40 GREAT MEN OF SCIENCE 



Schiller to Goethe or Liszt to Richard Wagner. But Kepler 

 and Galileo never met; only their correspondence tells us that 

 they were two great men who, surrounded by the incompre- 

 hending, were able to understand one another as individuals 

 and appreciate one another's merits. This is particularly 

 true in a high degree for Kepler; for he had a very fine and 

 delicate mind, and was always ready with admiration and 

 recognition for contemporaries and predecessors; in which 

 respect, quite by way of contrast to the custom of his time, he 

 sometimes even went too far in his writing.^ 



Kepler was the great investigator in optics, as well as of the 

 ideas of the Copernican planetary system. In the first 

 capacity he was the founder of geometrical optics, that is to 

 say, all knowledge based upon the rectilinear propagation, 

 reflection, and refraction of light. In the second capacity 

 he was the discoverer of the three planetary laws, which were 

 fundamental for all further progress in the mechanics of the 

 heavens. While it was still forbidden even to think of the 

 earth as performing its orbit, Kepler had already discovered 

 the more exact form of its path around the sun, together with 

 the paths of the other planets, their mode of motion in their 

 orbits, and the relations of the periods of revolution of the 

 difi^erent planets to one another. 



The three well-known laws of Kepler are as follows: 

 (i) The paths of the planets are ellipses, with the sun at 

 one focus ; (2) The line drawn from the sun to the planet 

 sweeps over equal areas in equal times ; (3) the squares of 

 the periods of revolution of the diff"erent planets are to one 

 another as the cubes of their mean distances from the sun. 

 This refinement of knowledge was taken by Kepler from 

 Tycho Brahe's fine observations with an expenditure of 

 labour and acuteness which had never been known before, 



^ Thus for example in the case of the writer and self-advertiser Porta, 

 who at that time attained a certain degree of importance; indeed. Kepler 

 generally allows even Aristotle too great significance. 



