64 DISPLACEMENT INTERFEROMETRY. 



stroboscopic .vertical lines or slit-images, very definite but faint. At a" the 

 lines again appeared, this time very bright and present throughout the field. 

 The same occurred at c'", the note being strident. After this the wave-trains 

 became regular again, and were traced almost as far as c lv . 



No doubt these are indications of resonance in the telephone plate, the vibra- 

 tions being of the type, figure 84, and implying a rotation of mirror. In fact, 

 at a" there were fine lines between the main lines, indicating contemporaneous 

 presence of very high rotational overtones. Moreover, in different adjust- 

 ments the frequency evoking resonance was not quite the same. 



