a jimple Micrometer. 2S7 
the hundredths of an inch is quite fufficient, and one of its 
divifions is equal to little lefs than three minutes, fo that an 
angle of a minute may be meafured by it. 
In looking through a telefcope, furniffied with fuch a micro- 
meter, the field of view appears divided by the micrometer 
Icale, the breadth of which occupies about oue-feventh part of 
the aperture, and as the fcale is femitranfparent, that part of 
the object, which happens to be behind it, may be difcerned 
fufficiently well to afcertain the divifion, and even the quarter 
of a divifion, with which its borders coincide. Fig. 2. (hews 
the appearance of the field of my telefcope with the micro- 
meter, when directed to the title page of the Philofophical 
Tranfa&ions, wherein one may obferve that the thicknefs of 
the letter C is equal to three-fourths of a divifion, the dia- 
meter of the O is equal to three divifions, and fo on. 
At firft view one is apt to imagine that it is difficult to 
count the divifions which may happen to cover or to meafure 
an objed ; but upon trial it will be found, that this is readily 
performed; and even people, who have never been ufed to 
obferve with the telefcope, foon learn to meafure very quickly 
and accurately with this micrometer ; for, fince every fifth and 
tenth divifion is longer than the reft, one foon acquires the 
habit of faying five, ten, fifteen, and then, by adding the 
other divifions lefs than five, completes the reckoning. Even 
with a telefcope, which has no ftand, if the objed end of it 
be refted againft a fteady place, and the other end be held by 
the hand near the eye of the obferver, an objed may be mea- 
fured with accuracy fufficient for feveral purpofes, as for the 
eftimation of fmall diftances, for determining the height of a 
-houfe, &c. 
After 
