NEWTONIAN CONSTANT OF GRAVITATION. 21 
on the index. Safety fingers of wire were also attached to the headstock, but not in 
contact with the scale, so as to prevent it from falling, if it should by accident get 
displaced. One of the microscopes of the optical compass was allowed to rest in 
brass V’s bolted to a solid iron casting, which rested on the same slate-topped pier 
as the machine. The microscope was moved in its V’s until one end of the scale was 
in focus; the tail headstock was then traversed on the bed of the machine until the 
other end was opposite the microscope. The scale was then moved until this end was 
in focus, but the process, being only carried out by the fingers, was difficult to 
perform, as besides fixing the scale parallel to the bed as tested by the focus of the 
microscope, it was necessary also to see that it remained parallel in the vertical plane, 
and to adjust this by pressing out or adding to the soft wax wafers by which the 
scale was lightly held. At first this quadruple adjustment, in which the setting of 
one right generally put the other three out, seemed as if it would require for its 
successful accomplishment some mechanical contrivance more under control than the 
fingers. However, by a happy accident, I succeeded in soon getting the scale so that 
IT could detect no want of parallelism either way with the fairly powerful microscope 
that I was using. The actual distances were determined along a line about 95 of an 
inch below the upper ends of the short divisions. 
The distances which it was necessary to know with the greatest accuracy, were 
those from 0 to 6, from 1 to 5, and from 24 to 34. These were determined as follows. 
The loose headstock was traversed on the bed, and clamped when the division at one 
end of the distance to be measured was on the cross-wire of the microscope. <A bar 
was then put in, the feeling piece put in its place, and the micrometer head turned 
until the feeling piece was just prevented from slipping, when the reading was taken, 
The headstock was unclamped, moved, and the process repeated until two or three 
readings had been taken. The bar was then removed, the loose headstock moved 
until the division at the other end was on the cross-wire, and a new bar of suitable 
length put in, and the micrometer turned until the feeling piece was again just held. 
When the three readings had been taken in the second position, the headstock was 
set back to the first position, the first bar placed in position again, and three new 
readings taken ; then in the same way three more were taken in the second position. 
The microscope was not touched at all during the process. 
In connection with these measures, the following important details may be referred 
to. I found great difficulty in setting the loose headstock by means of the high- 
p-tched leading screw, especially as the wheel was almost out of reach, and in seeing 
the cross-wire in the eyepiece projected upon any division of the scale so as to bisect 
that division exactly. These difficulties were practically removed by placing the 
microscope so that its cross-wire was slightly inclined to the vertical in which 
direction the divisions are ruled, and by setting the ruled lines symmetrically between 
a pair of microscopically fine specks of dust upon the cross-wire which, with the 
particular inclination then, just lay on either edge of the line. The width of the line 
