532 THE REFLECTING TELESCOPE. 



existence of this image may be demonstrated by means of a piece of 

 oiled paper, as in the case of the microscope. Looking from the 

 side at the paper, the image which appears magnified when seen 

 through the eye-glass o will be seen in its real size. In the 

 case of the telescope the lenses are first adjusted until the candles are 

 distinctly seen ; the paper is then placed between the lenses and 

 moved to and fro until the real image appears sharply defined. That 

 the final image formed by the telescope appears larger than the object 

 when seen direct, may easily be proved by looking with the right eye 

 through the eye-piece o and with the left eye direct at the candles. 



In the terrestrial telescope, fig. .B, a is again the place of the real 

 image produced by the object glass, while the second image, formed 

 by the action of the lens u, is at c. This secondary image is, with the 

 distances assumed here, half the size of a primary one. A paper 

 screen for receiving these images maybe placed at a, and afterwards 

 at c. If the experiment is made in the evening, there being no 

 other light except the three candles, and screens of fine tissue paper 

 be used, both images may even be shown at the same time. The 

 screen for a is then placed and adjusted first, and afterwards the 

 screen for c. 



In the Galilean telescope it may easily be shown that no real 

 image is formed, by moving a slieet of paper first from the object 

 glass to the eye-piece o, then holding it on the other side of o and 

 moving it slowly farther and farther from the eye-piece ; in any 

 position only a patch of light with undefined outlines will be seen 

 on the paper. In fig. (7, a denotes the position of the image which 

 would be formed without fhe interposition of the eye-piece ; it can 

 obviously only be received on a screen when the lens o is removed. 



For many astronomical purposes telescopes are used 

 in which the object lens is replaced by a concave 

 mirror. Such instruments are called reflectors, or 

 'reflecting telescopes,' and there are various kinds of 

 reflectors in use. Fig. 291 explains the construction 

 of the ' Newtonian reflector.' The end h i of the tube 

 is quite open and directed towards the object to be 

 observed; at the opposite closed end of the tube is the 

 concave mirror c d. This mirror would produce a small 

 inverted image of the object at j ^; but the small in- 

 clined mirror reflects the rays forming this image into 



