SPECTACLES :—SENSITIVE SPOT OF RETINA. 429 
influence of using spectacles of too high a power, soon mani¬ 
fests itself in the strained feeling which the eyes experience 
for some time ; but this feeling at last subsides, in conse¬ 
quence of the eye having adapted itself to the glasses, and 
having thus undergone a change which it might otherwise 
take years to produce. In this manner the eyes of a person 
at sixty may he brought to the state which, under more 
careful management, might have been deferred ten or fifteen 
years longer.—Similar remarks apply to the use of concave 
lenses by short-sighted persons. They should never he em¬ 
ployed of a higher power than is requisite to see objects with 
distinctness, when at a moderate distance ; and on no account 
should any glasses he used that diminish their apparent size. 
As age advances, the eyes of short-sighted persons usually 
become more flattened, and are then able to adapt themselves 
to objects at a variety of distances ; so that persons who have 
been short-sighted when young, are not unfrequently able to 
see distinctly at an advanced age, without the assistance of 
convex glasses. 
55i. The power of receiving and transmitting visual im¬ 
pressions is by no means uniform over the whole retina. In 
the whole field of vision which at any time lies before the 
eye, we only see with perfect distinctness that part to which 
its axis (namely, that diameter of the sphere which passes 
through the centre of the pupil) is directed, and of which the 
image, therefore, is formed upon “the yellow spot” (§ 535) 
which lies at the posterior pole of the axis. Nevertheless we 
have a sufficiently distinct perception of the remainder of the 
field, to enable us to judge of the general relations of its 
objects to each other and to those which we distinctly see : 
thus, whilst reading or writing, we can only recognise letters 
and words at any one moment within a spot which a sixpence 
or a shilling would cover, but we may distinguish the hues 
over the whole area of the page, and can plainly see the 
position of the book or paper upon the desk or table, together 
■with the position of this in the apartment. In the act of 
reading or writing, as in surveying the different parts of a 
landscape or a picture, or in examining any solid object that 
is brought under our notice, we direct the axis of the eye 
successively to one point after another, until we have satisfied 
ourselves that we have gained a distinct view of every part, 
