CONVEX MIRRORS. 453 



therefore a is the image of A. Similarly b is the point 

 of intersection of the rays h B and i k produced back- 

 wards ; therefore b is the image of B. The image of 

 an object plowed in front of a spherical convex mirror is 

 formed behind the. mirror, is erect, smaller than the object, 

 and virtual. 



The distance of the image formed by a convex mirror 

 is found by multiplying the focal length into the distance 

 of the object and dividing by the sum of these two quanti- 

 ties. The ratio of the magnitudes of object and image is 

 for convex as well as for concave mirrors equal to the ratio 

 of their respective distances from the centre of curvature, 

 Let an object, 10 cm high, be 15 cm distant from a convex 

 mirror of 40 cm radius and therefore of 20 cm focal length. 



1 R v 90 



The distance of the image is then *~ - = 8 cm '57 ; 



J. O ~T~ *-t\) 



since the object is in front of the mirror, and the image be- 

 hind it, their distances from the centre of curvature are 

 respectively 40 + 15 = 55 cm and 40 - 8*57 = 31 cm -43, 

 and the magnitude of the image follows therefore 

 from the proportion 55 cm : 31 cm '43 :: 10 cm : x 

 = 31-43 x 10 = 5cm>?L 

 55 



In the case of convex mirrors it is more easy to find common 

 substitutes than is the case for concave mirrors, and as their images 

 present very little variety, the student need not go to the trouble 

 of producing a regular convex mirror. A common hollow watch- 

 glass supplies a pretty good convex reflector if the hollow surface 

 is blackened with lamp-black. This may be done by means of the 

 name of a piece of cotton wool, about the size of a pea, fixed to a 

 wire, dipped into oil of turpentine, and lighted. Other flames are 

 not so suitable ; they do not deposit so much soot, and heat the 

 watch-glass so much that it often cracks before it is blackened. The 

 bulb of a thermometer filled with mercury, or a glass flask of globular 

 shape (such as often used for chemical experiments) filled with ink, 

 or even a bright metal button of convex shape, may serve for our 

 purpose. 



