ERRORS OF REFRACTION AND ACCOMMODATION. 153 



of rays, either because they have already passed, or because they have not yet 

 reached their focus. The unlike meridians are not necessarily arranged in this 

 way, the vertical is not always that of greatest curvature, although this is the most 

 common form, nor are the meridians which differ most always at right angles to 

 one another. When they are the astigmatism is a REGULAR one; when the angle 

 between is not a right angle it is an IRREGULAR astigmatism. Regular astig- 

 matism is easy to correct by glasses ground so as either to converge the rays which 

 are to pass through the meridian of least refractive power or to render more 

 divergent those directed towards the meridian of greater refractive power. 

 Irregular astigmatism is more difficult to correct. In some eyes the astigmatism 

 is of neither of these types but consists in irregularities in the curvature of the 

 same meridian, a condition which cannot be made right with glasses. Even in a 

 normal eye there is something of this irregularity. The image of a star is not a 

 single point, as it would be if the refracting surfaces were of even curvature 

 throughout, but it has an irregular shape which is different for each individual, 

 because of the slight defects in the curves of the various surfaces. 



Accommodation. AVe have seen how parallel rays from a dis- 

 tant object are brought to a focus on the retina of a normal eye. 

 When however the gaze is directed to an object close at hand the 

 eye receives rays which diverge from one another by a considerable 

 angle. If no change takes place in the refractive power of either 

 cornea or lens these rays tend after refraction towards a focus 

 lying behind the retina. In consequence when they are intercepted 

 by the retina they form a blur and not a sharp point. To make the 

 image of the near object a clear one either the distance between 

 retina and lens must be made greater than it is for distant vision, 

 just as one increases the distance between lens and plate in a 

 camera when focussing for objects close at hand, or else the refrac- 

 tion by the eye media must be increased. The latter is the change 

 which is brought about in the mammalian eye by the act of accom- 

 modation. The curvature of the anterior surface of the lens is 

 made sharper and its refraction therefore greater. 



To understand the most generally accepted explanation of the way in which 

 this is done, one must have clearly in mind the anatomical relationship which the 

 lens bears to neighbouring structures in the eye (Fig. 43). It is held in its place 

 by the numerous fibres which together make up the suspensory ligament of the 

 lens. These are more or less radially arranged about the lens, being continuous 

 at their inner ends with the capsule near the margin, while the outer ends of the 

 threads are connected to the surface of the ciliary body near its free inner edge 

 throughout its entire circle. The arrangement is such that those fibres which 

 come from the posterior surface of the lens capsule go to the anterior surface of the 

 ciliary body and those which arise from the anterior surface of the lens are at- 



