A NEW ASTRONOMY 215 



body, the earth ; and wherever the earth is transported heavy bodies 

 will always seek it ; but if the earth were not round they would not 

 from all sides seek the middle of it, but would from different sides be 

 carried to different points. If two stones were situated anywhere in 

 space near each other, but outside the reach of a third cognate body, 

 they would after the manner of two magnetic bodies come together 

 at an intermediate point, each approaching the other in proportion 

 to the attracting mass. And if the earth and the moon were not kept 

 in their orbits by their animal force, the earth would ascend towards 

 the moon one fifty-fourth part of the distance, while the moon would 

 descend the rest of the way and join the earth, provided that the two 

 bodies are of the same density. If the earth ceased to attract the 

 water all the seas would rise and flow over the moon. Dreyer. 



Kepler's last important published work was his Rudolphine 

 Tables (1627), embodying the accumulated results of Tycho's 

 work and his own, and remaining a standard for a century. It is 

 noteworthy that during Kepler's work on these tables, mathe- 

 matical computation was peacefully revolutionized by the intro- 

 duction of logarithms, newly discovered by Napier and Biirgi. 



In 1628, after vain attempts to collect arrears of his salary as 

 imperial mathematician, he even joined Wallenstein as astrologer, 

 but died soon after at Regensburg in 1630. 



Kepler also wrote an important work on Dioptrics with a mathe- 

 matical discussion of refraction and the different forms of the 

 newly invented telescope, the whole constituting the foundation 

 of modern optics. In it he develops the first correct theory of 

 vision, "Seeing amounts to feeling the stimulation of the retina, 

 which is painted with the colored rays of the visible world. The 

 picture must then be transmitted to the brain by a mental cur- 

 rent, and delivered at the seat of the visual faculty." He sup- 

 poses that color depends on density and transparency, and that 

 refraction is due to greater resistance of a dense medium. He 

 enunciates the law that intensity of light varies inversely as the 

 square of the distance. " In proportion as the spherical surface 

 from whose centre the light proceeds is greater or smaller, so is 

 the strength or density of the light-rays which fall on the smaller 



