Mr. Troughton on dividing Instruments. ng 
are fastened to the main frame, as at W and Z, by short pil- 
lars, having also the off leg of the angle secured in the same 
manner ; Y is a fine conical steel point for making the dots, 
and X is a feeler, whereby the point Y may be pressed down 
with a uniform force, which force may be adjusted, by bend- 
ing the end of the bar just above the point, so as to make the 
dots of the proper size. The point Y yields most readily to a 
perpendicular action ; but is amply secured against any eccen- 
tric or lateral deviation. 
The apparatus, so far described, is complete for laying our 
foundation, i. e. making 256 primary dots ; no matter whether 
with perfect truth, or not, as was said respecting the divisions 
of the roller; precision in either is not be expected, nor 
wished ; but it is of some importance, that they should be all 
of the same size, concentric, small, and round. They should 
occupy a position very near the extreme border of the circle, 
as well to give them the greatest radius possible, as that there 
should be room for the stationary microscope and other me- 
chanism, which will be described hereafter. 
It must be noticed, that there is a clamp and adjusting screw 
attached to the main plate of Fig. 1 ; but, as it differs in no 
respect from the usual contrivances for quick and slow motion, 
it has been judged unnecessary to incumber the drawing 
with it. 
Now, the roller having been adjusted, with one microscope 
H upon its proper dot on the circle, and the other microscope 
at the first division on the roller ; place the apparatus of Fig. 3 
so that the dotting point Y may stand directly over the place 
which is designed for the beginning of the divisions. In this 
position of things, let the feeler X be pressed down, until its 
