A HISTORY OF SCIENCE 



much larger field of view than the Galilean telescope, 

 but did not give as clear an image, and in consequence 

 did not come into general use until the middle of the 

 seventeenth century. The first powerful telescope of 

 this type was made by Huygens and his brother. It 

 was of twelve feet focal length, and enabled Huygens to 

 discover a new satellite of Saturn, and to determine 

 also the true explanation of Saturn's ring. 



It was Huygens, together with Malvasia and 

 Auzout, who first applied the micrometer to the 

 telescope, although the inventor of the first micrometer 

 was William Gascoigne, of Yorkshire, about 1636. 

 The micrometer as used in telescopes enables the 

 observer to measure accurately small angular dis- 

 tances. Before the invention of the telescope such 

 measurements were limited to the angle that could 

 be distinguished by the naked eye, and were, of course, 

 only approximately accurate. Even very careful ob- 

 servers, such as Tycho Brahe, were able to obtain only 

 fairly accurate results. But by applying Gascoigne's 

 invention to the telescope almost absolute accuracy 

 became at once possible. The principle of Gascoigne's 

 micrometer was that of two pointers lying parallel, 

 and in this position pointing to zero. These were 

 arranged so that the turning of a single screw separated 

 or approximated them at will, and the angle thus 

 formed could be determined with absolute accuracy. 



Huygens's micrometer was a slip of metal of variable 

 breadth inserted at the focus of the telescope. By 

 observing at what point this exactly covered an object 

 under examination, and knowing the focal length of 

 the telescope and the width of the metal, he could 



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