SEQUEL TO COPERNICUS. 279 



of that theory had endeavored to account for this, by supposing thai 

 the rays of the sun passed freely through the body of the planet ; and 

 Galileo takes occasion to praise him for not being deterred from adopt- 

 ing the system which, on the whole, appeared to agree best with the 

 phenomena, by meeting with some appearances which it did not en- 

 able him to explain. 11 Yet while the fate of the theory was yet un- 

 decided, this could not but be looked upon as a weak point in its 

 defences. 



The objection, in another form also, was embarrassing alike to the 

 Ptolemaic and Copernican systems. Why, it was asked, did not Venus 

 appear four times as large when nearest to the earth, as when furthest 

 from it ? The author of the Epistle prefixed to Copernicus's work had 

 taken refuge in this argument from the danger of being supposed to 

 believe in the reality of the system ; and Bruno had attempted to an- 

 swer it by saying, that luminous bodies were not governed by the 

 same laws of perspective as opake ones. But a more satisfactory an- 

 swer now readily offered itself. Venus does not appear four times as 

 large when she is four times as near, because her bright part is not four 

 times as large, though her visible diameter is ; and as she is too small 

 for us to see her shape with the naked eye, we judge of her size only 

 by the quantity of light. 



The other great discoveries made in the heavens by means of tele- 

 scopes, as that of Saturn's ring and his satellites, the spots in the sun, 

 and others, belong to the further progress of astronomy. But we may 

 here observe, that this doctrine of the motion of Mercury and Venus 

 about the sun was further confirmed by Kepler's observation of the 

 transit of the former planet over the sun in 1631. Our countryman 

 Horrox was the first person who, in 1639, had the satisfaction of seeing 

 a transit of Venus. 



These events are a remarkable instance of the way in which a dis- 

 ^uvery in art (for at this period, the making of telescopes must be 

 mainly so considered) may influence the progress of science. We shall 

 soon have to notice a still more remarkable example of the way in 

 which two sciences (Astronomy and Mechanics) may influence and 

 promote the progress of each other. 



11 Drinkwater-Betliune, Life of Galileo, p. 



