88 SEVENTEENTH CENTURY. pt. in. 



It is related that Jansen's children when playing one day with 

 two powerful magnifying glasses, happened to place 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 at once what valuable help it might 

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

 diately, 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 ;;?, 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' n'. From here, as you will remember (see p. 49), we 

 follow them out in straight lines, and see the image at the 

 angle m ^ n, so that it appears greatly magnified. If you 

 look at any object through one tube of an opera-glass, and 



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