351 
Brightness of the Satellites of 'Jupiter . 
part of the disc, equal to about i of the diameter of the satel- 
lite, is said to be visible. It seems that observations of this 
kind, made with very good telescopes, charged with high 
powers, are capable of great precision. For the remark that a 
margin of Jupiter, equal to about £ of the diameter of the 
satellite, became visible in 45" of time, adds great support to 
the accuracy of the observation of the foregoing 4 minutes : 
and, at all events, it is evidently proved, from the whole of the 
entrance upon the disc, that the diameter of this satellite is less, 
by one half at least, than what from the result of the measures 
of former observers it has been supposed to be. 
A method has also been used, of deducing the diameter of 
the satellites from the time they employ to immerge into the 
shadow of the planet ; but. this must be very fallacious, and 
ought not to be used; 
I should not pass unnoticed the apparent magnitude of the 
satellites. The expressions that have been given of them may 
be collected into the following narrow compass, 
1,4,2 4 ; 1 3 -, 4 ; 1 - 2 4,2,1 3--4 ; 1 ; 2 
1,4,2 3-2,1-, 4 3 - - 2 - 1 , 4 1T2 4.1-2 
1*2 -4 3 — 1 > 2 2 - 1 
From which we may conclude, that the 3d satellite is con- 
siderably larger than any of the rest; that the 1st is a little 
large than the 2d, and nearly of the size of the 4th; and that 
the 2d is a little smaller than the 1st and 4th, or the smallest 
of them all. . 
WM. HERSCHEL. 
Slough, near-Windsor, 
April 30, 1797. 
