THE ASTRONOMICAL TELESCOPE. 



525 



Of telescopes there are three princi- 

 pal kinds : the c astronomical,' the 

 ' terrestrial/ and the ' Galilean.' 



The astronomical telescope resembles 

 the microscope in this, that the object 

 glass produces a real inverted image 

 of the object, which is magnified by 

 the eye- piece. The image formed by 

 the object glass is, however, not 

 already magnified, as is the case in 

 the microscope, but is smaller than 

 the object, because the distance of the 

 object from the lens is much larger 

 than twice the focal length of the 

 object glass. This focal length is 

 therefore made as large as possible, 

 lest the image produced by it should 

 be too small; and since distant objects 

 cannot be artificially illuminated, the 

 object glass must have a large aper- 

 ture so as to collect as many rays as 

 possible from the objects to be seen, 

 and thus to render the images of them 

 formed by the telescopes as bright as 

 possible. Thus the large telescopes in 

 astronomical observatories, usually 

 called c refractors,' have object glasses 

 of from 15 to 30 and even 40 0111 dia- 

 meter, and from 2 to 7 m focal length. 



Fig. 286 shows the principle of the 

 astronomical telescope ; a b is the 

 object, <?! b l the real image produced 



