1^6 Gen. Roy’s Account of 
into 60 equal parts, each of which reprefents one fecond of 
angular motion of the telefcope. By examining the particular 
plan of this Reel Aide, it will be feen, that it is attached by a 
chain to the fpring of a watch, coiled up in the ufual manner, 
within a fmall barrel adjacent to it in the frame. By this pro- 
vifion no time whatever is loft; the fmalleft motion of the 
head being inftantly (hewn, by a proportionable motion 
of the wire, to one hand or the other, in the field of the 
microfcope. 
It is necefiary to remark, that the whole microfcope be- 
tween its pillars can be raifed or deprefled a little more or lefs, 
with regard to the plane of the circle, by the help of two fteel 
levers, feen one on each fide of the elevation, which for that 
purpofe are applied in the holes reprefented above and below 
the proje&ing plate that unites the tops of the pillars. By 
means of this motion, diftindtnefs is obtained at the wires ; 
and by the motion of the proper fcrew of the object lens, 
which neceffarily follows that given to the whole microfcope, 
the fcale is fo adjufted as that fifteen revolutions of the head 
fhall move the wire over fifteen minutes, or one grand divifion, 
on the limb, equal to nine hundred feconds, each degree on 
the circle being only divided into four parts. This operation 
being delicate, requires great patience and many repetitions, 
before the purpofe can be exactly, or even nearly, effedted : 
for at the fame time that the fixed wire mu ft bifedt the dot on 
the gold tongue, the moveable wire muft alfo bifedt the dot at 
i8o c on the limb, as well as the firft notch in the magnified 
fcale at the bottom of the plate, where the minutes in the field 
of the microfcope are reprefented in the proportion of between 
fifteen and fixteen to one as painted on the eye of the obferver. 
In this adjuftment there is yet another circumftance to be 
5 attended 
