SPECTACLES :—SENSITIVE SPOT OF RETINA. 429° 
nfluence of using spectacles of too high a power, soon mani- 
is itself in the strained feeling which the eyes experience 
or some time ; but this feeling at last subsides, in conse- 
ence of the eye having adapted itself to the glasses, and 
having thus undergone a change which it drach otherwise 
ke years to produce. In this manner the eyes of a person 
{ sixty may be brought to the state which, under more 
areful management, might have been deferred ten or fifteen 
ee. —Similar remarks apply to the use of concave 
es by short-sighted persons. They should never be em- 
oyed of a higher power than is requisite to see objects with 
lis finctness, when at a moderate distance ; and on no account 
jould any glasses be used that diminish their apparent size. 
3 age advances, the eyes of short-sighted persons usually 
scome more flattened, and are then able to adapt themselves 
‘objects at a variety of distances ; so that persons who have 
een short-sighted when young, are not unfrequently able to 
@ distinctly at an advanced age, without the assistance of 
mnvex glasses. 
554. The power of receiving and transmitting visual im- 
fessions is by no means uniform over the whole retina. In 
whole field of vision which at any time lies before the 
fe, we only see with perfect distinctness that part to which 
axis (namely, that diameter of the sphere which 
ough the centre of the pupil) is directed, and of which the 
age, therefore, is formed upon “the yellow spot” (§ 535) 
lich lies at the posterior pole of the axis. Nevertheless we 
ave a sufficiently distinct perception of the remainder of the 
id, to enable us to judge of the general relations of its 
jects to each other and to those which we distinctly see : 
us whilst reading or writing, we can only recognise letiers 
d words at any one moment within a spot which a sixpence 
ba shilling would cover, but we may distinguish the lines 
yer the whole area of the page, and can plainly see the 
sition of the book or paper upon the desk or table, together 
th the position of this in the apartment. In the act of 
ding or writing, as in surveying the different parts of a 
dscape or a picture, or in examining any solid object that 
meseht under our notice, we direct the axis of the eye 
sively to one point after another, until we have satisfied 
elves that we have gained a distinct view of every part, 
