96 SEVENTEENTH CENTURY. Ff. HI. 



in 1 60 1, he succeeded him as principal mathematician to 

 the Emperor. 



Kepler on Optics, 1604. Although Kepler is chiefly 

 known as an astronomer, his first work, published in 1604, 

 was on Optics, and in it he points out most beautifully the 

 true use of the different parts of the eye. He was much 

 struck with Porta's idea that the eye is like a camera obscura, 

 and he proved that the rays of light, after passing through the 

 lens of the eye, form a real picture upside down on the fine 

 network of nerves called the retina, at the back of the eye, 

 and are then conveyed by the optic nerve to the brain. He 

 also pointed out that the reason why we do not see things 

 upside down is that since our mind follows out each ray in a 

 straight line, the rays appear to cross back again on the lens 

 of the eye, and we see them as if they had never been in- 

 verted. This is, however, a question still undecided by 

 physiologists. 



Kepler invented a much more powerful telescope than 

 the one which Galileo had made. You will see by turn- 

 ing back to p. 88 that the fault of Galileo's telescope was that 

 it made the rays diverge or bend outwards, just as they 

 reached the eye, and in this way many of them passed out- 

 side and were lost. Kepler avoided this by using two convex 

 lenses. In his telescope (see Fig. 9), the rays from the object 

 m n, after converging on the lens A B come to a focus at m' n', 

 where they make a real image of the arrow upside down. If 

 you could put a piece of thin transparent paper at the point 

 m' ri in a telescope, you would see a picture of the object 

 upon it. The rays from this image falling on the lens c D, 

 are again bent inwards, as by the ordinary magnifying glass 

 (see p. 49), and thus by following them out in straight lines 

 the eye sees a magnified arrow upside down at some point 



