Chap. XXIX.] RA NGE OF AcCOMMODA TION. 387 



The range of accommodation. For parallel 

 rays, then, the normal eye requires no adjustment. 

 Practically, rays falling on the eye for any distance 

 not less than sixty-five metres do not necessitate 

 accommodation. For any object within this distance, 

 however, increased convexity is necessary. At this 

 distance, and up to infinity, we have, therefore, the 

 puncium remotum of distinct vision. The nearer 

 within the limit the object coires, the more is the 

 accommodating power caJ'.eJ. '.rite play, the lens 

 becomes more and more convex. Bat ib is apparent 

 that there must be another Hmit. A point must be 

 reached beyond which any approach of the object to 

 the eye cannot be compensaved for by the lens. The 

 accommodation is strained to its uttermost ; and, if the 

 object comes nearer, its raps cannot be focussed on 

 the retina. This is the punctum proximum, and 

 normally is distant 1 2 centimetres from the eye. 



Between the two limits is the range of accommo- 

 dation of the eye for distance. 



The power of accommodation of an eye would be 

 measured by the converging power of a lens which 

 produced distinct vision of an object placed at the 

 punctuin proximum, without calling in the accom- 

 modation of the eye, a lens, that is, which would so 

 act on the rays diverging from the near point as to 

 give them the direction of rays coming from the far 

 point, a parallel direction, namely, for which accom- 

 modation is not required in the normal eye. The 

 focal length of such a lens is given by the formula 



_ 

 P - I = /' 



where / = the focal length of the lens, P - the 

 distance of the pimctum proximum (normally 12 

 centimetres), and R = that of the punctum remotum. 



