INTRODUCTION TO PHYSIOLOGICAL OPTICS 431 



front of the window. This lens has a focal dis- 

 tance of two dioptres or one-half metre (see 

 page 435). 



The image will be clear again. The myopia 

 in this case is therefore 2 D. 



Hypermetropia. In the hypermetropic eye at 

 rest parallel rays and even those to a certain 

 degree convergent meet the retina before they 

 have come to a focus. The most frequent cause 

 of hyperinetropia is the abnormal shortness of the 

 antero-posterior diameter of the eye. The defect 

 can be remedied by placing a convex lens before 

 the eye. The entering rays are thereby rendered 

 convergent, or their convergence is increased. 

 The degree of the hypermetropia is measured 

 by the strength of the convex lens which, placed 

 before the eye, will so increase its convergent 

 power that parallel rays will come to a focus on 

 the retina. 



Place the screen 2.5 cm. nearer the lens than 

 the principal focus. 



The image will be blurred. The screen will 

 intersect the rays before they have converged 

 to the focal point. 



Hold the weak convex lens, marked + 2, in 

 front of the window. 



The image will be clear. The hypermetropia 

 in this case is therefore + 2 D. 



