254 



OPTICAL PROJECTION 



downwards) the resonance -box of a fork mounted in the usual 

 manner. The top of the frame has an aperture to allow the 

 fork to come through. A smooth spherical ball of pith or 

 light cork, varnished, is hung as in the figure from an eye at 

 the side, and the frame adjusted so that the ball barely touches 

 the fork, which is focussed on the screen. On now sounding 

 another fork in unison, also mounted on a resonance-box, the 

 fork, focussed will be thrown into vibration, and the ball will 

 be driven away as shown on the screen. (For conditions of 

 success see next paragraph.) 



139. Doppler's Principle. By the same simple means 



Doppler's principle 

 or law (that the 

 colour in light, or 

 the pitch in sound, 

 depending solely on 

 the frequency of the 

 periodical impulses 

 falling upon eye or 

 ear, pitch is height- 

 ened when a vibrat- 

 ing body approaches, 

 and falls when it 

 recedes) can be de- 

 monstrated, pro- 

 vided the necessary 

 conditions of ac- 

 curacy be attended 

 to. These have been 

 well described by 

 Professor A. M. 

 Mayer. 1 He found 



c forks of 256 vibrations the most suitable, and provided two in 



exact unison, a third fork of 254 giving two inferior beats, and 



1 See American Journal of Science, April 1872. 



FNJ. 133 



