APPENDIX. 



I. Preliminary outline on lenses, lighting, focusing, 

 covering, finding, etc. 



1 . The crystalline lens of the eye is adjusted by change 

 of shape, effected by voluntary muscles. Hold a pencil 

 between the eye and a distant window and try to see pencil 

 and window sash at the same time. Note the distinct mus- 

 cular effort within the eye at each shift of vision from one 

 object to the other. Repeat with the window sash and a 

 tree on the horizon. 



2. Artificial lenses, being of permanent shape, are ad- 

 justed or focused by change of position, altering the dis- 

 tance from the object. Move a large simple lens forward 

 and backward between the eye and the letters of a printed 

 page until a clear and enlarged image of the letters is ob- 

 tained. Note that there is but one place of clear vision : at 

 this place the lens is in focus. Observe whether two lenses 

 of different size focus at the same distance from the page. 



3 . Catch the nearly parallel rays of light from a distant 

 window in the larger lens, and focus them on a sheet of white 

 paper held behind the lens. When focused, a clear minia- 

 ture picture of the window will appear upon the paper. 

 Measure the distance from the optic centre of the lens to the 

 paper; this is the focal distance of the lens. Try the other 

 lens and determine its focal distance. The curvature of the 

 surface of a lens mainly determines its focal distance, and 

 also its magnifying power. The magnifying power of any 

 simple lens is easily computed by the following formula: 



- = M, wherein 10 is ten inches, the focal distance of the 



normal, unaided eye, when viewing small objects, / equals 

 the focal distance of the lens in question (measured as above). 



