028 OPT 
obfervation. The next imperfefti'on is, that every mi¬ 
crometer hitherto ufed requires either a.fcrew, or a di¬ 
vided bar and pinion, to meafure the diftance of the wires 
or the two images. Thofe acquainted with works of this 
kind are fenfible how difficult it is to have fcrevvs perfe&ly 
equal in every thread or revolution of each thread; or 
pinions and bars that fhall be fo evenly divided as to be 
depended upon in every leaf and tooth, to the two or 
three thoufandth part of an inch : and yet, on account of 
the final 1. fcale of thofe micrometers, thefe quantities are 
of the greateft confeqncnce ; an error of a tingle thou¬ 
fandth part inducing in moft inftruments a miftake of 
feveral feconds. The greateft ini perfection of all is, that 
the wires require to be iiiuminated; and, when Dr. 
lierfchel had double ftars to meafure,..one of which was 
very obfcure, he was obliged to be content with lefs 
light than is, neceffary to make the wires diftindl; and 
feveral ftars on this account could not be meafured at all, 
though not too clofe for the micrometer. 
Dr. Herfchel, therefore, was led to direft his attention 
to the improvement of thefe inftruments ; and the refult 
of his endeavours has been a very ingenious lamp-micro- 
tneter, which is not only free from the imperfections above 
fpecified, but alfo poffeffes the advantages of a large fcale. 
This inftrument he applied to a Newton’s reflecting te- 
lefcope. (See Phil. Tranf. vol. Ixxii. and Ixxiii.) Two 
movable lamps, the light of which comes through two 
fmall holes, are placed at a convenient diftance from the 
telefcope, in the direction at which you look at the image. 
Thefe points of light are looked at by the left eye, and 
brought to the oppofite Tides of a planet looked at by the 
right eye ; and, by meafuring their diftance from each 
other, and from the eye, the angle under which the mag¬ 
nified diameter appears will be known, which, divided by 
the magnifying power of the telefcope, gives the apparent 
diameter required. 
The conltruCHon of this micrometer is as follows : 
ABGCFE, fig. 6, is a ftand nine feet high, upon which a 
femi-circular board, qhogp, is movable upwards or down¬ 
wards, in the manner of (ome fire-fcreens, as occafion may 
require, and is held in its fituation by a peg p, put into 
any one of the holes of the upright piece AB. This 
board is afegment of a circle of fourteen inches radius, 
and is about three inches broader than a femi-circle, to 
give room for the handles, ;D, eP, to work. The ufe of 
this board is to carry an arm L, thirty inches long, which 
is made to move upon a pivot at the centre of the circle, 
by means of a firing which pafies in a groove upon the 
edge of the femi-circle pgohi/; the firing is faftened to a 
hook at o, (not expreffed in the figure, being at the back 
of the arm L,) and, palling along the groove from oh to q, 
is turned overa pulley at </,and goes down to a fmall barrel 
at e, within the plane of the circular board, where a double- 
jointed handle, eP, commands its motion. By this con¬ 
trivance we fee the arm L may be lifted up to any altitude, 
from the horizontal polition to the perpendicular, or be 
fnft’ered to delcend by its own weight below the horizon¬ 
tal to the reverfe perpendicular fituation. The weight of 
the handle P is fufficient to keep the arm in any given 
pofition ; but, if the motion Ihould be too eafy, a friifion- 
fpring applied to the barrel will moderate it at pleafure. 
In front of the arm L, a fmall Aider, about three inches 
long, is movable in a rabbet from the end L towards the 
centre backwards and forwards. A firing is faftened to 
the left fide of the little Aider, and goes towards L, where 
it pafies round a pulley at m, and returns under the arm 
from m, n, towards the centre, where it is led in a groove 
on the edge of the arm, which is of a circular form, up¬ 
wards to a barrel (raifed above the plane of the circular 
board) at r, to which the handle, rD, is faftened. A 
lecond firing is faftened to the Aider, at the right fide, 
and goes towards the centre, where it pafies over a pulley 
n ■ and the weight w, which is fufpended by the end of 
this firing, returns the Aider towards the centre, when a 
contrary turn of the handle permits it to adt. 
