PHYSICS: C. BARUS 
339 
The quartz fiber q to carry the needle mm, hangs from a long one-eighth 
brass rod d, which may be raised or lowered in view of the sleeve c held by a 
separate adjustable arm, without. The rod d must fit the perforation in the 
cork nicely, so that the former may be smoothly raised or lowered and held 
in any position in virtue of friction. 
To swing the needle this is first placed on cork Y's, below the opening of the 
case AA, the felting gg having been removed. The quartz fiber q is then low- 
ered on the long stem d, until the lower hook on the fiber is in position to 
grasp the clasp on the needle mm still below the case. The needle is then 
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cautiously lifted by raising d. After this the felt strip gg is inserted to close 
the case, and the necessary adjustment made at c and d to swing the needle 
freely in the restricted space provided for it. 
In the rectangular interferometer used (fig. 4) the breadth of the ray parallel- 
ogram was b — 10 cm. and the angle of incidence i = 45 degrees, so that if the 
micrometer displacement AN corresponds to the small twist 6 of the 
needle, 0 = 0.071 AN. 
3. Observations. — The quartz fiber used (diameter 2 r) was L = 17 cm. long. 
Supposing the distance apart of attracting weights M and attracted weights 
