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POPULAR SCIENCE REVIEW. 
German astronomer. Later came a yet more remarkable and 
important discovery, through the observation of Jupiter’s sys- 
tem, — the discovery that light does not travel as had been 
supposed instantaneously, but with a measurable, however in- 
conceivable, velocity. Through this discovery, supplemented by 
Bradley’s discovery of the aberration of the fixed stars, came a 
proof — which is absolutely beyond cavil or question — of the true 
theory of the solar system. Supplementary proofs of Newton’s 
views have been derived, also, as might be expected, from the 
influence exerted by a planet whose disturbing agency so 
largely exceeds that of all the other members of the solar 
system. 
Let us return to Galileo’s discovery of the satellite-system of 
Jupiter, and the influence of that discovery on the views of 
astronomers. It was immediately felt, by those who opposed 
the new views of Copernicus, that the discovery of Jupiter’s 
moons was fatal to their objections. Accordingly they spared 
no efforts in casting doubts on the observations of Galileo. 
Some asserted that the Tuscan had seen no such sights as he 
pretended. Others that he had indeed seen them, but in illu- 
sive dreams, that he was the sport of demons specially sent to 
punish him for a prying, inquisitive, and truth-doubting spirit. 
“We have looked,” they said “for hours through his telescope, 
and have seen no such sights as he and his friends have described.” 
When at length it was impossible to deny the existence of 
Jupiter’s moons, it became the fashion to dispute the real 
character of their movements. It was argued that these objects 
do not revolve around the planet, but, travel backwards and 
forwards behind its disc. Down to the middle of the seven- 
teenth century, many refused to believe that the satellites 
actually circulate around Jupiter.* 
The discovery by Cassini, in 1665, that the satellites can be 
traced wdien their orbital motions carry them between the 
planet and the earth, placed the true character of these bodies 
beyond a doubt. By means of Campani’s object-glasses of 100 
and 136 feet focal length, Cassini was able to see the satellites 
projected as small bright spots on the disc of the planet. He 
found also that their motions when thus situated, are precisely 
those due to an orbital motion around the planet, and therefore 
very different from those of bodies attached to the planet. This 
* Por aught I know the motion of the satellites may he denied to the 
present day. In the preface to the last edition (1823) of the Principia , 
edited by the learned Jesuits Le Sueur and Jacquier, there occurs the fol- 
lowing remarkable passage : — 11 In adopting the theory of the earth’s motion, 
to explain Newton’s propositions, we assume another character than our 
own, for we profess obedience to the decrees of the popes against the motion 
of the earth.” It is, therefore, not wholly impossible that decrees may have 
been promulgated against the circulation of Jupiter’s satellites also. 
