222 THE EYE 



The eyes of a considerable proportion of persons do not produce 

 sharply focused 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 focused 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. 



Presbyopia is the result of a gradual diminution of the power 

 of accommodation through loss of elasticity of the lens as age 

 advances, the retina retaining its normal position relative to 

 the lens. 



Astigmatism is the variation of the radius of curvature of the 

 cornea in different planes. 



The Dioptre. 



The measurement of the refraction of the eye is made in terms 

 of lenses which have their focal lengths expressed in metres. 

 A lens of 1 metre focal length is a lens of 1 dioptre, of -5 metre, 

 2 dioptres, etc. Errors of refraction are thus measured by the 

 clinician, e.g. a hypermetropic patient who requires a lens of -3 

 metre focal length in order to render his eye emmetropic is said 

 to have 3 dioptres of hypermetropia. 



Movements of the Eyeball. 



The eyeball lies at the front of the bony orbit, a cone-shaped 

 canal with its apex directed backwards and pierced by the 

 optic foramen. The antero-posterior or visual axis of the eye, 

 i.e. the line passing through the centre of the cornea and the 

 fovea makes an angle of about 20 with the long axis of the orbit. 

 The movements of the eyeball are carried out by the extrinsic 

 muscles, which are six in number. Of these, four (called the 

 recti) originate in a common tendon surrounding the optic foramen, 



