CONCAVE MIRRORS. 443 



ripples, which are the more imperceptible the nearer the metal was 

 to becoming solid again at the moment when the glass was 

 pressed upon it, and the thinner the metallic layer happened to be. 

 The surface has a bright polished appearance when new, which may- 

 be preserved very long if care is taKen to protect the surface against 

 the touch of the fingers and dust. It is immediately tarnished when 

 touched and wiped, and should therefore be kept with the polished 

 side turned downwards. 



A screen of cardboard, covered with white paper, is clamped verti- 

 cally in a retort-stand, and placed in a dark room as close to a candle 

 as is possible without burning the cardboard. The concave mirror 

 is fixed by its handle in a second retort-stand at a distance of a few 

 decimetres from the candle, at the same height as the latter, so that 

 the light reflected by it impinges upon that part of the cardboard 

 which is in close vicinity to the candle. Object and image should 

 be as near as possible to the axis of the mirror. The position of the 

 latter should therefore be such that a line drawn from its centre to 

 the edge of the screen nearest to the candle may be a perpendicular 

 both to the mirror and to the screen. This is indicated in the fol- 

 lowing diagram. 



Screen. 





 Candle. 



The mirror is now moved in the same position towards or from 

 the screen. If the image of the flame becomes larger during this 

 motion, move the mirror in the opposite direction, and a position 

 will soon be found in which a well-defined inverted image of the 

 flame and the more brightly illuminated upper portion of the candle 

 makes its appearance on the screen. 



The distance at which the image has its best definition is equal 

 to the radius of curvature of the mirror ; our lens would produce 

 a mirror of O m '3 radius. The image of the flame appears at its 

 best definition somewhat smaller than the flame itself, because the 

 flame has no sharp boundary line, and the intensity of the light 

 of the image is always less than that of the luminous object. This 

 disadvantage may be avoided in the following manner. Cut 

 a small triangular aperture in the screen, 6 or 8 mm broad and 

 20 or 25 mm high, the vertex of the triangle being above. Place the 

 candle behind the aperture, so that it may throw its light through 

 the opening upon the mirror, which will thus reflect an image not 



