SOUND 251 



Its motions round the circle. With a glass carefully selected, 

 very fine effects are obtainable in this way. With a suitable 

 glass, circular ripples of quite another character, but of great 

 beauty, may often be obtained by pressing the finger very hard 

 upon the rim while drawn round sloiuly, so as to produce a 

 * groaning ' kind of note. And with somo glasses (see note 

 on page 241)) very similar ripples to these last, but more 

 evanescent, may be produced by tapping some particular point 

 on the goblet or its stem with a metal rod. 



For a perpendicular screen, the easiest arrangement is the 

 apparatus shown in fig. 142. The funnel and its frame are 

 withdrawn, and the glass in its square metal foot substituted, 

 arranged for height so that the surface of the fluid occupies 

 the position usually taken by the phoneidoscopic film. 



The vibrations in mercury, with a suitable glass, are very 

 fine when projected by this method. For this heavy liquid a 

 bow is sometimes advisable, and only certain glasses will 

 vibrate readily when so heavily weighted. 



136. Trevelyan's Rocker, The vibrations producing the 

 note of this apparatus are easily projected by cementing a 

 small mirror on the face it should not however be too small. 

 A small parallel pencil of light is focussed on the screen, 

 either from such an apparatus as was shown in fig. 95, or 

 the rays being made parallel from the condensers, a small 

 aperture may be placed in the stage of the optical front, and 

 focussed by the lenses of that arrangement. The rocker may 

 then be arranged so that the mirror meets the pencil at an 

 angle of nearly 45, throwing the pencil upwards, whence it is 

 re-reflected by the adjustable plane mirror to the screen. The 

 motionless spot will, when the rocker is in action, be drawn 

 out into a line of light ; and by turning the plane mirror side- 

 ways, this will further be analysed into a sinuous line, making 

 the individual vibrations visible. 



187. Vibrating Rods. All the leading phenomena of 

 these are easily projected, konig's apparatus for showing 



