CHAP, iv.] THE TELESCOPE. 277 



the observatory of Paris. (See Plate XL) The mirror is 1*20 

 metres (4 feet) in diameter.. 



The illustration which we give represents the telescope in a position 

 for observation. The wheeled hut under which it usually stands, a 

 sort of waggon seven metres high by nine long and five broad, is 

 pushed back towards the north along double rails. The observing 

 staircase has been fitted to a second system of rails, which permits it 

 to circulate all round the foot of the telescope, at the same time that 

 it can turn upon itself, for the purpose of placing the observer, 

 standing either on the steps or on the upper balcony, within reach of 

 the eye-piece. This eye-piece itself may be turned round the end of 

 the telescope into whatever position is most easily accessible to the 

 observer. 



The tube of the telescope, 7*30 metres in length, consists of a 

 central cylinder, to the extremities of which are fastened two tubes 

 3 metres long, consisting of four rings of wrought iron wrought together 

 by twelve longitudinal bars also of iron. The whole is lined with 

 small sheets of steel plate. The total weight is about 2400 kilo- 

 grammes. At the lower extremity is fixed the cell which holds the 

 mirror ; at the other end a circle, movable on the open mouth of the 

 telescope, carries at its centre a plane mirror, which throws to the 

 side the cone of rays reflected by the great mirror. 



The weight of the mirror in its barrel is about 800 kilogrammes ; 

 the eye-piece and its accessories have the same weight. 



Silver-mirror telescopes are made of small dimensions, which 

 magnify 60 to 200 times. Fig. 207 represents one of this model, 

 with the mirror 10 centimetres in diameter and only 60 centimetres 

 focal distance. With a similar instrument astronomical amateurs may 

 divide numerous double stars, observe Jupiter's satellites, Saturn's 

 rings, sun spots, and distinguish very interesting details in the 

 lunar mountains. If we wish to form an exact idea of the important 

 services that the invention of telescopes has rendered for the last two 

 centuries to the science of observation, and particularly to the 

 astronomer, it is necessary to read the history of these sciences 

 themselves ; at each page one is arrested in wonder before the 

 grand results. 



We have collected in Plate XII. a few examples of the 

 details which the telescope gives us of the structure of the sun, 



