34 2 Dr. Herschel's Observations of the 
The air is so tremulous that the power of 600 is too high, 
and the necessary uniformity required in these observations 
will not permit a lower to be used. Perhaps one of 400 might 
be more generally employed ; and it may be proper to use it 
constantly. 
November 3, 1 796. 23 h 55'47". 10-feet reflector; power 600. 
The 4th satellite of Jupiter is large and bright. 
The 3d satellite is large and bright. 
The 1st satellite is pretty small, and not very bright. 
The 2d satellite is small, and considerably bright. 
The brightness and magnitude of each satellite refer to its 
own usual brightness and magnitude. 
Before we can proceed to draw any conclusions from these 
observations, we ought to take notice of many causes of decep- 
tion, and of various difficulties that attend the investigation of 
the brightness of the satellites. 
The difference in the state of the atmosphere between two 
nights of observation, cannot influence much our estimation of 
the brightness of a satellite, provided we adopt the method of 
comparative estimations. If we endeavour by much practice to 
fix in our mind a general ideal standard of the brightness of 
each satellite, we shall find the state of the atmosphere in dif- 
ferent nights very much disposed to deceive us; but if we learn 
to acquire a readiness of judging of the comparative brightness 
of each satellite with respect to the other three, we may arrive 
at much more precision, since the different disposition of the 
air will nearly affect all the satellites alike. But here, as we 
get rid of one cause of deception, we fall under the penalty of 
another. The situation of those very satellites to which we are 
