GALILEAN TELESCOPE. 345 



Expt. 373. To Construct a Galilean Telescope. Make a tube 

 of pasteboard of convenient diameter by rolling brown paper, &c., 

 on a cylindrical wooden rule, metal tube, &c., and pasting together 

 the successive layers ; preferably several such tubes may be made 

 of such sizes that one will slide stiffly within the next when com- 

 pletely dry, so that by pulling out the series a considerable length 

 of tube may be obtained, whilst by pushing each tube within the 

 next the whole is reduced to a comparatively short length.* At 

 one end of the widest tube fix a convex lens of from 6 to 1 2 inches 

 focal length or even more. This is conveniently effected by select- 

 ing a tube, the diameter of which is a shade larger than that of the 

 lens, and fixing therein a bit of the next-sized tube, only about J inch 

 in length, cut off for the purpose. This inner ring is cemented 

 inside the wider tube, and furnishes a rest or support for the edges 

 of the lens, which is kept in position by a piece of brass wire bent 

 nearly into a circle of slightly larger diameter than the wider tube, 

 so that its springiness fixes it tolerably firmly in position when it 

 is slightly compressed, and so made to enter within the wider tube. 

 In this way the lens is retained in position for use, but can be re- 

 moved for cleaning when required by simply removing the brass 

 spring ring and allowing the lens to drop out. 



At one end of the smallest tube used a concave lens is to be 

 similarly fixed, the focal length of this being about 1 inch or less ; 

 the tubes are then to be pulled out or pushed in until the two 

 lenses are separated by a distance nearly equal to the difference 

 between their focal lengths ; so that if the convex lens have a focal 

 length of 6 inches and that of the concave lens be 1 inch, the 

 two lenses will be nearly 5 inches apart. If a distant object be 

 now looked at through the lenses and tube, adjusting their distance 

 apart accurately by pushing in or pulling out the tubes with a 

 screwing motion until clear definition is obtained, an erect magni- 

 fied virtual image will be perceived. Ordinary opera glasses con- 

 sist of a pair of similar telescopes of this description, mounted side 

 by side, forming a "binocular telescope." Owing to the compara- 

 tively considerable shortness of the instrument when the lenses 

 are adjusted so that the focal lengths are respectively 6 or 8 inches 

 and 2 to 3 inches, this form of telescope is extremely portable. Fig. 

 190 represents the course of the rays of light whilst passing through 



* This device of preparing tubes (of metal or other materials), sliding one 

 within the other, is extensively used for many purposes other than optical 

 instruments ; thus, sliding tube gasaliers, &c., are generally spoken of as 

 " telescopic " on this account ; similarly, a dining table that can be extended 

 by pulling out the frame so as to admit of extra "leaves" being inserted is 

 generally termed a "telescopic" table. 



