SINr.LE-LENS MICROMETER. 
371 
The external appearance of the whole instrument is that of description of 
, . , . r,., , the instrunitut. 
a common telescope, consisting ot three tubes. 1 he scale 
occupies the place of the object glass, and the little lens is 
situated at the smaller end, with a pair of plain glasses sliding 
before it, between which the subject of e.xamination is to be 
included. I'his part of the apparatus is shewn separately in 
fig- 3. It has a projection at «, with a perforation through 
which a pin is inserted, to connect it w'ith a screw represented 
at b, fig. 2. This screw gives lateral motion to the object, so 
as to make it correspond with any particular part of the scale. 
The lens has also a small motion of adjustment by means of the 
cap. t, fig. 3, which renders the view of the magnified object 
distinct. 
Before the instrument is completed, it is necessary to deter- 
mine with precision the indications of the scale, which must 
be diflerent according to the distance to which the lube is drawn 
out. In my instrument, one division of the scale corresponds to- 
of an inch, when it is at the distance of lG,tj inches 
from the lens } and since the apparent magnitude in sm.aM 
angles varies in the simple inverse ratio of this distance, each 
division of the same scale will correspond to --5^7 *l><i 
distance of 8 inches, and the intermediate fractions 
TT-'-- &c. are found by intervals of I, dO iuch marked 
on the outside of the tube. The basis on whicli these indica- 
tions were founded in this instrument, w.ig 3 wire carefully 
ascertained to be -j-t- of an inch in diameter, the magnified 
image of which occupied fifty divisions of the sc.n1e, when it 
was at the distance of ld,d inches, and hence one division 
_ ^ _ ... Since any error in the original estimate 
~5.)X.'00 unuo 
of this wire must pervade a'l .subsequent me.asures derived from 
it, the substance employed was pure gold drawn till fifty-two 
inches in length, weighed exactly five grain*. If we assume 
the specific gravity of gold to be 10, 3d, a cylindrical inch will 
weigh 3837 grains, and we may thence infer the diameter of , 
such a wire to be of an inch, more neariy than can be 
ascertained by any other method. For the sake of rendering 
tire 
