



552. In advanced life, however, from the diminution in 

 the convexity of the cornea and in the refracting power of the 

 humors, the eye can no longer accommodate itself to near 

 objects ; their rays not being brought to a focus by the time 

 that they reach the retina. But as it is still able to see dis- 

 tant objects clearly, it is said to be long-sighted. By the use 

 of a convex glass, however, adapted to supply that additional 

 amount of refraction which is required, near objects may be 

 distinctly seen. A contrary state of the eye not unfrequently 

 exists, in which the cornea is too convex, and the refracting 

 power of the humors is too high ; from which it happens 

 that the rays proceeding from distant objects are brought to a 

 focus too soon, so as to cross each other before they reach the 

 retina. But as such an eye can form a very distinct picture 

 of a near object, it is said to be near-sighted. This imper- 

 fection is remedied by interposing a concave lens between the 

 object and the eye, by which its refracting power is dimi- 

 nished to the necessary degree. 



553. In the choice of spectacles or eye-glasses for these 

 purposes, particular care should be taken that they are not too 

 powerful ; since great mischief is frequently done to the eye, by 

 the employment of lenses of too great a curvature. A person 

 who in youth and middle age has enjoyed good sight, very 

 commonly finds it necessary to employ spectacles for assist- 

 ance in reading and writing, as his age advances towards fifty 

 years; and he should be very cautious, when first availing 

 himself of their assistance, to employ those of the longest 

 focus. As his age advances, it will be necessary to substitute 

 more powerful lenses for these ; but this should be done very 

 gradually; and in no instance should a glass be employed 

 that produces any apparent enlargement in the object, its 

 proper use being only to render the object distinct. The evil 



