OPTICS 



527 



mally situated screen (retina). For short-sightedness oculists prescribe 

 concave lenses. 



Expt. 100. Hyperopia and its Correction. (Apparatus: -f- 10 D. 

 lens, +2 D. lens, round and L-diaphragms, screen.) (A) Hyperopia 



FIG. 297 



Myopia and its relief. In myopia, owing usually in large part to a too great antero-posterior 

 depth of the eye, the rays focus before they reach the retina and an indefinite image results 

 (O O ). This common defect is corrected by concave, minus spectacle-lenses, these diverging 

 the rays before reaching the eye so that the rays focus farther back on the retina. 



FIG. 298 



Hyperopia and its relief. In hyperopia, owing usually to a too great flatness of the lens and 

 cornea, the rays coming from a point (P) are not converged sufficiently to meet a focus on the 

 retina, but would do so behind it were the latter transparent. This defect ("normal" about 

 fifty years of age) is corrected by convex spectacle lenses, these converging the rays so that they 

 focus on the retina. 



and Presbyopia. Let a pencil of parallel rays enter the eye. Place the 

 screen at the focus of the eye-lens as before, then move the screen for- 

 ward toward the lens 2.5 cm. This represents the abnormal hyperopic 



