528 



APPENDIX 



eye, except that more often the lens is too convex rather than the eye-ball 

 too shallow. 



(J5) Correction of Hyperopia. Place the +2 D. (thin double convex 

 lens) outside the cornea but close to it. The focus is now brought for- 

 ward to the abnormally situated screen (retina), the rays being more 

 converged. 



FIG. 299 



The visual defects coming from astigmatism. A figure like / tends to appear either like 

 or like ///, because of the unequal refraction in the respective axes of the eyes. (Imbert.) 



FIG. 300 



Astigmatism and its relief. In (regular) astigmatism the refractive media of the eye (espe- 

 cially the cornea) are more convex in one axis than in another. Thus, in the diagram the rays- 

 in the vertical plane are not converged at all and the image (O O') is very indistinct in that 

 axis, being a line instead of a point. This defect is corrected by cylindrical spectacle-lenses with 

 the greater refraction in the axis in which it is lacking in the eye-lens, and equal to that defect 

 in degree. In the diagram the spectacle-lens c c refracts only in the axis in which the eye- 

 lens refracts none. 



Expt. 101. Astigmatism. (Apparatus: 10 + D. lens, two + 10 D. 

 cylindrical lenses, the three diaphragms, screen.) (A) Astigmatism. 

 Place the cylindrical lens (curvature lateral) in the other side of the frame 

 in which the -f 10 D. (or eye) lens is. Let parallel rays pass into the 

 eye through this "astigmatic lens." Place the screen exactly in the focus 

 of the lens. The image of the circular aperture in the diaphragm is 

 elongated vertically. 



(E) Correction of Astigmatism. Now place before the cornea of the 

 eye the other cylindrical lens with its curvature vertical. The image 

 becomes round. Turn the correcting lens 90 degrees, and the elongation 

 of the image is increased. (See the diagram.) 



