APPENDIX. 



I. Preliminary outline on lenses, lighting, focusing, cover- 

 ing, 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 sa,me 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: 



10 



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) , 



