162 0. N. Rood—Nature and Duration of 
Micrometer.—Let the spark be generated at S, fig. 1; its light, 
fallmg on the stationary mirror M, wi 
oe form a spark-image at I, in the plane of the 
observing plate. If at the same instant a 
s second spark be generated at S’, its im 
will fall at I’. Let us suppose that during 
e simultaneous production of the two 
sparks, the mirror is rotating so rapidly as to be able to draw 
2. artially out into streaks, then we 
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This paper was supported by a 
moved till the right distance had been attained. 
Total duration of the discharge. 
As was to be expected, this was found subject to some varia- 
