200 SPECIAL PHYSIOL GOT 



X. THE RANGE OF ACCOMMODATION. 



The amount of refractive change produced by the eye in adjusting 

 for its punctum proximum after it has been at rest i. e., after it 

 has been adjusted for its punctum remotum is termed the range of 

 accommodation. In a previous chapter the punctum proximum and 

 punctum remotum were determined. It was reserved for this place 

 to express the position. of these limits of accommodation in terms 

 of dioptres, and thus most readily determine and definitely express 

 the range in simple dioptres. The relation of this to what has just 

 preceded will be evident. 



Let r equal the punctum remotum expressed in dioptres e. g., if 

 the punctum remotum is located at infinity that would represent 

 zero dioptres (r=0); if the punctum remotum is located J m. distant 

 from the eye r would equal 2 D. 



Let p equal the punctum proximum expressed in dioptres e. g., 

 if the punctum proximum is located at 10 cm. (y^ m.) p =10 D. 



Let a equal the range of accommodation in dioptres; then a=p r. 



To apply this formula to the above example we have 

 a 10 D 2D 8D. 



1. Experiments and Observations. (1) Determine the range of 

 accommodation for each member of the class. 



(a) Determine the punctum remotum and punctum proximum. 



(6) Record these quantities in metres. 



(c) Substitute these values in formula (5), expressing the distances 

 in the corresponding dioptres i. e., using the reciprocals of the 

 distances. 



(2) Range of Accommodation in Myopia, (a) Is r positive or 

 negative in myopia? 



(6) Is a always less than p, or may it sometimes be greater? 

 (c) What is the average range of accommodation of the myopes 

 of the class? 



(3) Range of Accommodation for Emmetropia. (a) What is the 

 value of r in emmetropia? 



(6) What is the relative value of a and p in this class of cases ? 



(c) What proportion of emmetropes in the class? 



(d) Have they all the same range of accommodation? 



(e) Can any probable cause be assigned for any variations which 

 may be found? 



(/) How does the average range for emmetropes compare with 

 the average range for myopes? 



(4) Range of Accommodation for Hyperopia. (a) If the punctum 

 remotum is "beyond infinity" (!), that is equivalent to saying that the 

 eye when at rest does not focus parallel lines (from infinity) upon the 

 retina, but the lines must be more than parallel i. e., from beyond 



