I 
130 Gen . Roy’s Account of 
micrometer fcrew to be formed of it ; and it carries a vertical 
wire placed in the focus of the eye-glaflfes, in which pofition 
it is moved parallel to itfelf from left to right, by turning the 
micrometer head. This ilide is alfo attached to a watch fpring 
which a£ts in a contrary dire&ion to the head, as in other mi- 
crofcopes of this fort. 
Each degree of the femicircle being divided into two parts 
or 30', and one revolution of the micrometer head moving 
the wire in the field of the microfcope f ; therefore in 10 re- 
volutions it changes its place half a degree or 30^ which are 
fhewn by a fcale of 10 notches in the upper part of the field 
of the microfcope, and alfo reprefented towards the top of the 
plate. Each notch correfponds to 3 minutes or 180 ieconds, 
and the head being divided into 3 minutes, and each minute 
into 12 equal parts, therefore each part is of the value of five 
feconds. 
Art. XVL Concerning the Semicircle . 
With regard to the femicircle, which has been repeatedly 
mentioned in the courfe of this defcription, it is yet neceffary 
to make fome remarks ; and particularly to (hew how, by its 
means, the axis of vifion of the telefcope, when adjufted, is 
brought and kept truly horizontal, which is effected in the 
following manner. 
Gn the oppofite fides of the horizontal bar that carries the tele- 
fcope there are fixed four fmall, but finely polifhed bell-metal 
planes, two on each fide, on the right and left of the top of the 
vertical axis, in fuch a manner as that the furfaces of the two 
on either fide are directed to or in the fame plane with the 
center of the axis of the telefcope. Thefe planes will be befit 
conceived 
