284 Mr. Cavallo’s Defcription of 
pocket ones. 3dly, They do not meafure the angle without 
fome lofs of time, which is neceffary to turn the fcrew, or to 
move fome other mechanifm. 4thly, and laftly, They are 
confiderably expenfive, fo that fome of them coft even more 
than a tolerably good telefcope. 
After having had long in view the conftrudtion of a micro- 
meter, which might be in part at leaft, if not intirely, free from 
all thofe objections ; and, after various attempts, I at laft fuc- 
ceeded with a Ample contrivance, which, after repeated trials, 
has been found to anfwer the defired end, not only from my 
own experience, but from that alfo of feveral friends, to whom 
it has been communicated. 
This micrometer, in fhort, confifts of a thin and narrow 
flip of mother of pearl finely divided, and fituated in the focus 
of the eye-glafs of a telefcope, juft where the image of the 
objeft is formed. It is immaterial whether the telefcope be a 
refradlor or a refleftor, provided the eye-glafs be a convex lens, 
and not a concave one, as in the Galilean conftrudtion. 
The limpleft way of fixing it is to flick it upon the dia- 
phragm, which generally ftands within the tube, and in the 
focus of the eye-glafs. When thus fixed, if you look through 
the eye-glafs, the divifions of the micrometrical fcale will ap- 
pear very diftindl, unlefs the diaphragm is not exadtly in the 
focus ; in which cafe the micrometrical fcale mu ft be placed 
exadly in the focus of the eye-glafs, either by pufhing the 
diaphragm backwards or forwards, when that is practicable ; 
or elfe the fcale may be eafiiy removed from one or the other 
furface of the diaphragm by the interpohtion of a circular 
piece of paper or card, or by a bit of wax. This conftruction 
is fully fufficient when the telefcope is always to be ufed by 
the fame perfon ; but when different perfons are to ufe it, then 
