52 DISPLACEMENT INTERFEROMETRY APPLIED TO 



and wider than the strips 5 may have to be closed laterally, wholly or partly, 

 except at cc', by the hands, for instance. The sounding-boxes of tuning-forks 

 respond very nicely to the apparatus 77. If two small closed pipes (say locm. 

 long and 3 cm. diameter) be placed at c and c', figure 77, strong beats are 

 almost always obtained, unless the embouchures at c and at c' are quite alike. 

 This attended to, one may get notes out of drain-pipes 5 feet long as well as 

 out of tin cups or funnels. Though this was not the case in my apparatus, it 

 would be an advantage to have the plane scc's' quite flat, so as to repose on the 

 tube end of any size. If obtainable, pp' would then preferably be made of 

 triangular tubing. 



47. Reed pipes, voice. In the endeavor to obtain waves of larger amplitude 

 the device of figure 78 was resorted to. Here the pipe PP, 32 cm. long and 3 cm. 

 in diameter, through which the component ray L of the interferometer passes, 

 is provided with a tubulure about i cm. in diameter at its middle and con- 

 nected by flexible tubing t (to be made longer or shorter at pleasure) with 

 the reed-box R. The vibrations of the reed are thus impressed on PP and 

 conversely. The note was coarse, like a bassoon. Without PP the reed 

 merely wheezed. 



With t very short, the first experiments gave a display of enormous waves, 

 20 or 30 fringes high. I suspected that this could be only a direct mechanical 

 effect of the vibrating reed on the interferometer. The reed pipe was therefore 

 mounted on a separate scaffolding, entirely independent of the interferometer, 

 although both were ultimately attached to the large pier of the laboratory. 

 The result was an immediate reduction of double amplitudes to 4 or 5 fringes. 

 On replacing the reed R by a clarionet mouth-piece, the results were similar. 

 The waves are naturally compound wave-trains, and could only be repro- 

 duced photographically. When blowing strongly, deeply incised fundamentals 

 appear, decorated throughout by the overtones. On blowing more gently the 

 fundamentals receded, and the compound wave-train defied further analysis. 



The pipe was now moved, as a whole, out of the range of the ray L, and again 

 sounded. To my astonishment, strong 



waves were again produced, though not Q Q 



as much so as when the pipe was in | | 



position for interference . In other words , U/ 



these reed notes act directly on certain y g _L1 



parts of the interferometer and excite 

 the parts selected by resonance. 



To test this further, I made use of 



T 



79 



r 



the voice, singing a foot or more away 

 from the interferometer. I was again surprised to find that at certain 

 chest notes (b 1 c"}, the interference bands broke into marked waves near a fringe 

 in double amplitude, the effect being absent from the remainder of the 

 scale. A clarionet played about a yard from the interferometer evoked the 

 following response : 



