NEWTONIAN CONSTANT OF GRAVITATION. 19 
is provided, this simple construction, though perhaps less convenient, being far more 
perfect than any possible kind of slide. 
The focussing slide F rests upon R by means of the two V’s, v,v, which fit into 
a straight V groove in R, and by means of the flat surface fi, which rests upon 
a planed surface parallel to the v groove. This focussing slide is stiffened by 
longitudinal ribs above and below the general level, one on one edge, and the other on 
the other edge. It also carries a focussing screw 8, of 50 threads to the inch 
roughly divided on the head. This merely pushes against the tail rib of R, causing the 
slide F to retreat from the centre of the apparatus. It can be moved the other way by 
hand, or by a gentle forward pressure on the screw head when it is being turned back- 
wards. Asit is necessary to be able to give a fine focussing movement to this slide in two 
separate positions, about one inch apart, a focussing block b of the required length is 
pivotted on R, so that it may either remain out of use as shown in the figures, or may 
be brought under the focussing screw after the focussing slide has been withdrawn. 
A turn or two isall that is necessary then for the purpose of focussing in either 
position. 
Two traversing slides T,, T, each rest upon the focussing slide by five small 
surfaces, of which four in each case are due to the long projecting V’s v, on the front 
edge, of which the middle parts are scraped so as not to bear upon the longitudinal 
V groove in the traversing slide. 
The fifth point in each is formed by a small friction wheel w, which lies in a 
recess in the traversing slide, and runs upon a planed surface on T parallel to its 
V groove. ‘The reason that I introduced a wheel here is, that while a very small 
vertical uncertainty is of no consequence, I was thus able to cause the whole of the 
frictional resistance to traversing to lie in the V itself, which is the more necessary as 
the distance between the ends of the V is necessarily less than the distance perpen- 
dicular to it. This is taken advantage of further, for I have arranged that the force 
that draws the two traversing slides together is produced by a long, very small helical 
spring of steel lying in a hole drilled in the V’s themselves, thus being in the line of 
friction and producing no tendency in either to depart from its geometrical bearings. 
For the same reasou I have made the fine adjusting screw which separates them act 
in the same line. This consists of a fine steel screw §,, fitting rather loosely in its 
very short nut, carried by T, at one end, and with a fine polished conical point at the 
other, which rests between a little V carried by T, and a vertical surface on T,. The 
screw and cone piece, therefore, are free from constraint, but simply push the 
traversing slides apart in the same line where the friction and the opposition of the 
spring act. Thus, when the screw is turned forwards, the slides simply separate to a 
minute extent, but have no tendency to lose their parallelism. Each traversing slide 
is furnished with three grooves cut away so as to support a microscope lying in any 
of them at each end only over small surfaces at 45° on either side, thus allowing it 
two movements, one of rotation, and one fore and aft. The latter is prevented by 
Dp 2 
