f 4 ] 
II. On the Diameter and Magnitude of the Georgium Sidus; 
•with a Defcription of the dark and lucid Difk and Periphery 
Micrometers, By William Herfchel, Efq. F. R, S. 
Read November 7, 1782. 
I T is not only of the greateft confequence to the aftronomer, 
but alfo gives the higheft pleafure to every intelligent per- 
i'on, to have a juft idea of the dimenficns of the l’olar fyftem, 
and the heavenly bodies that belong to it. As far then as they 
fall within the reach of our inftruments, they ought carefully 
to be examined and meafured by all the various methods wo 
can invent. Almoft every fort of micrometer is liable to fome 
inconveniences and deceptions : it will, however, often happen, 
that we may corredt the errors of one inftrument by the oppo- 
iite defects of another. The meafures of the diameter of the 
Georgium Sidus, which were delivered in my firft paper, differ 
confiderably from each other. However, if we fet afide the 
three fir ft, on a fuppofition (as I have hinted before) that every 
minute objedt, which is much fmaller than what we are fre- 
quently ufed to fee, will at fir ft fight appear lefs than it really 
is; and take a mean of the remaining obfervations, we fhall 
have 
