530 KEFRACTING TELESCOPES. 



scopes. A is the form of the astronomical (on a small 

 scale), B of the terrestrial, and C of the Galilean tele- 

 scope. In all of them b denotes the tube which carries 

 the eye-piece, a that which carries the object glass. 

 The terrestrial telescope has usually more than the two 

 necessary tubes ; it may thus be considerably shortened 

 and more conveniently carried about. 



A telescope, like a microscope, can only be constructed by skilled 

 hands. For our purpose, which is the demonstration of the princi- 

 ples on which the instrument acts, a concave lens of 2 cm diameter 

 and 5 cm focal length will be required, besides the three convex 

 lenses of 28, 5, and 3 cm focal length previously used. The three 

 small lenses are provided with cork frames, 3 cm in diameter, as for 

 the microscope ; the larger lens is fixed in a tube of cardboard, from 

 6 to 10 cm long, and kept in position by means of two rings of card- 

 board, as explained in the construction of the camera obscura (see 

 page 478). Right and left below this tube pieces of cork are glued 

 upon it, suitably cut and filed so as to ensure a stable position for 

 the tube when placed on the board. The form of these pieces is 

 seen in fig. 290 at / fc. The two small boards required, 13 and 6, 

 should be procured from a joiner, each 6 cm wide and l cm '5 thick ; 

 one 10 cm and the other 50 cm long. 



The experiment should be made in the evening or in a darkened 

 room. A long table will be required, or two tables will have to be 

 joined, in order to obtain the requisite distances. Three small 

 pieces of candle are arranged near the end of the table, as in 

 fig. 290 D, one of them being placed behind the two others and 

 raised above them by means of a piece of cork about 3 cm high. The 

 board B (fig. 290 A, B, 0) is placed upon the table, at a distance of 

 3 m from the pieces of candle, and directed towards them. The 

 smaller board b carries the eye-piece, the frame of which is not 

 glued to the board, but fixed between two pins stuck in the wood. 

 The cardboard tube with the object glass is placed upon the larger 

 board at a suitable distance from the eye-piece. Looking at the 

 candles through the eye-piece, the board 6 is moved to and fro until 

 distinct magnified images of the candles are seen. Care should be 

 taken in moving b that its edges are always in the same plane as 

 those of I?, otherwise the axes of the lenses will not be in a straight 

 line. 



For the astronomical telescope, A in fig. 290, the convex lens of 



