of a L amp- Micro meter. r 69 
obferver goes to meafure their didance, it will not be neceffarv 
to fay more upon the fubjedh 
I am now to (hew the application of this inflrument. It is 
well known to opticians and others, who have been in the habit 
of ufing optical indruments, that we can with one eye look into 
a microfcope or telefcope, and fee an object much magnified, while 
the naked eye may fee a fcale upon which the magnified picture 
is thrown. In this manner I have generally determined the 
power of my telefcopes ; and any one who has acquired a 
facility of taking fuch obfervations will very feldom midake- fo 
much as one in fifty in determining the power of an indru- 
ment, and that degree of exadtnefs is fully fufficient for the 
purpofe. 
The Newtonian form is admirably adapted to the ufe of this 
micrometer ; for the obferver (lands always erect, and looks in 
a horizontal direction, notwithftanding the telefcope (hould be 
elevated to the zenith. Bolides, his face being turned away 
from the object to which his telefcope is directed, this micro- 
meter may be placed very conveniently without caufing the 
lead obftrudtion to the view : therefore, when I ufe this indru- 
ment I put it at ten feet didance from the left eye, in a line 
perpendicular to the tube of the telefcope, and raife the move- 
able board to fuch a height that the lucid point of the central 
lamp may be upon a level with the eye. The handles, lifted 
up, are paffed through two loops fadened to the tube, jud by 
the obferver, fo as to be ready for his ufe. I fhould oblerve, 
that the end of the tube is cut away fo as to leave the left eye 
intirely free to fee the whcjle micrometer. 
Having now directed the telefcope to a double dar, I view it 
with the right eye, and at the lame time with, the left fee it pro- 
Vom LXXII, Z jetted 
