NEWTONIAN CONSTANT OF GRAVITATION. 45 
are not diametrically opposed to one another, then the lead balls will not rotate about 
the torsion fibre as an axis. If, however, the optical compass is removed and the lid 
is turned round 180°, and the measurement repeated, then half the movement 
necessary to bring the fibres into focus as measured by the head of the focussing 
screw is the diametrical inaccuracy of the lead balls, and half the inequality of the 
right and left intervals is their eccentricity with respect to their mechanical axis 
measured in their own plane. The torsion head may now be moved half of each of 
these amounts separately, and then the vertical line half way between the pair of 
fibres, which by construction is necessarily the same as the torsion fibre within less 
than yo'09 inch, is also in the mechanical axis about which the lead balls actually 
turn. 
Exampues.—Plane of Fibres and Wires. 
Focus for wires, ‘05 on focussing screw. 
eee LCS eca 
23 23 
Therefore the fibres are behind (away from microscope) the wires by 5p = 004 inch, 
Eccentricity of Pair of Fibires.—After setting the pair of microscopes to one 
interval and sliding to the other, the left appears in the eye-piece to be smaller than 
the right by about 007 inch. 
By measurement it is found to be ‘00792 less,-or the pair of quartz fibres are 
00396 out of centre. 
The necessary observations are :— 
5°98607 Centre to centre, wires. 
°89330 Centre to centre, fibres. 
02314 Thickness right wire. 
6°90251 
00102 Thickness left fibre. 
2)6-90149 
3°45074 Mean of right and left intervals. 
345470 Right edge left fibre to right side right wire. 

‘00396 Excess of right interval above mean. 
00792 Right interval greater than left. 
Operation 10. 
Prepare for observations of deflection and period. Remove the optical compass 
and replace the windows by fig. 12 at the back and fig. 11 in front. Place the 
tubular screens, fig. 15, in position, Screw in the pillars R, R, and arrange the guys, &e. 
