428 LONG AND SHORT SIGHT :—SPEOTACLES. 
distinctly at the same time; but that, on whichever of the 
two objects we fix our eyes, we shall see 7 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 additions 
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 pict 
of a near object, it is said to be near-sighted. ‘This impe 
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 the: 
purposes, particular care should be taken that they are not to 
powerful ; since great mischief is frequently done to the eye, by 
the employment of lenses of too great a curvature. A perso 
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 fifi 
years ; and he should be very cautious, when first availin| 
himself of their assistance, to employ those of the longesy 
focus, As his age advances, it will be necessary to substitu ie 
more powerful lenses for these ; but this should be done very 
gradually ; and in no instance should a glass be employe 
that produces any apparent enlargement in the object, if 
proper use being only to render the object distinct. The ev§ 
