94 SEVENTEENTH CENTURY. PT. in. 



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 ray appears to cross back again on 

 the lens of the eye, and we see them as if they had never 

 been inverted. 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 turning 

 back to p. 87 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 comes 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 n in a telescope, you would see an in- 

 verted 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 follow- 

 ing them out in straight lines the eye sees a magnified arrow 

 upside down at some point between c D and M N. Kepler's 

 telescope is called the ' Astronomical telescope.' It has a 

 much larger ' field of view ' than Galileo's ; that is, it enables 



