a Jimple Micrometer. 285 
the diaphragm, which fupports the micrometer, muft be con- 
ftruded fo as to be eafily moved backwards or forwards, though 
that motion needs not be greater than about a tenth or an 
eighth of an inch. This is necelfary, becaufe the diftance of 
the focus of the fame lens appears different to the eyes of dif- 
ferent perfons, and therefore, whoever is going to ufe the tele- 
fcope for the menfuration of any angle, muft firft of all un- 
fcrew the tube, which contains the eye-glafs and micrometer, 
from the reft of the telefcope, and, looking through the eye- 
glafs, muft place the micrometer where the divifions of it may 
appear quite diftindt to his eye. 
In cafe that any perfon Ihould not like to fee always the 
micrometer in the field of the telefcope, then the microme- 
trical fcale, inftead of being fixed to the diaphragm, may be 
fitted to a circular perforated plate of brafs, wood, or even 
paper, which may be occafionally placed upon the faid dia- 
phragm. 
I have made feveral experiments to determine the moft ufe- 
ful lubftance for this micrometer. Glafs, which I had fuc- 
cefsfully applied for a fimilar purpofe to the compound micro- 
fcope, feeraed at firft to be the moft promifing ; but it was at 
laft rejected after feveral trials : for the divifions upon it gene- 
rally are either too fine to be perceived, or too rough ; and 
though with proper care and attention the divtiions may be 
proportioned to the fight, yet the thicknels of the glafs ltfeit 
obftrudts in fome meafure the diftindt view of the objedt. 
Ivory, horn, and wood, were found ufelefs xor ihe conftruc- 
tion of this micrometer, on account of their bending, fwel- 
ling, and contracting very eafily ; whereas mother of pearl is 
a very fteady fubftance, the divifions upon it may be marked 
very 
