JOHANNES KEPLER 45 



Dioptrics) which collected all that had been known from the 

 time of Euclid, and made quite clear how, for example, the 

 pinhole image (Camera obscura) is formed, but then went 

 very much further. Thus we have there for the first 

 time the decrease in the strength of light according to the 

 inverse square law in the case of free propagation from a 

 small source, the correct theory and calculation of mirror 

 images, the refraction of light and the theory of lenses; and 

 also the explanation of the path of light in the eye, and thus 

 all the essential facts of sight, as far as light itself is concerned, 

 including the correct explanation of stereoscopic vision with 

 two eyes. It was true that Kepler still made use of a law of 

 refraction which was only approximate, but this did not 

 prevent him from recognising total reflection and grasping it 

 directly. Finally, also, he was the first to state the principle 

 of the telescope with two convex lenses, which is to-day used 

 exclusively and quite generally for astronomical work in 

 place of Galileo's telescope (which had a combination of a 

 convex and a concave lens), and also serves many other 

 ptirposes, being called the 'Kepler' telescope. The present- 

 day telescopic objective is also given. 



In the meantime, Prague had become less and less endur- 

 able for Kepler, The Imperial Mathematician had no money 

 to live upon, and nevertheless the Emperor (Rudolf II) would 

 not allow his Kepler to escape, A further misfortune was the 

 death of his wife and one child. So Kepler had gradually 

 brought himself to face the need for leaving the place he 

 loved so well, and looked around in his own home country. 

 The Court of Stuttgart would have been glad to have him at 

 the University; but Kepler's independent cast of mind made 

 him objectionable to the ecclesiastical authorities, although 

 these were Lutheran,^ So in 161 2, he went to a school at 



^ In this we may compare the fate of Bohme in Silesia, at the same 

 period, where the new enthusiasm awakened by Luther had taken a still 

 more unfortunate and erroneous direction, even among his own fol- 

 lowers. 



