POETION OF THE COSMOS. VISUAL POWER. 57 



their application to celestial observation, before therefore 

 the memorable years 1608 and 1610, an exceedingly im- 

 portant part of the astronomy of our planetary system was 

 already established. The inherited treasures of Greek and 

 Arabian knowledge were augmented by the careful and ex- 

 tensive labours of George Purbach, Regiomontanus ( Johann 

 Muller), and Bernhard Walther of Nuremberg. Their efforts 

 were followed by the bold and comprehensive intellectual de- 

 velopment of the Copernican system ; and to this succeeded 

 an abundant mass of exact observations by Tycho Brahe, 

 and the acuteness in combination, and indomitable perseve- 

 rance in calculation, of Kepler. Two great men, Kepler 

 and Galileo, stand at the most important epoch which the 

 history of practical astronomy presents, that which separates 

 observation with the naked eye, but with greatly improved 

 measuring instruments, from telescopic vision. Galileo was 

 then 44, and Kepler 37 years old; Tycho Brahe, the most 

 exact practical astronomer of that great period, had been 

 dead seven years. I have already remarked in the 2nd 

 volume of Cosmos (English edition, p. 324), that Kepler's 

 three great laws, which have made his name for ever illus- 

 trious, did not receive the praise of any one of his cotempo- 

 raries, not even of Galileo. Discovered by a purely em- 

 pirical path, but more rich in consequences to science at 

 large than the isolated discovery of previously unseen celes- 

 tial bodies, they belong entirely to the period of natural 

 vision, to the period of Tycho Brahe, and even to the Ty- 

 chonian observations ; although the printing of the " Astro- 

 nomia nova, seu Physica coelestis de Motibus Stellse Martis" 

 was not completed until 1609, and the third law ; according 

 to which the squares of the periodic revolutions of two 



