CH. XII. 



KEPLER'S 'THREE LAWS' 



you to see over a larger space at one time ; but, on the other 

 hand, it turns everything upside down. In making astrono- 

 mical observations it is not of much importance which part 

 of a star is uppermost ; but for terrestrial telescopes another 

 lens has to be put in to bring the images back to their right 

 positions, and since Kepler's time many other improvements 

 have been made. 



FIG. 9. 



Kepler's Telescope. 1 



A B, Object-glass ; c D, eye-piece ; m n, real arrow ; m' ', picture of the arrow 

 formed at the focus of the rays ; M N, magnified arrow. 



Kepler's first Law, 1609. After Tycho Brahe's death 

 Kepler went on working at the * Rudolphine Tables,' and 

 this led him to consider again the movements of the planets, 

 and to try and find a theory to explain the path or orbit of 

 the planet Mars. Mars is the planet which stands fourth 

 from the sun ; Mercury is nearest to the sun, then comes 

 Venus, then our earth, and then outside our earth is Mars. 

 Tycho had noted in his tables the places at which the planet 

 had been seen at certain periods ; and from these observa- 



1 This figure and also Fig. 8 were kindly drawn for me by Mr. A. x 

 R. Wallace. 



