332 Observations relateve to. 
deepest of these than by the other, yet on its being first 
used, the deepest number always occasions an uneasy sen- 
sation, as if the eye was strained. If, therefore, the glass 
that is most concave be at first employed, the eye, in a Jit- 
tle time, will be accommodated to it, and then a glass one 
number deeper may be used with similar advantage to the 
sight; and if the wish for enjoying the most perfect vision 
be indulged, this glass may soon be changed for one that is 
a number still deeper, and so in succession, unti] at length 
it will be difficult to obtain a glass sufficiently concave to 
afford the assistance that the eye requires *. 
Although near-sightedness is in gencral gradual in its 
progress, instances occasionally occur of its existence, in a 
considerable degree, even in children; in whom it is some- 
times discovered almost as soon as they begin to take notice 
of the objects around them. This may be occasioned by 
some degree of opacity in the transparent parts of the eye; 
but such a cause of near-sightedness is easily discovered by 
an examination, and is quite different from that state of the 
eye to which the term myopia, or near-sighteduess, is 
usually applied; by which is simply meant, too great a 
convexity either in the cornea or in the crystalline, in pro- 
portion to the distance of these parts from the retina. In 
such cases of extreme near-sightedness in children, it is 
sometimes necessary to deviate from a rule, which in slighter 
cases I always follow, of discouraging the ase of spectacles 5 
since, without their assistance, it would be impossible for 
them to prosecute their learning with ease or convenience. 
Extreme near-sightedness is sometimes occasioned by an 
evident change in the spherical figure of the cornea, and its 
assumption of a conical shape. This morbid state of the 
cornea is not only productive of near-sightedness, but, 
when the projection is considerable, vision 1s so much con- 
fused, that it affurds little or no service, and cannot be 
amended by any glass. The cornea, in most of these cases, 
is preternaturally thin, and not unfrequently it is accom- 
panied with symptoms of general debility; under which 
last circumstance chalybeate medicines, and bracing appli- 
sevice to the eye, have been found to afford considerable 
enefit, 
* T have observed, that most of the near-sighted persons, with whom I 
have had an opportunity of conversing, have had the right eye more near- 
sighted than the left; and I think it not improbable, that this difference be- 
tween the two eyes has been occasioned by the habit of using a single con- 
cave hand-glass; which, being most commonly applied to the right eye, 
contributes, agreeably to the remark above mentioned, to render this eye 
more near-sighted than the other. 
Near- 
