70 THE ART OF PROJECTING. 



a band of light will appear upon the screen, three to 

 five feet long, depending upon the amplitude of vibra- 

 tion and the distance to the screen. If now the mirror 

 m be turned so as to swing the beam at right-angles to 

 the band of light, a long sinuous line of light will be 



Fig. 46. 



spread upon the wall. It may be seen to be forty or 

 fifty feet long if the mirror be moved fast enough. At 

 the time the fork is struck attention may be called to 

 the sound. If two beams of light, about half an inch 

 apart, and one above the other, be made to fall upon 

 the first mirror while it is vibrating, and the mirror m 



Fig. 47. 



(Fig. 45) be moved as before, two undulating lines will 

 appear, one above the other (Fig. 47), with phases ex- 

 actly corresponding. Let the two beams of light be 



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Fig. 48. 



brought side-by-side and they will appear to have op- 

 posite phases (Fig. 48), and will show as beautiful in- 

 terlacing lines. A double image prism put in the path 



