SEQUEL TO COPERNICUS. 417 



fixed 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 answer it by saying, that luminous 

 bodies were not governed by the same laws of per- 

 spective as opaque ones. But a more satisfactory 

 answer 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 telescopes, 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 discovery in art, (for at this period, 

 the making of telescopes must be mainly so con- 

 sidered,) may influence the progress of science. We 

 shall soon have to notice a still more remarkable 

 example of the way in which two sciences (Astro- 

 nomy and Mechanics) may influence and promote 

 the progress of each other. 



VOL. i. E E 



