EXPERIMENTS WITH CONCAVE MIRRORS. 449 



The real images produced in the cases where the object is at a 

 distance either greater or less than twice the focal length may be 

 represented in a similar manner to that shown in the first case, 

 where the distance was equal to the radius of curvature. The rela- 

 tive positions of screen, candle and mirror for either case will appear 

 from the following diagrams. 



IMAGE LARGER THAN OBJECT. 



Screen. 





 Candle. 



IMAGE SMALLER THAN OBJECT. 



Screen. 







Candle. 



The magnified image may be received on the screen of tissue 

 paper described in the preceding article, but the smaller image 

 should be received on a piece of thin writing-paper. Both images will 

 then be visible on either side of the screen. The small image can- 

 not be received on the paper screen with wooden frame if it is de- 

 sired to see it from both sides, for if it is to be produced near the axis 

 of the mirror it would fall on the opaque frame. The proper dis- 

 tances between mirror, screen and object required for producing 

 well-defined images, can be found after having them approximately 

 arranged as in the above diagrams, by moving only one of the 

 three to and fro until the image obtains the best definition. 

 In the case where the image is smaller it is best to move the screen 

 only ; in the case where the image is magnified move the candle. 



For a mirror of 15 cm fecal length (radius 30 cm ) a distance of the 

 object of 20 cm corresponds to a distance of the image of 60 cm ; and 

 if the former distance is 25 cm , the latter is 37 cm '5. In the first 

 case the image is 3 times, and in the second case 1^ times, as 

 large as the object. In all cases the positions of object and image 

 are interchangeable : if the object be placed at the distance given for 

 the image in either of the foregoing examples, the image will be 

 found to be formed at the distance given as that of the object in the 

 same example. 



G G 



