OPTICS. 
the micrometer is fixed to telefcopes of different focal 
lengths, a new table mull be made. The whole fyftem 
of wires is turned about in its own plane, by moving the 
eye-tube round with the hand; and by that means the 
wire nm can be thrown into any pofition, and confequently 
angles in any pofition may be meafured. Dr. Bradley 
adde'd a fmall motion by a rack and pinion to let the wires 
more accurately in any pofition. 
But the micrometer, as now contrived, is of ufe, not 
only to find the angular diltance of bodies in the field of 
view at the fame time, but alfo of thole which, when the 
telefcope is fixed, pafs through the field of view fuccef- 
fively; by which means we can find the difference of 
their right afcenlions and declinations. Let A a, B b, Cc, 
fig. 2. be three parallel and equi-dillant wires, the mid¬ 
dle one bifefting the field of view ; HOR a fixed wire per¬ 
pendicular to them, palling through the centre of the field ; 
and Ff, Gg, two wires parallel to it, each movable by a 
micrometer-ferew, as before, fo that they can be brought 
up to HOR, or a little beyond. Then, to find the angu¬ 
lar dillance of two objects, bring them very near to B b, 
and in a line parallel to it, by turning about the wires, 
j and bring one upon HOR, and by the micrometer-fcrew 
make Ff or Gg- pafs through the other; then turn the 
(crew till that wire coincides with HOR, and the arc, which 
the index has palled over, (how's their angular dillance. 
It the objects be farther remote than you can carry the 
dillance of one of the wires Ff, Gg, from HOR, then 
bring one object to Ff and the other to Gg; and turn 
each micrometer-fcrew till they meet, and the fum of the 
arcs, palled over by each index, gives their angular dis¬ 
tance. If the objefts be two liars, and one of them be 
made to run along HOR, or either of the movable -wires 
as occafion may require, the motion of the other will be 
parallel to thefe wires, and their difference of declina¬ 
tions may be obferved with great exaflnefs; but, in taking 
any other alliances, the motion of the liars being oblique 
to them, it is not quite fo eafy to get them parallel to B b; 
becaufe, if one liar be brought near, and the eye be ap¬ 
plied to the other to adjull the wires to it, the former liar 
will have gotten a little away from the wire. Dr. Bradley, 
in his account of the ufe of this micrometer, publilhed 
by Dr. Malkelyne in the Philofophical Tranfaitions for 
177a, thinks the bell way is to move the eye backwards 
and forwards as quick as polfible ; but it feems to be bell 
to fix the eye at fome point between, by which means it 
takes in both at once fufficiently well defined to compare 
them with B b. In finding the difference of declinations, 
if both, bodies do not come into the field of view at the 
fame time, make one run along the wire HOR, as before, 
and fix the telefcope and wait till the other comes in, and 
then adjull one of the movable wires to it, and bring it up 
to HOR, and the index gives the difference of their de¬ 
clinations. The difference of time between the paffage 
of the liar at either of the crofs movable wires, and the 
tranfit of the other liar over the crofs fixed wire (which 
reprefents a meridian), turned into degrees and minutes, 
will give the difference of right afeenfion. The (tar has 
been here fuppofed to be bife&ed by the wire ; but, if the 
wire be a tangent to it, allowance mull be made for the 
breadth of the wire, provided the adjullment be made for 
the coincidence of the wires. In obferving the diameters 
of the fun, moon, or planets, it may perhaps be moll con¬ 
venient to make ufe of the outer edges of the wires, be¬ 
caufe they appear mod diftinit when quite within the 
limb ; but, if there lliould be any fenfible inflection of 
the rays of light in palling by the wires, it will be bed 
avoided by uling the inner edge of one wire and the out¬ 
ward edge of the other; for by that means the inflection 
at both limbs will be the fame way, and therefore there 
will be no alteration of the relative pofition of the rays 
palling by each wire. And it will be convenient in the 
micrometer to note at what divifion the index dands when 
the movable wire coincides with HOR ; for then you need 
not bring the wire, when a flar is upon it, up to HOR, 
Vol. XVII. No. 1203. 
