12 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



The force at the distance R = 6 1.5 cm. from the axis (this being also the 

 position of the line of light passing through the plate) is, for like 6 and p, since 



on introducing the values given, 



Thus, if AA/"= io~ 4 cm., F R = io~ 3 X 13 dynes, or about io~ 3 X4 dynes per van- 

 ishing interference ring. 



7. Observations. The data are given for convenience in fig. 9, on the same 

 plan as the above, the reflection method being used. Though the work was 

 continued through several weeks, only an example is shown, as it was neces- 

 sary to readjust the apparatus frequently, and the behavior was throughout 

 anomalous. Since 0=x/4D, 



a = <?9 = o. 0146 radian = o.8i# second of arc, 



since D = goocm., M = 971 grams, MV=*i6g grams, 



/? = 6i.s cm., /t=i3.icm. r=aosec. 



In fig. 9 the ordinate x is given in arbitrary units, which must be divided 

 by 5.74 to reduce them to seconds of arc. 



160 

 140 



80 

 60 

 40 

 &0 



s 



Ja/ru 



9 11 



FIG. 9. 



13 15 1T 



The conical sockets for the horizontal pivots of the pendulum being of steel 

 and not quite smooth, it is possible that the relatively enormous values of the 

 changes of inclination, a, registered may have been due to displacements of 

 the pivots in their sockets ; but readjustments of the fiducial zero (before the 

 observations marked n in the graph) were as frequently necessary when there 



