'The Microfcope explained. xli;c 
piecd of ivory, about 2-ioths broad, the fir ft inch being 
intirely ivory, is fubdivided into io equal parts. See 
%• 23. 
A piece of glafs, fixed in a brafs or ivory Aider, on 
which is drawn two parallel lines, acrofs its diameter, 
about 3-ioths of an filch long; each tenth being di¬ 
vided, viz. one into three, the fecond into four, and 
the third into five parts : this is called a micrometer. 
See fig. 24. 
The ufc of thefe are as follow ; place the micrometer 
fig. 24. on the middle of the flage, and the rule fig. 23. 
on one fide, but parallel to it; then look into the mi¬ 
crofcope with one eye, keeping the other open, obferve 
how many parts, one tenth of a line in the microfcope 
takes in upon the parts of the rule feen by the naked 
eye. For example, fuppofe with a fourth magnifier, 
that i-ioth of an inch magnified; anfwers in length to 
forty tenths or parts on the rule when feen by the naked 
eye; which fhews that this magnifier increafes the dia¬ 
meter of the objedt forty times. 
The fimplicity of this method, gives general fatisfac- 
tion to thofe who can manage it; but there are fome : 
people who find it difficult, from their not having been! 
aCcuftomed to obferve with both eyes open, for fuch per- 
fons another micrometer is provided,* 
A micrometer for a compound microfcope. 
F ' IG. 28. reprefents this micrometer; it is applicable 
to the body of any compound microfcope,- It was 
made for his majefty in the year, 1761, and with it was 
then prefented a manufcript of its ufe, of which the fol¬ 
lowing is an extra# with feme alterations. 
The ferew has fifty threads to an inch, it carries ah 
index, pointing to the divifions on a circular plate, fixed 
e a* 
