80 THE ART OF PROJECTING. 



gular deviation. The beam of light will describe a 

 circle. 



If it moves slowly, the path and direction of the 

 moving beam can be nicely observed. These two ad- 

 vantages are not to be had with forks ; for, first, it is 

 accidental if one gets a circle or any other desired re- 

 sultant figures from forks in unison, for the obvious 

 reason that the phases cannot be regulated, and second, 

 the vibrations of the fork are so rapid that the analysis 

 of the motion can only be made in a mechanico-mathe- 

 matical way. 



By moving the fixtures on the left side toward the 

 centre of the plate a, the pulley / will not revolve so 

 fast. If moved half-way it will make one revolution 

 while the other makes two, and the vibrations stand in 

 the ratio i : 2 represented by forks in octave. Such 

 ratio is shown upon the screen by a form very much 

 like the figure 8, and known as the lemniscate. 



Between these two places, every musical ratio in the 

 octave can be got, and the resultant motions projected 

 in their proper curves. More than that, while the mir- 

 rors are both vibrating, any of the ratios desired can be 

 moved to at once by merely turning the thumb screw d, 

 which is wholly impossible with any forks which require 

 stoppage and adjustment of lugs for each different 

 curve. 



Again, if the fixture c is moved still farther toward 

 the centre than half-way, the curves projected will be 

 those belonging to the second octave, until the pulley 

 reaches three-fourths of the way, when the ratio will be 

 i : 4, and the resultant figure will be like a much-flat- 

 tened double eight. 



If one would show the phenomenon of beats, it will 

 be necessary to have the mirror m and its attachment 



