GALILEO AND HIS JUDGES. 63 



himself observed, proved it as regards that planet ; 

 and the fact of neither of these bodies ever being 

 seen far apart from the Sun, greatly strengthened 

 the conclusion in respect of both of them. A transit 

 of Mercury over the Sun's disc had, in fact, been 

 observed in the year 1631, by Gassendi ; but Galileo 

 was doubtless not aware of it when he wrote the 

 Dialogue. 



It being clear then that Venus and Mercury revolve 

 round the Sun, Galileo shows what strong ground 

 there is for inferring that the superior planets, Mars, 

 Jupiter, and Saturn (the others not being then 

 known), do so also ; this he judges from the greater 

 size of these latter, and particularly of Mars, when in 

 opposition than when in conjunction ; whence we 

 may conclude that the Earth, which as well as the Sun 

 is contained within their orbits, is not in the centre of 

 them, or nearly so. It is remarkable that Galileo 

 treats all the planets as revolving in circles, though 

 one would think he must at that time have been 

 aware of Kepler's discovery that they move in 

 ellipses. He makes Simplicio grant these last- 

 mentioned points, which is curious ; and he also 

 explains how the telescope showed phenomena, such 

 as the phases of Venus, which were unknown to 

 Copernicus. Simplicio has hitherto had no confidence 

 in this new instrument, and following in the footsteps 

 of his friends the Peripatetic philosophers, has sup- 

 posed the appearances in question to be optical 

 illusions arising from the lenses used ; he will, however, 



