CH. XL 



GALILEO'S TELESCOPE. 



89 



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 of your glass. But 

 when you do not compare them in this way you do not 

 realise how much the object is enlarged, because it appears 



Fig. 8. 



31 



Galileo's Telescope. 



A B, Convex lens ; C d, concave lens next the eye ; vi «, real arrow ; M N, 

 apparent size of arrow ; nJ m' and «'«', end of the cones of rays m and « as 

 they reach the eye ; m <> n, angle at which the magnified arrow is seen. 



to come nearer, so as to be at some point between m n 

 and Oj and to be less magnified in consequence. I must 

 warn you that both in this diagram and the one at p. 97 

 the proportions are very much distorted, because a star or 

 even a house would be an immense distance off as compared 

 with the length of a telescope, whereas the arrow is obliged 

 to be drawn here as near to the lenses as they are to each 

 other. 



Secondary Light of the Moon. — Galileo's first telescope 

 only magnified three times, that is, made an object three 

 times larger ; but he made a second which magnified eight 

 times, and then he turned it to the moon and began to 



