JUPITER WITHOUT HIS SATELLITES. 
251 
circumstance, and the fact that the bright spots remain un- 
changed in form as they pass over the disc, proved incontestably 
that he had not mistaken bright spots, such as are sometimes 
seen on the body of the planet itself, for the satellites whose 
ingress on the disc he had previously watched. But he was 
able to detect another evidence of the true nature of these 
bodies, since he discovered that the shadows which they cast 
upon the body of the planet, are visible as small dark spots 
upon the disc. 
Forty years later Maraldi observed that the fourth satellite does 
not always present the same appearance as it traverses the disc 
of the planet. Sometimes it appeared to him as a bright spot, 
at others it appeared darker than the planet. He noticed also 
that when the satellite seemed to be projected as a dark spot,, 
this spot was smaller than the shadow of the satellite. “ Accord- 
ing to the laws of optics,” he says, and others have followed him 
in the statement, u it ought to have appeared larger.” Assuming 
this view to be correct, and that the observations of Maraldi 
were rightly interpreted by him, we are led to a somewhat 
singular result. It has been proved, (incontestably, I think) 
by Sir W. Herschel’s observations, that all the satellites of 
Jupiter follow the law observed in the case of our own moon — 
turning constantly the same face towards their primary. He 
observed that each satellite varied in brightness in different 
parts of its orbit, but that when it arrived at the same position 
in its orbit, “ it exhibits always the same degree of brightness.” 
It would follow from this, that each satellite in transiting the 
disc of Jupiter should exhibit invariably the same appearance — 
since when so situated we always see the same half of the 
satellite, that half namely which is invisible from Jupiter. This 
at least, would always happen, unless a satellite were subject to 
transient variations of brilliancy arising from physical change 
occurring on its own face. Maraldi’s observation would seem 
therefore to point to the occurrence of such changes on the 
fourth satellite, and corresponding observations of variations of 
brilliancy in the other satellites, by Cassini, Maraldi, and Pound 
would lead to the same conclusion as respects these bodies also. 
The observation by Bianchini, that in other parts of their orbits 
the satellites are subject to considerable variations of brilliancy, 
would seem to confirm this result. 
How without asserting the impossibility that the above ex- 
planation is the true one, I cannot but consider that it is highly 
improbable that the satellites of Jupiter are actually subject to 
physical changes of the kind implied. The observations of 
Sir W. Herschel are decidedly opposed to Bianchini’s view, and 
scarcely less directly contradictory of Maraldi’s. It appears to 
me far more probable that the apparent loss of brilliancy 
