VISION. 239 



Let A equal the range of accommodation; Donders 

 expressed the range of accommodation thus: 



Take an example: Let the punctum remotum be 50 

 cm. (I/? m.) from the eye, the punctum proximum 

 10 cm. (y 1 ^ m.); substitute the distances expressed in 

 meters in formula (4) and one obtains A = \ m. The 

 range of accommodation, i.e., the accommodative power 

 of the eye is equal to a lens of \ m. focal distance. But 

 a lens of | m. focal distance is an 8 dioptre lens. A much 

 simpler way of arriving at this result is to use: 



r (= 1) and p ( =-p). If we let a = -i, then we may 

 write: 



(5) a = p r. 



To apply this formula to the above example we have 

 a 10 D. 2 D. = 8 D. 

 7. Experiments and Observations. 



(1) Determine the range of accommodation for each 

 member of the class. 



(0) Determine punctum remotum and punctum proxi- 

 mum. 



(b) Record these quantities in meters. 



(V) 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 ? 



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



greater ? 

 (<:*) What is the average range of accommodation of 



the myopes of the class ? 



