40 PROFESSOR C. V. BOYS ON THE 
sponding quantities found in operations 2 and 3, ah and dg, and thus find the levels 
of the centres of the gold and of the lead balls. If the gold balls are on the whole 
high or low, lower or raise the torsion rod the necessary amount and re-measure 
a, a and d, d, 
Operation 8. 
If tbe mirror rests so that the foot of the perpendicular is about the position of 
rest as seen by reflection in the telescope, when the lead balls are, so far as it is 
possible to judge, in the same plane as the gold balls, all is ready for the next 
operation ; otherwise it will be necessary to turn the support of the torsion rod, which 
will not raise or lower it, until observation at the telescope shows that this is the case. 
The mirror will then be symmetrically placed with respect to the window. 
Operation 9. 
This is a long operation carried out with the optical compass, the object being to 
find the horizontal distances between the axes of the wires and between the axes of 
the fibres, to centre the torsion fibre with respect to the axis of the instrument, and 
to find the corresponding readings of the vernier of the lid and of the glass scale 
seen by reflection in the telescope when the plane of the wires and fibres is 
identical. Owing to the great length of this operation, the description of it is 
divided into sections. 
Section a.—Remove the two pillars RR from the lid, and the first spur-wheel of 
the driving train W, which is made to simply lift off. Put the optical compass in its 
place on the lid so that the line of traverse is apparently parallel to the line joining 
the wires. 
Focus the positive eye-piece of each microscope upon its cross-wire or scale, and 
then shift the focussing collars ¢ of each, so that when the microscopes change 
places in the same groove and are pressed up to their collars, they are each in focus 
on the same object. 
Section b.—Using one microscope in any one groove of one of the traversing slides 
T, or T,, focus alternately on the edges of the two wires, using the focussing screw 
S; to move both the same way, and turning the base of the optical compass upon the 
lid to move them opposite ways. When both are found in exact focus the line of 
traverse is parallel to the plane joining the axes of the wires. 
Section c.—Measure this distance. To do this the two traversing slides are placed 
together, and the fine steel spring passed through the hole in their V’s. The spring 
is then stretched and prevented from contracting by a pin passed through it at one 
end. The two slides T,, T, then are pulled together by the spring, but are separated 
more or less by the adjusting screw cone S,. The two microscopes are laid in the 
outer grooves (or the middle pair in case a 4-inch distance has to be measured), and 
the slides moved until each is directed upon one side of the corresponding wire. The 
