NEWTONIAN CONSTANT OF GRAVITATION. 43 
does not affect the parallelism of the line of traverse with the plane of the wires, for 
the optical compass is carried round with the lid. When the fibres seem about 
equally out of focus and in the same direction, bring them towards or away from the 
microscopes, as may be necessary, by the adjusting screws of the torsion head. 
However carefully this may be done, the suspended beam and gold balls are sure to 
be set swinging slightly as a pendulum. This is easily overcome by resting a wax 
taper or very flexible piece of wood against the central tube, and while watching the 
motion in the microscope bearing very lightly upon it in time with the oscillations. 
If a torsional swing has begun, there is nothing to be done except to wait until the 
amplitude is reduced to a very small amount. It is impossible to obtain real quiet in 
this sense, as owing to gravitation the observer’s body and the gold balls will attract 
one another unequally. After one or two trials the two fibres wili be found 
simultaneously in focus, at any rate, if not continuously owing to the small torsional 
oscillations, yet between the elongations at successive half periods. I prefer to leave 
the apparatus at this stage and to return after an hour, when the mirror is more 
quiet, to verify the accuracy of focus. If this is correct since the focussing screw 
has not been touched, the plane of the fibres coincides with the plane of the wires. 
If more than u very small focussing correction is made, Operation 8 will have to be 
repeated afterwards. 
Section e.—Read the great scale in the telescope and take the vernier reading of 
the lid. Then, since the wires and fibres are in the same plane, the lid reading and 
corresponding scale reading are known for one position, and, since the angular value 
of a scale division is known, also for all positions. 
Section ~—Measure the distance between the fibres and their thicknesses, using 
exactly the same procedure as described under Section c. I may mention here that 
as the beam mirror is only ‘9 instead of 1 inch as intended, the traversing slides as 
made would not come near enough together for both fibres to be seen at the same 
time. I therefore made use of the traversing slides in their other position, @.e., with 
T, to the right of T, instead of to the left, after having had the right side of T, 
reduced by slot drilling so as to allow the now inner grooves to come } inch nearer to 
one another. The change from one position to the other is made in a moment. 
b> 
G 
