VISION 391 



which lines the eye behind the retina. By the joint action of 

 these two sets of plain muscle-fibres the suspensory ligament 

 is slackened, and the extremely elastic lens, previously com- 

 pressed, bulges forwards. The radius of curvature of its 

 anterior surface changes from 10-3 millimetres for distance 

 to 6 millimetres for vision at the " near point." It was 

 stated, in connection with the development of the lens (p. 374), 

 that the cells of the posterior half of the hollow sphere out of 

 which it is formed grow forwards into extremely long fibres, 

 which traverse its whole thickness. These fibres are bent like 

 the segments of a carriage-spring. Their anterior ends rest 

 against the flattened ligament of the lens ; the vitreous humour, 



B 



Fia. 32. THE FORMATION OP AN IMAGE BY THE REFRACTING MEDIA OF THE EYE. 



x, The common centre of curvature (nodal point of the several media). Rays which pass 

 through this point are not deflected, y, The principal focus of the system. All rays which 

 are parallel to the optic axis converge to this point. The image of the point A is formed at 

 a, the spot at which a ray parallel with the optic axis meets an unbent ray the image of B 

 at 6. 



which is always under tension, compresses their posterior 

 ends. When removed from the eye, the lens becomes rounder 

 than it is in situ, even when accommodated for near objects. 

 But in later life it grows stiff. It ceases to bulge forwards 

 when its ligament is slackened. Hence it becomes necessary 

 to aid the presbyopic eye with convex glasses when it is used 

 for near objects, although for distant vision it remains as 

 effective as ever. If the ciliary muscle is constantly and 

 completely relieved of the labour of accommodation, it grows 

 lazy, or rather wastes from want of use. A person who relies 

 on spectacles loses his power of accommodation ; but ophthal- 

 mologists agree that self-focussing, if it give rise to a sensation 

 of strain, is bad for the eyes. In myopic persons the eyeball 

 is too deep ; objects are focussed in front of the retina. In 

 hypermetropia (" long sight ") the eyeball is too shallow ; 



