122 DISPLACEMENT INTERFEROMETRY. 



patible with the given tenacity of the fiber and the importance of a long 

 needle is manifest. M and R do not affect the theoretical result, but they do 

 determine the micrometer reading, for 



<=x/n or 



If /=ioo sec., 6=10 cm. 



AN = 0.0107 



A whole fringe would thus be about 0.4 per cent, while fractions of a fringe 

 could certainly be estimated in case an ocular micrometer or the like is used. 

 There is, however, no need of totally neglecting r/ 2 /4^ 2 - It may be treated as 

 a correction to eliminate the outstanding 17 per cent, if t is as large as 100 

 seconds. The problem is not easy, but it seems worth a serious trial. 



93. Recent work. In the further pursuit of this subject I endeavored to 

 exhaust the apparatus, believing that even if the viscosity of air does not 

 appreciably change, the annoyance due to convection currents could be elimin- 

 ated. The experiment was disastrous, however; for in spite of the thick plate 

 of glass, the vessel suddenly burst at high exhaustions, quite destroying the 

 fine instrumental contents, among which I particularly regretted the quartz 

 fiber. Another case of much greater strength and adapted for high vacua was 

 then constructed and tested; but I have not, up to the present date, been able 

 to find a suitable quartz fiber. In those examined, even if the motion of 

 the needle was little more than creeping and with the old annoying ten- 

 dency to rest persistently on the sides, the effect of gravitational attraction 

 proved to be quite inadequate. The work, however, will be pursued during 

 the present summer. 



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