a Lamp- Micrometer. ; 
extremely didiucl, I obtain a fcale of more than half an inch 
* * » • i » • J 
to a fecond, without increafing the didance of the micrometer ; 
whereas the mod: perfect of my former micrometers, with the 
fame in dr u meat, had a fcale of lefs than the two thoiifandth 
part of an inch to a fecotid. 
The meafures of this micrometer are not confined to double 
dars only, but may be applied to any other objects that require 
the utmod accuracy, fuch as the diameters of the planets or 
their fatellites, the mountains of the moon, the diameters of 
the fixed dars, &c. 
For indance, October 22, 1781, I meafured the apparent 
diameter of <* Lyras ; and judging it of the greated impor- 
tance to increafe my fcale as much as convenient, I placed the 
micrometer at the greated convenient didance, and (with fomc 
trouble, for want of longer handles, which might eafily be 
added) took the diameter of this dar by removing the two 
lucid points to fuch a didance as jud to inclofe the apparent 
diameter. When I meafured my radius it was found to 
be twenty-two feet fix inches. The didance of the two 
lucid points was about three inches ; for I will not pre- 
tend to extreme nicety in this obfervation, on account of the 
very great power I ufed, which was 6450. From thefe mea- 
fures we have the magnified angle 38' 10" \ this divided by 
the power gives 0^,355 for the apparent diameter of & Lyne. 
The fcale of the micrometer, on this occafion, was no lefs 
than 8,443 inches to a fecond, as will be found by multiplying 
the natural tangent of a fecond with the power and radius in 
inches. 
November 28, 1781, I meafured the diameter of the new 
dar ; but the air was not very favourable, for this fingular dar 
was not fo didind with 227 that evening as it generally is 
Z 2 with 
