THE GALILEAN TELESCOPE. 



inverted image a l bi of the object 

 (left out of the figure to save space). 

 This image is again inverted by the 

 lens hi, which therefore produces an 

 erect image a 2 6 2 , which is finally 

 viewed through the eye-piece eg, which 

 acts as magnifier and gives the erect 

 image a B b 3 . The foci of the lenses 

 cd, hi, and eg are /i,/ 2 , and / 3 re- 

 spectively. 



The Galilean telescope gives at once 

 an erect image, without the interposi- 

 tion of a third lens. Opera glasses 

 are constructed on this plan. The 

 object glass cd (fig. 288) resembles 

 that of the astronomical and terrestrial 

 telescopes, and produces an image a^ l>i 

 of the object a b. The position of 

 this image is found, as in all previous 

 cases, by drawing, from one extremity 

 a of the object two rays, a hf\ a^ and 

 a i di, of which the former is parallel 

 to the axis and passes through the 

 focus, while the other passes through 

 the centre of the lens. Besides these 

 two rays two additional rays are drawn, 

 iz. a c a 1 and aka l ; and to avoid con- 

 fusion the rays which produce the 

 image of b are left out from the figure ; 

 the image b^ can clearly be found easily 

 by drawing rays in the same way as 

 is done for the point a. 



