86 SEVENTEENTH CENTURY. FT. in. 



them one behind the other in such a position that the 

 weathercock of a church opposite the house seemed to them 

 nearer and larger than usual, and their father, when he saw 

 this, fixed the glasses on a board and gave them as a 

 curiosity to Prince Maurice of Nassau. Whether this story 

 be true or not, it is certain that in the year 1609, both 

 Jansen and Lippershey made these rough telescopes as toys, 

 though they did not know how useful they might be. But 

 when Galileo heard of it he saw what valuable help it might 

 afford in studying the heavens ; and he set to work immedi- 

 ately, and soon succeeded in making a useful instrument. 



A diagram of Galileo's telescope is given in Fig. 8. It 

 was made on the same principle as opera-glasses are now, 

 with one convex lens A B, which makes the rays from the 

 object bend inwards or converge, and one concave lens c D, 

 which makes them bend outwards or diverge before they 

 come to a focus. In Fig. 8 one complete cone of rays is 

 drawn coming from the point m, and the outline of another 

 cone from the point n ; there are really similar cones coming 

 from all points along the arrow, but it is impossible to give 

 these in a diagram. Each set of rays, as they fall on the 

 lens A B, are made to converge, so that they would end in a 

 point or focus, if they were not caught by the lens c D. 

 But this lens having its surfaces curved inwards makes the 

 rays bend outwards or diverge again, so that the end of the 

 cone m reaches the eye in parallel lines at m m and the 

 cone n at n ;/. From the eye, as you will remember 

 (see p. 49), we follow them out in straight lineSj and see 

 the image at the angle M o N, so that it appears greatly 

 magnified. If you look at any object through one tube of 

 an opera-glass, and keep the other eye open so as to see the 

 object at its natural distance, you can cover the real image 

 with the magnified one, and thus see the magnifying power 



