474 REFRACTION THROUGH LENSES. 



as 28 cm ; the thickness of such a lens in the middle exceeds that of 

 the edge by about 3 mm . Any other lens, not too small nor too convex, 

 will however serve quite as well. 



In the middle of a circular piece of cardboard about 15 or 20 cm 

 in diameter, of the same thickness as the edge of the lens, cut a 

 round hole of the exact size of the lens, which should just fit into it, 

 neither too loosely nor too tightly. Prepare two rings of cardboard, 

 having an internal diameter somewhat smaller than the lens (5 '6 

 or 5 cm> 8) and an external diameter of about 9 cm . Glue one of the 

 rings round the hole so as to form an edge around it ; the edge will 

 project l mm or 2 mm and prevent the lens when placed upon it from 

 falling out on that side. The other ring is placed similarly upon 

 the opposite side of the cardboard, but fastened only with three or 

 four drawing pins, or with a few drops of sealing-wax, so as to be 

 easily removed when the lens is to be taken out again. 



For the experiments, the round piece of cardboard with the lens 

 is clamped in the fork of the retort stand, which is turned until the 

 lens is perpendicular. The flame of a candle may again be the 

 luminous object, and the paper screen used in the shadow experi- 

 ments may serve for receiving the image. The broad edge of the 

 cardboard round the lens serves for throwing a shadow upon the 

 greater portion of the screen on which the image is received, for the 

 bright image appears, by contrast, much more distinct if the neigh- 

 bourhood is comparatively dark. The lens must in these experi- 

 ments be between the candle and the screen, for the real images 

 formed by lenses are on the side opposite to that on which the object 

 is. Care must also be taken, 1st, that the light be incident upon the 

 lens in the direction of its axis ; the candle and the middle of the 

 lens should therefore be at the same height ; 2nd, that the cardboard 

 frame of the lens should be at right angles to the straight line from 

 the flame to the centre of the lens. The necessity of these pre- 

 cautions becomes apparent when a well-defined image has been 

 obtained ; then the slightest displacement of the lens from the 

 correct position at once impairs the distinctness. 



When the lens has a focal length of 28 cm , the candle must be at a 

 distance of 56 cm on one side of it, and the screen at the same dis- 

 tance on the other side, if the image is to be of the same size as the 

 object. If the screen is moved farther from the lens, the candle 

 must be moved nearer to it in order to produce again a distinct 

 image, which will be magnified. If the candle is placed at a greater 

 distance, the screen must be moved nearer to the lens, and the 

 image will then be smaller than the object. As in the case of a 



