8 Mr. her sc h el on the Diameter and 
the method of altitudes, where two equally diftant extremes 
give us a true mean. 
Nov. 19, 1781. The diameter meafured 32 1 parts of my 
micrometer, the wires being outward tangents to the difk. On 
ihutting them gradually bv the fame light, they clofed at 24; 
therefore the difference is8| parts, which, according to my 
fcale, gives 5 // 2 /v for the diameter. This was taken with 
227, and the meafure feemed large enough. Not perfectly 
pleafed with my light, which was rather too ftrong, I repeated 
the meafure, and had 33 \ parts; then (hutting the wires gra- 
dually, by Z/6/h light they clofed at 25: the difference, which 
is 8f parts, gives 5" n'" m 
Aug. 29, 1782. 15 h. I faw the Georgium Sidus full as well 
defined with 460, as Jupiter would have been at that altitude 
with the fame power. 
Sept. 9, 1782. Circumffances being favourable, I took a 
meafure of the diameter of the Georgium Sidus with the power 
of 460, and filk- thread micrometer. After a proper allowance 
for the zero, I found 4." 1 1"'. 
OCt. 2, 1782. I had prepared an apparatus of lucid difks, 
and meafured the diameter of the Georgium Sidus with it. 
Having only white oiled papers, I placed two of them toge- 
ther, and ufed only a fingle lamp ; but could not exactly imi- 
tate the light of the Planet. When I fir ft faw the Sidus and 
luminous circle together, I was ft ruck with the different co- 
lours of their lights ; which brought to my recollection y An- 
dromedas, e Bootis, cc Herculis, (3 Cygni, and other coloured 
ftars. The Planet unexpectedly appeared blueifh, while the lucid 
difk had a ftrong tinCture of red ; but neither of the colours 
were fo vivid and fparkling as thofe of the juft mentioned ftars. 
1 The 
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