Brightness of the Satellites of Jupiter. 349 
allowance Tor the influence of the superior light of the planet *, 
but it seems that we can hardly abstract sufficiently on such 
occasions. Two similar cases occur, in fig. 3. at 179; and 
fig. 4. at 5 degrees. 
It is indeed not impossible but that occasional changes, on 
the bodies of the satellites themselves, may occasion some tem- 
porary irregularity of their apparent brightness : it will, how- 
ever, not be necessary to make such an hypothesis, till we have 
better authority for it. The brightest side of this satellite is 
turned towards us when it is between the greatest eastern elon- 
gation and the conjunction. It revolves, consequently, on its 
axis in 3 d i8 h 17', 9. 
The 3d satellite suffers but little diminution of its bright- 
ness, and is in full lustre at the time of both its elongations. 
It is however not impossible but that, after having recovered 
its light, on the return from the opposition, it may suffer a 
second defalcation of it in the nearest quadrant about half way 
towards the conjunction. The two independent observations 
at 151 and 152 degrees, fig. 3. seem to give some support to 
this surmise. It revolves on its axis in 7 d 3 11 59', 6 . 
The 4th satellite presents us with a few bright views when 
it is going to its opposition, and on its return towards the 
greatest eastern elongation ; but otherwise it is generally over- 
cast. Its colour also is considerably different from that of the 
other three; and it revolves on its axis in i6 d i8 h f,i. 
It will not be amiss to gather into one view, all the obser- 
vations that relate to the colour of the satellites. 
The 1st is white; but sometimes more intensely so than at 
others. 
The 2d is white, bluish, and ash-coloured. 
Z z 2 
