252 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



subtended by the object, the image and the object both being at the 

 same distance from the eye. In refractors, on the contrary, and this 

 applies to all kinds of telescopes, the image is always of smaller 

 dimensions than the object itself; bat it is larger than the image 

 furnished by the naked eye, and this constitutes the magnifying power 

 of refractors. 



The eye-piece is movable in the tube which holds the object-glass ; 

 a milled head, which works with rack and pinion, enables the distances 

 between the glasses to be adjusted. In this way an image of perfect 

 sharpness is obtained. This is called focussing. Short-sighted 

 people shorten the tube and long-sighted lengthen it in order to see 

 distinctly. 



The magnifying power in all telescopes depends upon the ratio 

 of the focal length of the object-glass to that of the eye-piece. 



The telescope we have just described, with two lenses, has received 

 the name of Galileo's telescope, it shows objects erect at the same time 

 that it brings them nearer and magnifies them. 



Galileo's first telescope only magnified from four to seven times 

 in diameter ; the most powerful that was made and used by the 

 illustrious astronomer magnified thirty-two times. That enabled him 

 to make a number of discoveries which then were justly considered 

 wonderful ; the mountains in ttye Moon, the spots and rotation of the 

 Sun, Jupiter's satellites and the phases of Venus, the breaking up of 

 the .great nebulosity called the Milky Way into stars, &c. His 

 Nuntius Sidereus, which he published to inform the scientific world 

 of his results and researches, scarcely sufficed to record these dis- 

 coveries, which soon formed a branch of astronomical study unknown 

 to the ancients. 



In the present day, Galileo's arrangement is no longer used for 

 astronomical instruments, its magnifying power is too feeble ; but it 

 is employed as a terrestrial glass, and especially for the examination 

 of near objects ; it is nothing more than the Opera-Glass, a very con- 

 venient instrument, because, with equal magnifying power, it is of 

 much shorter length than refractors with converging eye-pieces. The 

 field is small, and as .the rays diverge on leaving the eye-piece, it is 

 necessary to place the eye very near the latter so as not to lessen the 

 field still more. 



It is now time to say a word on the improvement made in the 



