60 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



vertical harmonic motion, as shown by the arrows with periods often as high as 

 4 seconds, but varying in the lapse of time. The waves observed in the upper 

 sections became strong and clear ; in the lower section there was usually much 

 vagueness. Shadow waves were nearly absent . The slit-images at rest showed 

 no lateral separation, but evident vertical vibration. 



These faulted wave-forms occur only when the vibration of the telephone is 

 intense (resistance 1,000 ohms) and its note jarring. The sections practically 

 jumped from one quiescent position, high or low, to the other, until the 

 period of induction was reduced from 0.08 second to o.oi second, whereupon 

 sections vibrated regularly up and down. When the resistance in circuit is 

 increased to 10,000 or 20,000 ohms, the even band of figures 81 and 82 reap- 

 pears. In the parts w', however, gaps or blank spaces frequently appear, so 

 that the sharp wavew, figure 85, vanishes laterally outward in both directions, 

 to a gap free from fringes, which gap in turn concentrates laterally inward to 

 a sharp wave w again. At very high resistances, the gaps may show waves of 

 small wave-length like overtones. 



r 



TO 



/N 



> 



Id 



Supposing these occurrences might be due to accidental looseness of mirror, 

 I took out the plates and refastened the mirror carefully. When the fringes 

 were again found there was no appreciable change in the phenomena. 



As already suggested, it seems probable that the above observations as a 

 whole are to be referred to a buckled plate of the telephone, resulting from the 

 relatively excessive expansion of the hard-rubber case which holds it. In 

 figures 85 and 86 there are two positions of equilibrium of the plate, and the 

 passage from one to the other is practically instantaneous. Possibly also when 

 the fringes are unbioken straight bands, there are strains in the plate which 

 resist displacement under the given forces, so that straight fringe bands result. 

 Again, two wave-trains may happen to annihilate each other. 



To guard against resonance vibrations in the interferometer itself, I discon- 

 nected the telephone on the latter, and placed the auxiliary telephone strongly 

 excited in different parts of the interferometer and even on the optical tele- 

 phone itself. The fringe bands remained even and non-sinuous throughout, 

 so that discrepancies of this kind are absent. Evidence with the same purport 

 is the frequent occurrence of unbroken fringe bands in case of a sounding tele- 

 phone, throughout the experiments. 



The differences between the effects of induction periods T O.I second to 

 r = o.oi second was chiefly this, that the former allowed each shock- wave more 

 time to subside and there are fewer groups in the field. The occurrence of gaps 



