Mr. Troughton on dividing Instruments. 117 
tion ; but they do not fall in with the ultimate divisions of the 
circle, which are intended to be equal to 5' each. 
The next thing to be considered is, how to make the roller 
measure the circle. As two microscopes are here necessary, 
and those which I use are very simple, I will in this place give 
a description of them. Fig. 6 is a section of the full size, and 
sufficiently explains their construction, and the position of the 
glasses ; but the micrometer part and manner of mounting it, 
are better shown at H, in Fig. 1 and 2. The micrometer part 
consists of an oblong square frame, which is soldered into a 
slit, cut at right angles in the main tube ; another similar piece 
nicely fitted into the former, and having a small motion at 
right angles to the axis of the microscope, has at one end a 
cylindrical guide pin, and at the other a micrometer screw; a 
spring of steel wire is also applied, as seen in the section, to 
prevent play, by keeping the head of the micrometer in close 
contact with the fixed frame. This head is divided into one 
hundred parts, which are numbered each way to 50 ; the use 
of which will be shewn hereafter. A fine wire is stretched 
across the moveable frame, for the purpose of bisecting fine 
dots. Two of these microscopes are necessary; also a third, 
which need not have the divided head, and must have in the 
moveable frame two wires crossing each other at an angle of 
about 30° ; this microscope is shewn at I, Fig. 1. In the two 
first micrometers, a division of the head is of the value of 
about o",2, and the power and distinctness such, that when 
great care is taken, a much greater error than to the amount 
of one of these divisions cannot well be committed in setting 
the wire across the image of a well made dot. The double 
eye-glass has a motion by hand, for producing distinct vision 
