Brightness of the Satellites of Jupiter. 343 
to refer the light of the satellite under estimation, being change- 
able, permits us no longer to trust to their standard, without 
a full scrutiny of the causes that may have produced an alte- 
ration in them. 
In the foregoing observations it will also be seen, that I at- 
tempted to compare the intenseness of the light of the satellites 
with the different brightness of the disc of Jupiter ; but these 
endeavours will always fail, on account of the little assurance 
we can have that the parts of the disc, setting aside its quick 
rotation, will remain for any time of the same lustre. 
Avery material difficulty arises from the magnifying power 
we use in our estimations. If it be a low one, such as for instance 
180 (for a lower should not even be attempted), then we run 
the risk of being disappointed in bright nights by the sparkling 
of the brilliant light of the satellites. Besides, we cannot then 
see the bodies of them, and judge of their comparative magni- 
tude, with the same power that we view their light. If we 
choose a high magnifier, we shall be often disappointed in the 
state of the atmosphere, which will of course occasion an inter- 
ruption in the series^ of our observation, of which the regular 
continuance is of the greatest consequence. If we change our 
power according to the state of the atmosphere, we introduce a 
far worse cause of confusion ; for it will be next to impossible 
to acquire, for each magnifying power, an ideal standard of 
comparative brightness to which we can trust with confidence. 
If the magnitudes are not attended to, and carefully contra- 
distinguished from the intenseness of light, we shall run into 
considerable error, by saying that a satellite is large, when we 
mean to express that it is bright. It is so common to call stars 
that are less bright than others, small, that we must be careful 
