of a Comet . 495 
whereas with a higher power the proportion of this- error to 
the whole becomes much lefs, and the meafure more exaff , even 
after we have made allowance for a fmall additional error occa- 
lioned by the want of that perfect diftinfinefs which is required 
for other purpofes. However, to enter deeply into an expla- 
nation of this would lead me to {peak of the caufes of the 
aberration of rays in the focus of an objeft fpeculum, of 
which there are fome that are feldom taken into confideration 
by opticians, and indeed are fuch as cannot be calculated ; but 
this not being my prefen t purpofe, fuffice it to obferve, that 
the method is juftified by experience. 
When the diameter of the Comet was in creafed to about 
I thought it advifable to leflen the power with which I mea- 
fured ; and, as I ma!fe ufe of two different micrometers, as 
well as eye-glafies, I took a meafure with both of them. The 
agreement of the micrometers to f ,f is no fmall proof of the 
goodnefs of the obfervations of the 28th of March, and very 
properly connects the meafures of the high powers with thole 
that were made with 227. 
Vol. LXXL 
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