INSTRUMENTS OF PRECISION 



then deduce the apparent angular breadth of the object. 

 Urn-gens discovered also that an object placed in the 

 common focus of the two lenses of a Kepler telescope 

 appears distinct and clearly defined. The micrometers 

 of Malvasia, and later of Auzout and Picard, are the 

 development of this discovery. Malvasia's microme- 

 ter, which he described in 1662, consisted of fine silver 

 wires placed at right-angles at the focus of his tele- 

 scope. 



As telescopes increased in power, however, it was 

 found that even the finest wire, or silk filaments, were 

 much too thick for astronomical observations, as they 

 obliterated the image, and so, finally, the spider-web 

 came into use and is still used in micrometers and 

 other similar instruments. Before that time, however, 

 the fine crossed wires had revolutionized astronomical 

 observations. " We may judge how great was the im- 

 provement which these contrivances introduced into 

 the art of observing," says Whewell, "by finding that 

 Hevelius refused to adopt them because they would 

 make all the old observations of no value. He had 

 spent a laborious and active life in the exercise of the 

 old methods, and could not bear to think that all the 

 treasures which he had accumulated had lost their 

 worth by the discovery of a new mine of richer ones." l 



Until the time of Newton, all the telescopes in use 

 were either of the Galilean or Keplerian type, that is, 

 refractors. But about the year 1670 Newton con- 

 structed his first reflecting telescope, which was great- 

 ly superior to, although much smaller than, the tele- 

 scopes then in use. He was led to this invention by 

 his experiments with light and colors. In 1671 he 



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