NEWTONIAN CONSTANT OF GRAVITATION. 44 
final exact adjustments are easy of execution, for the focussing screw §, supplies a 
slow fore and aft motion, while the corresponding slow lateral movement is given by 
the screw cone S, In order to allow one only of the traversing slides to move, the 
fingers of one hand are made to rest upon the slide which is to be kept still, and thus 
its friction on the base increased, the other one then only moves. When both 
microscopes are focussed upon (say) the right or apparent left sides of the wires, and 
their eross-wires are directed upon them also, the focussing slide is withdrawn about an 
inch, and the focussing block ) put into its place. The slide is then pushed forward 
until the focussing screw rests against the focussing block. The small glass scale is 
then placed so as to rest against the two parallelizing screws S, §,, and against the 
micrometer screw S,, and §,S, are moved until the scale is in focus in both microscopes 
at the same time. S, is then turned until the intended zero of the scale 04 is on 
the cross-wire of the left microscope and its head is read : it is then turned forward a 
fraction of a turn until a division at the other end of the scale 6°03 or 6:04 is on the 
cross-wire at that end. The amount of movement indicated by the head of the screw 
S,, added to the tabulated distance from ‘04 to 6°03 or 6:04, is the distance from the 
right side of one wire to the right side of the other. If the eye-piece micrometer is 
used instead of the cross-wire, then ‘04 is brought in the central division of the left- 
hand microscope, and the micrometer readings of the divisions 6:03 and 6:04, or 6°04 
and 6°05, are taken in the right microscope. Knowing the tabulated length of either 
of these divisions and the number of eye-piece divisions corresponding to it, the 
amount to be added to the tabulated distance between ‘04 and the lower of the two 
observed, is readily found. The distance between the left sides of the wires is found 
in the same way, and the mean of the two is taken for the distance between their axes. 
The thicknesses of the wires are also found by subtracting the readings of the apparent 
right from the readings of the apparent left sides, remembering that there are two 
whole turns of the screw. After each operation the focussing block is moved round, 
and the focussing slide moved up so as to bring the wires into view. If they are not 
exactly on the cross-wire as before, the measure is rejected and a new one taken, but 
this is rarely the case. 
The example taken from Experiment 8 does not show the confirmatory observations 
made with the eye-piece micrometer, for at that time the micrometer scales had not 
been made. As [I still rely upon the screw measure of the fractions and only take 
the eye-piece micrometer readings to satisfy myself that the screw observations have 
been correctly worked, this example will serve as well as a later one. 
MDCCCXCV.—A. G 
