S72 
SINGLE-LENS MICROMETER. 
MeasuremeiTt 
of a very mi- 
iiute object or 
angle. 
the scale more accurate, a similar method was in fact pursued 
with several gold wires, of different sizes, weighed with equal 
Crire ; and the subdivisions of the exterior scale were made to 
correspond with the average of their indications. 
In making use of this micrometer for taking the measure of 
any object, it would be sufficient at any one accidental position 
of the tube to note the number on the outside as denominator, 
and to observe the number of divisions and decimal parts, 
which the subject of examination occupies on the interior 
scale, as numerator of a fraction expressing its dimensions in 
proportional parts of an inch ; but it is preferable to obtain an 
integer as numerator, by sliding the tube inward or outward, 
till the image of the wire is seen to corresi>ond with some 
exact number of divisions, not only for the sake of greater 
simplicity in the arithmetical computation, but because we can 
by the eye judge more correctly of actual coincidence, than of 
the comparative magnitudes of adjacent intervals. 
The smallest quantity, which the graduations of this instru- 
ment profess to measure, is less than the eye can really ap- 
preciate in sliding the tube inward or outward. If, for 
instance, the object measured be really it nnay appear 
ToVo 7 "utW? which case the doubt amounts to 1 -50th 
part of the whole quantity. But the difference is here ex- 
ceedingly small in comparison to the extreme division of other 
instruments where the nominal extent of its power is the 
same. A micrometer with a divided eye-glass, may profess to 
measure as far as -t- 'tt- of an inch : but the next division is 
— or ijr- ; and though the eye may be able to distin- 
guish that the truth lies between the two, it receives no assist- 
ance within 5 part of the la%er measure. 
On 
