SCIENTISTS OF XVTH AND XVlTH CENTURIES. 103 



Fosidonius ; was one of the first to notice the rotation of the 

 SUN ON ITS AXIS, "like an orator in the midst of a crowd/' 

 he says, "who can face all his auditors simply by turning 

 upon himself" ; he indicated for the first time the UNDU- 

 .LATORY THEORY OF LIGHT; determined the refraction of 

 light to be nil at the zenith; expressed his belief in the 

 WEIGHT OF AIR ; in the sun being a magnet ; put forward 

 the empirical law, known as BODE'S LAW, by which the 

 distances of the planets seem to follow an arithmetical pro- 

 gression (see Piazzi, further on); he correctly foretold the 

 dates of the transits of Mercury and Venus ; he also explained 

 the correct use of the different PARTS OF THE EYE ; lastly, 

 he formulated the three laws of movement known as 

 KEPLER'S LAWS, which, constituting as they do one of 

 the great triumphs of mathematics, made the discovery of 

 Newton's law of gravitation a possibility. This, Kepler 

 accomplished after perseveringly observing the motions of 

 Mars and the laws, meant to apply to Mars only, he found 

 to apply to all planets. The result was obtained only after 

 years and years of labour but they are laws found for all 

 times ! Kepler's laws are : i. Orbits, of which the sun 

 occupies a focus, are elliptical (1609) > n - The areas de- 

 scribed by a line drawn from the planet to the sun are 

 proportional to the times, or, planets describe equal areas 

 about the sun in equal times (1609) ; in. The squares of 

 the period of complete revolution, or periodic times of any 

 two planets, are proportional to the cubes of their mean 

 distances from the sun or, the squares of the periodic times 

 of the planets are proportional to the cubes of their distances 

 {1618) a law which, in 1622, he proved to hold good for 

 the satellites of Jupiter as regards their primary. The third 

 law contains the principle of Newton's laws.* It is not too 

 much to say that the modern system of astronomy deserves 

 far better to be called the Keplerian system than the Coper- 

 nican. Kepler, speaking of attraction and gravitation, says : 

 " Everything is simple in the variety of nature. Thus, in a 

 river, the simple motion is the tendency of water to repair to 

 the centre of the earth, but as the way is not direct, the 



* See page 201. 



