REFRACTION IN THE EYE 483 



the range of accommodation, and = 



APE 

 becomes its numerical expression 1 



In myopia P may be 10 cm. and E 25 cm. 



= 4 D. The 



myopia is of 4 dioptres. -r=10D 4D 6D. 



A 



In this case Pis greater than A, 



In hypermetropia P may be 50 cm. and R neg- 



ative, - 25 cm. Then 1 = l -^= 2 D, and ~ = ^ 

 ou H Z5 



= 4 D. The hypermetropia is of 4 D. = 



JOk 



2 - (-4 ) = 2 + 4 = 6 D, which is the sum of 

 P and E. 



In emmetropia P may be 20 cm. and # in- 



finite. Then * = I = = 5 D. In 



1 Theoretically the auxiliary lens should be placed in the 

 eye and not in front of it, and its second nodal point should 

 coincide with the first nodal point of the eye. The placing of 

 the auxiliaVy lens in front of the cornea alters the position of 

 the cardinal points of the combined system, and also alters the 

 focal distance of the auxiliary lens. But the actual changes 

 in the lens during accommodation are nearly proportional to 



-r ~ 73 -, so that the formula serves practically for propor- 



A r A 



tional magnitudes, i. e., for comparing reciprocally the different 

 values of the range of accommodation under different circum- 

 stances. 



