g 1 1 



t)PTres. 



the image is most distant the lens may be exactly 

 between the other two ; then halve the distance 

 between the object or its image, and the lens is the 

 focal distance. 



3. If a small hole, about one-fourth, or one- 

 eighth of an inch, be made in the window-shutter 

 of a darkened room, and a lens and piece of paper 



^^ be held behind this hole, at proper distances, the 



Jfff place where the image of the hole is most distinct 



may be determined very exactly, and from them 



the focal length may be found by the foregoing 



rule. 



4. By the sun's image. Place the lens, so that 

 its axis may point as near as possible to the sun ; 

 then holding a paper opposite thereto, the burning 

 point, or where the image of the sun is smallest, 

 and the limb most distinct, is the focus. This 

 method is sufficiently accurate for spectacle-glasses, 

 and reading-glasses, and such as are broad in pro- 

 portion to their focal length ; but will not answer , 

 for lenses of a long focus, unless they are suffi- 

 ciently long to exhibit the solar spots ; because in 

 these cases, the image is only a glare of light, with- 

 out distinctness ; but the inconveniences may be 

 removed by the following method : 



5. Cover the lens with a piece of pasteboard or 

 paper, and make two round holes therein, at an 

 equal distance from the edge of the lens, and on 

 one of its diameters. The lens being thus covered, 

 point its axis to the sun : now if a paper be held 

 behind the lens, you will rind the two circles, or 

 white spots produced by the two holes, gradually 

 approach nearer to each other, as the paper is 

 moved further ; at last they will coincide ; and if 

 the paper be moved still further, they will again 

 separate. The distance of the paper from the 



14 



