118 



DISPLACEMENT INTERFEROMETRY BY 



throughout, though naturally starting with a different (arbitrary) zero posi- 

 tion on the two micrometers. At a the subdued sunlight used was intensified 

 and a marked break in the progress of the curve at once ensues. This is obvi- 

 ously a temperature or air-drift effect, acting contrariwise. As the mirrors 



TABLE 6. Slit-image and micrometer data, x =0.1423'; x'=o.8gAN; 

 grams; m = o.62 gram. Subdued sunlight. 



* Light subdued. 



are equidistant from the center, one beam is stronger than the other. The 

 total or final displacement, moreover, is too large an amplitude, being over 

 # = 0.15 cm. But the difficulties and dangers are well given by figure 114. 

 Hence in the following experiments part of a Welsbach mantle was used as 

 a source of light; and as the deflections x are relatively quite large, it was not 

 thought necessary to make fringe readings throughout for the purposes here 

 in question. The observations, moreover, will be given graphically for con- 

 venience, and the tables, which were computed in full, removed. 



90. Reversal at symmetrical positions. In the graph (figs. 115, 116, con- 

 structed with the m displacement in centimeters as ordinates, in relation to 

 time in minutes as abscissas), the large attracting mass M was reversed 

 when the deflection x had reached a certain mean departure (at # = 0.38 and 

 0.52 cm.) from the equilibrium position with which the series begin. Inertia 

 carries the needle beyond the turning-points indicated by circles, and the seg- 

 ments are at first sinusoidal in form. They finally became more nearly straight 



