REFRACTION THROUGH LENSES. 475 



mirror, a strongly magnified or greatly reduced image may be ob- 

 tained by projecting the image of the candle flame upon the wall. 



If the focal length of a lens is not known, it may be found in the 

 following manner. Place a candle on one side of a lens, about l m 

 from it ; move the screen on the other side to and fro until a well- 

 defined image of the candle is produced. Measure exactly the dis- 

 tances of flame and screen from the lens, multiply these distances 

 together, and divide the product by the sum of the distances ; the 

 quotient is the focal length of the lens. Thus suppose the distance 

 between the lens and the candle is 95 cm , that between lens and 

 screen 58 cm ; we should then have by the rule : 



95 x 58 5510 QAmq , 

 : 



that is, the focal length of the lens is almost exactly 36 cm . 



A convex lens is often used as a magnifying glass, the object being 

 placed between the lens and its principal focus. The image is erect, 

 virtual, and the more magnified the less the focal length of the lens. 

 In our lens, of about 30 cm focal length, the' image is very little 

 magnified ; in the most favourable position the image has not quite 

 twice the size of the object. But a lens having a focal length of 

 3 cm magnifies eight times if held close to the eye, and the object is 

 brought nearer and nearer until a distinct image is seen. A small 

 object is better and more completely seen through such a lens than 

 a larger one ; thus in the cross-section of a piece of cane, the 

 tubiform vessels may be distinctly seen by means of such a lens. 



The camera olscura is a contrivance for obtaining 

 distinctly visible and real images of objects which are 

 not brightly illuminated, as for example common objects 

 in diffused daylight. The principal aim of the apparatus 

 is to exclude all light which does not proceed from the 

 object itself, from the surface upon which the image of 

 the object is received. The 'camera' is especially used 

 in photography. A real image, usually smaller than 

 the object, is received upon a plate chemically prepared 

 so as to render it sensitive to the action of light, and a 

 permanent photographic picture is thus produced ; it is, 

 however, usual to introduce into the camera, before 



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