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APPENDIX 



for this lies in the fact that a double convex lens is essentially two prisms 

 placed base to base. 



(B) Spherical Aberration. Place in the clamp at the "cornea" a dia- 

 phragm with an aperture of 1 mm. This cuts off the spherical aberra- 

 tion by stopping the rays which cross within the pencil of rays. 



Expt. 96. Refraction by Convex Lenses. (Apparatus: 10-diopter 

 double convex lens in frame, round diaphragm, L-diaphragm, black 

 screen). (A) Let parallel rays enter the eye. Place the lens about 5 cm. 

 from the cornea. Place the screen 10 cm. from the lens and note the 

 focus, and that on either side of that one position the image is indistinct. 



FIG. 292 



Convergence of the rays in a beam of light is brought about by a convex lens, their point 

 of meeting being the focus F., and in case the pencil be oblique to the lens F' . 



Now replace the round diaphragm by the L-diaphragm and put the 

 screen in the focus. Note that the image of the L is inverted. A convex 

 lens converges parallel rays. (A lens of 10 diopters is one which is a 

 portion of a sphere whose radius is one-tenth of a meter; a + lens is 

 convex and a lens concave). 



(B) Adjust the draw-tube of the lamp so that the rays diverge into the 

 "eye" (have the tube close to the cornea). Place the round diaphragm 

 in the clamp outside the cornea; 10 cm. from this diaphragm place the 

 convex lens, the openings in the diaphragm being at the lens 7 focus. 

 The rays extending beyond the lens are rendered parallel by the lens 

 the same process as in A. 



