260 



OPTICAL PROJECTION 



by twitching with the finger. As far as regards the projection, 

 this is cheap and efficient, but is not audible ; it has the 



advantage, however, of 

 producing the figure by 

 motions of the same 

 kind as a couple of forks. 

 For projecting the 

 figure only, the ap- 

 paratus shown in fig. 

 137 has been devised 

 by Pfaundler, who de- 

 scribed it in the ' Pro- 

 ceedings ' of the Vienna 

 Academy (of Sciences) 

 and figures it in his 

 * Lehrbuch der Physik.' 

 Fl0 - 136 The vibration of two 



springs s, s', set rect- 

 angularly to each other, one variable by a weight, L, is given 

 to two discs, D D', with slots which also cross at right angles. 



The spot of light 

 where they cross, 

 evidently moves 

 in a path com- 

 pounded of the 

 two motions. 

 The discs are 

 arranged in front 

 of the condensers, 

 the spot being 

 focussed as an 

 ordinary dia- 

 gram, and the springs twitched. Stohrer has constructed a 

 more complicated apparatus for giving the same motions to 

 two discs by a wheel and gearing, methods of which will 



FIG. 137 



