DEVICES FOR SEEING 303 



vertical lines of the window frame that bound the panes of glass 

 appear not as perfectly straight lines but as more or less curved 

 lines. This is due to the fact that the rays passing through the 

 margin of such a lens and those passing through its center do not 

 come to a focus at exactly the same spot. If you will cut a small 

 circular opening one-half inch in diameter in a piece of cardboard 

 or thick paper and lay it on the lens so that all the lens is covered 

 except its central area and try the foregoing experiment again, 

 you will find that the image which you see is largely freed from 

 this spherical aberration. So you will find a diaphragm inserted 

 in the lens of many optical instruments to accomplish this 

 correction. The iris of the eye is in part for this purpose. When 



FIG. 150. Diagram of a stereopticon 



one is out at night, the pupil is very large to admit as much 

 light as possible, as you will readily see if you look at your eye 

 in a mirror immediately on coming in from the dark. Because 

 the pupil is so large, the image is not very distinct, and we often 

 mistake commonplace objects for terrifying things. 



The curved surfaces of a convex lens are segments of spheres. 

 If the surfaces could be paraboloid surfaces instead of spherical, 

 this defect would not occur. But it is very difficult to grind 

 lenses with paraboloid surfaces and very easy to grind them 

 with spherical surfaces. A piece of glass to be made into the 

 form of a lens is cemented to the end of a stiff rod; the other 

 end of the rod is pivoted at a point above a horizontal rotary 

 grindstone so that the glass presses on the surface of the grind- 

 stone. It is evident that the rod is the radius of a sphere, and 