625 
but only reckon from the divllrdn at which the index: 
then dands to the above divifion. 
Wolfius deferibes a micrometer of a very eafy and Am¬ 
ple druCture, fil’d contrived by Kircher, thus : In the 
focus of a telefcope fit a brafs or iron ring AB, fig 3, with 
female ferews diametrically oppolite to each other ; into 
thefe infert male ferews CE and FB, of fuch length as 
that they may be turned in the tube fo as to touch each 
other; and with this inflrument very fmall fpaces in the 
heavens may be accurately meafured ; for, when any ob¬ 
jects, viewed through a tube, appear contiguous to the 
ferews, if thefe be turned till they jud touch two oppo¬ 
lite points, vvhofe didance is to be meafured, it will be 
evident how many threads of the ferew they are apart. 
To determine how many feconds anfwer to each thread, 
applying the tube towards the heavens, turn the ferews 
tili they touch two points whofe diltance is already ac¬ 
curately known ; and obferve the number of threads cor- 
refponding to that interval; thus, by the rule of three, 
a table may be made of the feconds correfponding to the 
feveral threads; by means of which, without greater la¬ 
bour, the dillances of any points may be determined. 
The druCture of another micrometer, with the manner 
of fitting it to the telefcope, and applying it, is as follow's : 
ABCg, fig. 4, is a rectangular brafs frame, the fide AB 
being about three inches long, and the fide BC, as like- 
wife the oppolite fide Ag, about fix inches ; and each of 
the three (ides about eight-tenths of an inch deep: the 
two oppolite (ides of this frame are ferewed to the circu¬ 
lar plate, to be mentioned hereafter. The IcrewP, which 
has exaClly forty threads in an inch, being turned round, 
moves the plate GDEF, along two grooves made near the 
tops of the oppolite fides of the frame ; and the ferew Q, 
having the fame number of threads in an inch as P, moves 
the plate RNMY along two grooves, made near the bot¬ 
tom of the laid frame, in the fame direction as the former 
plate moves, but with only half the velocity of that other; 
thefe ferews are turned both at once, and fo the plates are 
moved along the fame way, by means of a handle turning 
the endlefs ferew S, whole threads fall in between the 
teeth of the pinions on the ferews P and Q. And note, 
that tw'o half revolutions of the endlefs ferew S carry the 
ferew P exaCtly once round. The ferew P turns the 
hand a, fallened to it, over a hundred equal divifions, 
made round the limb of a circular plate, to which the 
above-named two oppolite fides of the frame are ferewed 
at right angles; the teeth of the pinion on the ferew P, 
whofe number is 5, take into the teeth of a wheel on the 
back fide of the circular plate, vvhofe number is 25. 
Again, on the axis of this wheel is a pinion of two, u'hich 
takes into the teeth of another wheel, moving about the 
centre of the circular plate, on the outfide of it, and 
having fifty teeth ; this lad wheel moves the fmaller hand 
b once round the above-mentioned circular plate in the 
100th part of the time the hand a is moving round; for, 
as the number of teeth in the pinion of the ferew P is 5, 
and the number of teeth of the wheel thjs pinion moves 
is 20, therefore the ferew P moves four times round in 
the time that wheel is moving once round. Further, 
fince there is a pinion of two which takes into the teeth 
of a wheel whofe number is 50; therefore this wheel 
with 50 teeth will move once round in the time that the 
wheel of 20 teeth moves 25 times round ; and, confe¬ 
quently, the ferew P, or hand a, mull move a hundred 
times round in the fame time as the wheel of 50 teeth, or 
the hand b, has moved once round. 
Hence it follows, that, if the circular plate W, which 
is fallened at right angles to the other circular plate, be 
divided into two hundred equal parts, the index x, to 
which the handle is fallened, will move five of thofe parts 
in the fame time in which the hand a moves one of the 
hundred divifions round the limb of the other circular 
plate. Thus, by means of an index x, and plate W, every 
fifth part of each of the divifions round the other plate 
may be known. Further, fince each of the ferews P and 
7 U Q has 
