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On Micrometer Telescopes. 15 
affords a very pleasing variety in the use of microscopes and 
telescopes.—Of Dr. Brewster’s instrument I knew nothing, till 
1 saw an account of it, a few days ago, in his book. 
In the Doctor’s note it appears, that I use lines on a slip of 
mother-of-pearl, in my instrument: but this is a mistake ; for 
it is expressly mentioned in my paper, that my micrometer con- 
sists of a number of parallel lines drawn upon a piece of plane 
glass. j 
Soon after I had formed the design of this instrument, I ap- 
plied to the late Mr. Coventry of Southwark, for a set of his 
micrometers. Mr. ©. had none at that time, but promised to 
send me a set, which I never received though I waited for them 
upwards of two years. But, not being willing to give up the 
plan of my instrument, without trying how far it might be ren- 
dered useful in practice, 1 undertook to draw lines upon glass 
myself; and after an infinite number of unsuccessful attempts, 
I succeeded so far as to draw parallel lines +;,,th of an inch 
apart, which being intersected at right angles by equal lines 
form minute squares, each one-millionth part of a superficial 
inch. Whence I concluded that lines drawn, by the same me- 
thod, at ,1,th of an inch distant would be sufficiently accurate 
for my purpose. 
I am surprised to find so little use made of Mr. Coventry’s 
glasses ; for, if they were properly applied to optical instruments, 
micrometers might be made that would supersede the use of all 
others. For measuring small objects before the microscope, 
these micrometers are unquestionably the best, both for precision 
and readiness of application; and they may be applied to tele- 
scopes for measuring small angles with equal advantage. To 
illustrate this position, let the second example be taken from 
p. 128, vol. xxxviii. of The Philosophical Magazine. 
“ The magnifying power of the telescope is 45 times. 
“ Micrometer divisions ;1,th of an inch distant. 
*¢ Each division subtends an angle at the eye = 30”. 
“ One inch of the eye-tube is .. tet aa 
But if the divisions of the micrometer were drawn ;25th of 
an inch distant, which may be doue with great exactness: Then, 
Each division would subtend an angle .. = 99 = 15”, 
And one inch of the eye-tube would be .. = 52 = 26” 
Consequently, 
yoth of an inch on the eye-tube’ a6 ee ie? is 
wom bee Peg: rashart rt al eb 
tooth of ditto ., would be Eee ob” tS 3 
gecth of ditto .. would be aot. = OS”) 5 
As the divisions of the eye-tube may be read off to within 
vyoth part of an inch by a vernier scale, consequently the av 
wou 
