526 



APPENDIX 



In the three following experiments the 10 D. lens is to be left inside the 

 eye close to the cornea, representing the lens of the eye. 



Expt 99. Myopia and its Correction. (Apparatus: +10 D.lens, 2 

 D. lens, round diaphragm, L-diaphragm, black screen. (A) Myopia. 

 Let a pencil of parallel rays enter the eye. Place the black screen at 



FIG. 295 



0' 



Diagram of refraction by cylindrical lenses. The rays in the vertical plane are not refracted, 

 because the sides of the lens in that plane are parallel. In the horizontal plane the rays are 

 converged by the cylindrical surface of the denser medium. The cross P P f would therefore be 

 falsely focussed as merely a vertical line O O f . (F. R. Ireson.) 



FIG. 296 



Normal vision, or emmetropia, shows rays from a point (e. g., P), focussing exactly on the retina 

 in a point (O). The dotted line in front of the lens shows the latter's bulging anteriorly in 

 accommodation. 



the focus of the eye-lens representing normal vision. Move the screen 

 back from the focus-place 2.5 cm. This is now the usual condition of 

 a low degree of myopia or short-sight, a too great depth of the eye-ball. 

 (B) Correction of Myopia. Place the 2 D. lens outside the eye but 

 close to the cornea. The focus is now carried backward to the abnor- 



