ARTICLES 



THE 100-INCH REFLECTING TELESCOPE 

 OF THE MOUNT WILSON SOLAR 

 OBSERVATORY 



By H. SPENCER JONES, M.A., B.Sc, 

 Chief Assistant, Royal Observatory, Greenwich 



The completion of a great achievement in telescope construction 

 is now being witnessed, the final adjustment to and preparatory- 

 trials of a reflecting telescope far exceeding in size any telescope 

 that has previously been constructed being now in progress. 

 A brief account of the design and construction of the telescope 

 therefore appears desirable. 



Appropriately enough, this telescope belongs to an American 

 observatory, the Mount Wilson Solar Observatory, Pasadena, 

 California. Though we may envy our American allies the 

 fine instrument which they possess, we may console ourselves 

 with the fact that the skies in this country would not be 

 sufficiently good for such an instrument to be used with advan- 

 tage. In order that the full aperture of so large a mirror may 

 be advantageously utilised, the most perfect definition is needed. 

 For some purposes, however, large light-gathering power rather 

 than perfect seeing is required. 



The favourable atmospheric conditions at Mount Wilson 

 may be judged from the following table giving the number 

 of clear hours and nights spent in observation during the year 

 191 5-16, extracted from the last annual report of the Director. 



