284 OUTPOSTS OF THE INTELLIGENCE SERVICE 



This is a " defect " of the cornea and is due to the fact that the 

 surface is not uniformly part of a sphere. For example, the top 

 of an egg is spherical, but a portion of the shell removed from the 

 side would correspond in greater or less degree to the normal 

 astigmatic cornea. Astigmatism is termed rvgular when the 

 different meridians which cut the anterior surface present a gradual 

 change in passing from one meridian to the other, the curvatures 

 that are most different being at right angles to one another. The 

 eye has, therefore, two anterior focal points, the one corresponding 

 to the meridian of greater curvature nearer the eye than the one 

 corresponding to the lesser curvature. The emmetropic eye has a 

 certain amount of radial astigmatism, which is neutralised to a 

 considerable extent by a positive axial astigmatism, i.e. a circular 

 object appears as if compounded of two almost circular ovals in 

 the same plane with the same " centre," but at right angles to one 

 another. Such an astigmatism does not require correction by 

 lenses. 



Defects of the abnormal eye as an optical instrument. 



In the normal or emmetropic eye, parallel rays, which in 

 practice are those coming from a distance greater than 6 metres, 

 are focussed in the resting state. Within 6 metres accommodation 

 begins, and it increases as the object approaches, reaching its 

 maximum at about 10 or 15 cm. This is called the near point 

 of vision, and the point at which accommodation begins is called 

 the far point. 



The eyes of a considerable proportion of persons do not produce 

 sharply focussed pictures within normal limits. The defect 

 consists most commonly in the screen being (i ) too far from the 

 lens, or (ii.) too near the lens. In the first case the picture in the 

 resting eye falls in front of the retina. This is the condition of 

 Myopia or short sight. Divergent rays from near objects are 

 focussed with little or no accommodation, and images are sharply 

 defined when objects are well within the normal near point. 

 But parallel rays cannot be focussed unless they are artificially 

 rendered divergent before entering the eye. This is effected by 

 the use of concave lenses. In the second case, parallel rays are 

 focussed behind the retina in the resting eye. This is Hyper- 

 metropia or long sight. Such rays can be converged and brought 

 to a focus by using accommodation, but the near point is reached 

 while the object is still comparatively distant. To enable the eye 

 to focus objects near at hand we strengthen its converging powers 

 by interposing a convex lens. 



Presbyojjia is the result of a gradual diminution of the power 



