Mr. herschel on the Diameter and 
idea of taking only an illuminated periphery, inftead of the 
area of a circle. By this means I hoped to fee the circle well 
defined, and yet have but little light to interfere with the ap- 
pearance of the Planet. The breadth of my lucid periphery 
was ‘one- twentieth of an inch. The refult of this meafure 
proved 
3,3 =,0041486; and - 1 * — ■ Q = 3^77. 
?6s,45 
22 7 
Oft. 26, 1782. In my laft experiment I found the lucid pe- 
riphery much broader than I could have wifhed ; therefore, I 
prepared one of no more than one-fortieth part of an inch in 
breadth, the outer circle meafuring very exactly 4,00, and the 
inner circle 3,95. With this (lender ring of light illuminated 
with only one {ingle lamp, I meafured the Georgium Sidus, by 
removing the telefcope to various diftances ; and found at laft 
the following refult : 
1033,05 
= ,0038720; and 
13' 18", 6 
227 
Nov. 4, 1782. I was now fully convinced that light, be it in 
the form of a lucid circle, or illuminated periphery, would 
always occafion the meafures to be lefs than they fhould be, on 
account of its vivid impreffion upon the eye, whereby the mag- 
nitude of the objeCt, to which the Planet was compared, would 
be increafed. It occurred to me then, that if a lucid circle en- 
croached upon the furrounding darker parts, a lucid fquare bor- 
der, round a dark circle, w T ould in its turn advance upon the 
artificial difk. In my laft meafures, where the Planet had been 
compared to a lucid ring, I had plainly obferved that the Sidus, 
which was but juft equal to the illuminated periphery, was 
confiderably larger than the black area contained within the 
ring. This feemed to point out a method to difcover the quan- 
tity of the deception arifing from the illumination ; and confe- 
quently, to furnifh us with a correction applicable to fuch mea- 
4 fures ; 
