204 



ANNUAL REPORT SMITHSONIAN" INSTITUTION, 192 8 



spectroscope, discovering the mimerous dark lines now known as the 

 Fraunhofer lines. 



Through the addition of a means for moving the telescope con- 

 tinuously in right ascension, the mechanism reached a state in which 

 the observer could devote his thoughts and attention solely to his 

 work, and a great increase in the accuracy of measurements resulted. 

 Most telescopes are turned with a mechanical driving clock, but 



MIRROR 



LENS 



FiGUitE 1. — Action of a lens and a concave miiTor ou parallel light 



to-day some are electricall}^ driven. In the mechanical clock the 

 essential parts are a falling weight or an unwinding spring whose 

 rate of descent or release is regulated by a governor through a train 

 of gears. Foucault designed many of these governors, but the prin- 

 ciple in all of them is tlie introduction of a slight amount of resist- 

 ance to regulate the fall of the weights. I'he goveinor used in all our 

 telescopes on Mount Wilson is patterned after the one designed by 