ICS. 
a and h are two- final! lamps, two inches high, one inch 
and a half in breadth, by one inch and a quarter in depth. 
The tides, back, and top, are made fo as to permit no 
light to be feen ; and the front confifts.of a thin brails 
Aiding door. The flame in the lamp, tt, is placed three- 
tenths of an inch from the left fide, three-tenths from the 
front* and half an inch from the bottom. In the lamp b, 
it is placed at the fame height and diftance, meafuring 
from the right fide. The wick of the flame confifts only 
of a fingle very thin lamp-cotton thread ; for, the (mailed: 
flame being fufficient, it is eafier to keep it burning in fa- 
confined a place. In the top of each lamp muft boa little 
flit lengthways, and alfo a fmall opening in one fide near 
the upper parr, to permit air enough to circulate to feed 
the flame. To prevent every reflection of light, the fide- 
opening of the lamp a ftiould be to the right, and that of 
the lamp b to the left. In the Aiding door of each lamp 
is made a fmall hole, with the point of a very fine needle* 
juft oppofite the place where the wicks are burning, fa 
that, when the Aiders are flint down, and every thing dark, 
nothing fha.il be feen but two fine Lucid points of the fize 
of two ftars of the third or fourth magnitude. The lamp- 
a is placed fo that its lucid point may be in the-centre of 
the circular board, where it remains fixed. The lamp b 
is hung to the little Aider, which moves in the rabbet of 
the arm, fo that its lucid point, in a horizontal pofition 
of the arm, may be on a level with the lucid point in the 
centre. The movable lamp is fufpended upon a piece of 
brafs, faftened to the Aider by a pin exactly behind the 
flame upon which it moves as a pivot. The lamp is ba¬ 
lanced at the bottom by a leaden weight, fo as always to 
remain upright, when the arm is either lifted above, or 
depreffed below, the horizontal pofition. The double- 
jointed handles, rD, eP, confift of light deal rods, ten 
feet long, and the lowed: of them may have divifions 
marked upon it, near the end P, expreffmg exactly the dif- 
tance from the central lucid point in feet, inches, and 
tenths. 
From this conftru&ion we fee, that a perfon at a dif¬ 
tance of ten feet may govern the two lucid points, fo as 
to bring them into any required pofition, fouth or north, 
preceding or following, from o to go 0 , by uiingtbe handle 
P, and alfo to any diftance, from fix-tenths of an inch to 
five or fix and twenty inches, by means of the handle D. 
If any refleftion or appearance of light ihould be left from 
the top or rides of the lamp, a temporary fcreen, confid¬ 
ing of a long piece of pafteboard, or a wire-frame covered 
with black cloth, of the length of the whole arm, and of 
any required breadth, with a flit of half an inch broad in 
the middle, may be affixed to the arm by four bent wires, 
projefling an inch or two before the lamps, fituated fo 
that the movable lucid point may pafs along the opening 
left for that purpofe. 
Fig. 8. repreients part of the arm L, half the real fize; 
S, the Aider; m, the pulley, over which the cord xty\ is 
returned towards the centre ; v, the other cord going to 
the pulley, n, of fig. 7 ; R, the brafs piece movable upon 
the pin c, to keep the lamp upright. At R is a wire 
rivetted to the brafs piece, upon which is held the lamp 
by a nut and fcrew. Figs. 9 and 10 reprefent the lamps 
a, l, with the Hiding doors open, to fhow the fituation of 
the wicks. W is the leaden weight, with a hole d in it, 
through which the wire R, of fig. 8, is to be palled, when 
the lamp is to be faftened to the Aider S. Fig. 11 repre- 
fents the lamp a, with the Aiding door fhut; /, the lucid 
point; and ik, the openings at the top, and s at the fides, 
for the admiflion of air. 
The motions of this micrometer are capable of great 
improvement, by the appl carion of wheels and pinions, 
and other well-known mechanical refources; but, as the 
principal objedf is only to be able to adjuft the two lucid 
points to the required pofition and diftance, and to keep 
them there for a few minutes, while the obferver goes to 
meafure their diftance, it will not be neceffary to fay mor« 
upon the fubjedt. 
Mr. 
