240 LABORATORY GUIDE IN PHYSIOLOGY. 



(3) Range of accommodation for emmetropia. 

 (ci) What is the value of R in emmetropia ? 

 (J)} What is the value of r in emmetropia? 



(c} What is the relative value of a and p in this class 



of cases ? 



(d} What proportion of emmetropes in the class? 

 (e} Have they all the same range of accommodation ? 

 (/) Can any probable cause be assigned for any varia- 



tions which may be found ? 

 (g) How does the average range for emmetropes com- 



pare 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 at rest does 

 not focus parallel lines (from infinity) upon the 

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

 from beyond infinity; or, better, convergent; but if 

 they are convergent they would meet behind the 

 cornea. The p. r. for hyperopes is then nega- 

 tive in direction and is equal to the distance, 

 behind the cornea, at which the convergent lines 

 would meet if prolonged. It follows that -^- is in 



It 



the case of hyperopes negative. Our formula (3) 

 would then take the form: 



Therefore, formula (5) becomes (5') a = p -f- r. 

 Now, in determining r one may use a convex lens 

 of such a strength as to give the rays the requisite 

 convergence. The value of the lens in dioptres is, 

 of course, the value of r. In the hyperope a is 

 always greater than p. As the determination of the 

 punctum remotum of the hyperopic eye is a matter 



