I 
Magnitude of the Georgium Sidus, &c. 7 
alfo be fufFered. to pafs acrofs it ; but in this cafe, the lights 
will be lo blended together, that we cannot eafdy form a pro- 
per judgement of their magnitudes. By a good fcrew to the 
motions of my telefcope 1 have been able, at any time, to keep 
the Planet oppofite the lucid difk for five minutes together, and 
to view them both with the moil pcrfedt and undifturbed atten- 
tion. The apparatus 1 employed being now fufficiently explained, 
feveral alterations that were occafionally introduced will be 
mentioned in the obfervations and experiments on the Georgium 
Sidus, as they follow, in the order of time in which they were 
made. 
Obfervations on the Light , Diameter , and Magnitude , of the 
Georgium Sidus. 
061 . 22, 1781.. The Georgium Sidus was perfectly defined 
with a power of 227; had a fine, bright, fteady light; 
of the colour of Jupiter, or approaching to the light of the 
Moon. 
Nov. 28, 1781. I meafured the diameter of the Georgium 
Sidus by the lamp-micrometer, and took one mealure, which 
I was allured was too large ; and one, which I was certain was 
too little ; then taking the mean of both, I compared it with 
the diameter of the flar, and found it to agree very well. 
Hence 
Image rr 2,4 inches 
- = tang. ,0055684; and 
Angler: 19' 8", 
Power —227 (j 
jJiitance ~ 43 1 inches 
— the diameter f',o5. But the evening was foggy, and 
the flar having much aberration, I was induced to try the 
above method of extreme and mean diameters, fuggefled by 
the 
