MICROMETER. 
whereas the most perfect of my former 
micrometers, with the same instrument, had 
a scale of less tlian the 2000th pai t of an 
inch to a second. 
“ The measures of this micrometer are 
not confined to double stars only, but may 
be applied to any other objects that require 
the utmost accuracy, such as the diameters 
of the planets or their satellites, the moun- 
tains of the moon, the diameters of the 
fixed stars, &c.” 
We shall now give an account of a micro- 
meter by Mr. Troiighton, wdiich is applied 
to the eye-piece of a telescope to measure 
exceedingly small angles, as the diametei of 
the heavenly bodies, &c. Plate Microscope, 
&c.fig. 6, is an orthography projecting end- 
ways ; fig. 7, a section of the box containing 
the wires; and, fig. 8, a section lengthways : 
the same letters, as far as they can, are used 
in all the figures. Figs. 6 and 8, A is an eye- 
tube containing a convex lens at each end, 
this slides in another tube, d d, so as to ad- 
just the glass to distinct vision of the wires; 
the tube!^ dd, is screwed into another, bb, 
which is much larger, through this a thin 
long box, D D D, containing the wire slides. 
The micrometer is screwed to the telescope 
by a male screw, ee, (fig. 8.) in the same 
piece with which is a circular plate, //, cut 
all round with fine teeth, this plate fits 
against the flat bottom of the box, 6, and 
turns round concentycally with it by means 
of a ring, fc, fitting into a conical hole in the 
centre of the plate,//, and screwed to the 
box-; a small endless screw, h, (fig. 6.) turn- 
ing in two brass collars screwed to the box, b, 
works in the teeth cut round the plate,/, and 
by that means when the milled head on tlie 
arbor of the endless screw is turned, it turns 
the eye-tube and box, D D, round, to bring 
it to any convenient position for measuring 
the angles required ; the box containing the 
wires is shewn open in fig. 7, it containing^ 
two frames, /i/t/i, and llllj one sliding 
within another, which moves in the box, 
without lateral shake, yet fitted so as to 
slide easily backwards and forwards in the 
box, by the screws, m and n, in the same 
manner as the microscope in the upper part 
of the same plate ; o and p are springs to 
counteract the screws and make the motion 
pleasant. A wire is stretched across the 
frame, hh, at right angles to its sides, and 
another of the same size is fixed across the 
slider, llll, exactly parallel to the former; 
a small quantity of the underside of the lat- 
ter is cut away, and its wire is fixed in an- 
otlier plane to the wire of hhh, so that the 
wires can pass each other withont foiiching, 
but as near as possible ; when they arc 
placed by their screws over each other, and 
viewed through the eye-tube, they appear 
but as one wire : the divided circle, x, ou 
the nuts of screws are then slipped round, 
without the screw, to bring the first division 
on them to the index I ; the instrument is 
now adjusted for observing any angle, it is 
screwed to the telescope, and by the end- 
less screw, h, (fig. 6.) the micrometer is 
turned round so as to bring a fixed wire, w, 
which is perpendicular to the others, to 
cover the two objects ; the two wires are 
then separated by turning either of the nuts, 
F, until the wires include the angle to be 
measured: the whole box (fig. 7.) of the 
micrometer slides through the tube, in the 
direction of its length, to follow any moving 
object. When the observation is complet- 
ed it is read off' by a scale of notches in the 
box, (fig. 7.) determining the number of re- 
volutions the screw has made, and the divi- 
sions pointed out on the circles, x ; by the 
indexes, 1 1, the number of aliquot parts is 
denoted ; the circular plate, ff, is divided 
into degrees as shewn in fig. 6, and it is by 
this that the angle line measured makes 
with the horizon is registered. 
The circles are divided in 100 parts, and 
have no determinate value in angular mea- 
surement, but their value is determined ex- 
perimentally by observing tlirough the tele- 
scope, it is applied to the diameter of the 
sun, or any other body whose angular mea- 
sure has been previously and accurately 
determined by some other divided instru- 
ment, and from this the angle given by each 
observation is calculated. 
The micrometer has not only been applied 
to telescopes, and employed for astronomi- 
cal purposes ; but there have been various 
contrivances for adapting it to microscopical 
observations. M. Leeuwenhoek’s method 
of estimating the size of small objects, was 
by comparing them with grains of sand, 
of which 100 in a line took up an inch. 
These grains he laid upon the same plate 
with his objects, and viewed them at the 
same time. Dr. Jurin’s method was similar 
to this ; for he found the diameter of a 
piece of fine silver wire, by wrapping it very 
close upon a pin, and observing how many 
rings made an inch : and he used this wire 
in the same manner as Leeutvenhoek used 
his sand. Dr. Hook used to look upon the 
magnified object with one eye, while, at 
the same time, he viewed other objects, 
placed at the same distance, with the othei 
