228 Mr. Cavendish on the Manner of 
22 6 ; and then having first made the fixed wire of this micro- 
scope correspond exactly with the point, w£ must draw the 
scratches b and jG, d and S, & c. as before, and bring the 
fixed wire to the true point of quinque,section between b and ( 3 , 
n the manner directed in page 2-26, and with the point strike 
the scratch or dot: and if we please, we may, for further 
security, as soon as this is done, examine, by means of the 
moveable wire, whether this intermediate scratch or dot is 
well placed. 
The advantage of this method is, that when this is done, 
we may subdivide and cut the true divisions, by making the 
wire of the microscope bisect the intermediate scratches, in- 
stead of being obliged to use the more troublesome operation 
of placing it in the proper proportion of distance between the 
two extremes. 
This method certainly requires less attention than the for- 
mer, and on the whole seems to be attended with consider- 
ably less trouble ; but it is not quite so exact, as we are liable 
to the double error of placing the intermediate point and of 
subdividing from it. 
As in this method the intermediate points are placed by 
means of the micrometer, there is no inconvenience in placing 
the extreme scratches b and / 3 , &c. at such a distance from 
each other, that the intermediate one shall be in no danger 
of running into the bur raised by the extremes. 
Third Method. 
Let a a. ( Fig. 3 ) be the arch to be quinquesected ; lay down 
the arches a b, bd, and de , as in the first method; then turn 
the beam compass half round, and lay down the arches a /3 
