15 



in diameter on the slit of the spectrograph or 

 spectroheliograph. 



Loaned by the University of Chicago, and set up 

 on Mount Wilson at a time when the Solar Obser- 

 vatory was at work as an expedition from the 

 Yerkes Observatory, the Snow telescope was found 

 to have advantages and defects characteristic of a 

 new instrument. Currents of warm air, rising 

 from the hot soil of the mountain summit and 

 carried across the entering beam of light, decreased 

 the sharpness of the image during the hotter hours 

 of the day. The sun's direct rays warped the 

 telescope mirrors, changing the focus and blurring 

 the details of the image after a few minutes of 

 exposure. But the worst of these difficulties were 

 soon overcome by observing in the early morning 

 or late afternoon, shielding the mirrors from the 

 sun between exposures and cooling them with 

 blasts from electric fans. Thus controlled, the 

 Snow telescope yielded excellent photographs of 

 the solar atmosphere and justified the hopes we 

 had entertained of its performance. 



WHAT IS A SUN-SPOT? 



Sun-spots, though known and studied for 300 

 years, offered most promising opportunities for 

 research. Evidently there was much to be learned 

 from an investigation of their spectra, which had 

 never been attempted with adequate instrumental 

 means. To produce these spectra, the light of a 

 sun-spot was passed through a narrow slit, and 



