Chap. 9] 



SEISMIC METHODS 



455 



the observed deflection in scale divisions, the deflection angle of each 

 mirror is given by 



tan a = 



where A and D are expressed in 

 scale divisions. To obtain the ex- 

 tension of a bar, the angular mirror 

 deflection must be multiplied by the 

 effective lever arm of the rocking 

 mirror. A rocking-mirror arrange- 

 ment with a powerful telescope 4.5 

 m away from the scale has been 

 employed by Zisman. He obtained 

 magnifications of the order of one 

 million. 



Interference measurements for 

 strain determinations have been 

 applied in two ways. In the inter- 

 ference extensometer of Grueneisen 

 (Fig. 9-9), two tubes are clamped 

 to the sections whose change of dis- 

 tance is to be measured. The ends 

 of the tubes carry two closely 

 spaced polished glass disks, between 

 which interference patterns are pro- 

 duced. If the pattern changes by 



4A + 2D 



(9-20) 



Fig. 9-7. Bauschinger Martens mirror 

 extensometer. 



SuM 



Ttksctft 



Mimr 



Fig. 9-8. Double mirror reflection arrangement. 



one fringe width, the corresponding change in distance is one-half the wave 

 length of the light employed. 



