2S6 Mr . Cavallo’s Defcription of 
very easily, and, when it is made as thin as common writing 
paper, it has a very ufeful degree of tranfparency. 
Tab. VIII. fig. i. exhibits this micrometer fcale, but 
fhews it four times larger than the real fize of one, which 
I have adapted to a three-feet achromatic telefcope, that 
magnifies about 84 times. It is fomething lefs than the 
a 4 th part of an inch broad; its thicknefs is equal to that of 
common writing paper ; and the length of it is determined 
by the aperture of the diaphragm, which limits the field of 
the telefcope. The divifions upon it are the 2ocdths of an 
inch, which reach from one edge of the fcale to about the 
middle 01 it, excepting every fifth and tenth divifion, which 
are longer. The divided edge of it paffes through the center 
of the field of view, though this is not a necefiary precaution 
in the conftrudtion of this micrometer. Two divifions of the 
above delcribed fcale in my telefcope are very nearly equal to 
one minute ; and as a quarter of one of thole divifions may be 
very well diffinguilhed by eftimation, therefore an angle of 
one-eighth part of a minute, or of 7% may be meafured 
with it. 
When a telefcope magnifies more, the divifions of the mi- 
ciometer miift be more minute; and I find, that when the 
focus of the eye-glafs of the telefcope is fhorter than half an 
inch, the micrometer may be divided with the ^oodths of an 
inch ; by means of which, and the telefcope magnifying about 
200 times, one may eafily and accurately meafure an angle 
Imaller than half a fecond. 
On the other hand, when the telefcope does not magnify 
above 30 times, the divifions need not be fo minute : for in- 
flance, in one of Dollond’s pocket telefcopes, which when 
drawn out for ufe, is about 14 inches long, a micrometer with 
the 
