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 be 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 be 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 be 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. 



554. 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 lines 

 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, 



