SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. 1 57 



Carnegie Institution were installed. Since that time, with only such 

 interruptions as have been made necessary by the enforced absence of 

 the observers, the instruments have been read at stated hours by Mr. 

 Ferdinand Ellerman or Mr. W. S. Adams, who have also made 

 regular tests of the seeing with the telescope mentioned above. 



Through important financial assistance rendered by Mr. Arthur 

 Orr, of Evanston, Illinois, and 3.1r. John D. Hooker, of Los An- 

 geles, and the exceptional facilities kindl}' granted by the Atchison, 

 Topeka and Santa Fe Railway Company, through President Ripley, 

 it became possible to bring from the Yerkes Observatory the small 

 coelostat which had previously been sent to the eclipses of 1900 

 (North Carolina) and 1902 (Sumatra). It had been my purpose to 

 bring out the Snow telescope, but lack of sufficient funds prevented 

 me from doing so. The smaller coelostat was accordingly erected on 

 the mountain, where it yielded excellent photographs of the sun, 

 amply sufficient to give objective evidence of the high quality of the 

 observational conditions. 



During my first visit to Mount Wilson the only unfavorable feature 

 was the presence of fine dust in the air, w^hich was conspicuous not 

 only in the valley below, but also seemed to extend to a considerable 

 altitude above the mountain. This was by no means sufficient to 

 affect greatly the transparency of the sky, except very near the hori- 

 zon. Nevertheless, the Milkv Wav did not stand out with the degree 

 of contrast which one expects to see in a very transparent atmos- 

 phere. On my return trip to Chicago through the San Gabriel Valley 

 the dust seemed so conspicuous that I feared it might prove an im- 

 portant objection to Mount Wilson as a site for an observatory. In 

 most classes of solar observation dust does not play a very important 

 part, and the great steadiness of the image would far outweigh any 

 objections which might result from this cause. But in other classes 

 of work which were contemplated for the proposed observatory, this 

 dust, if persistent, would inevitably prove a serious obstacle. For 

 example, in determinations of the value of the solar constant and in 

 the photography of faint nebulae, the absorption and scattering of 

 light produced by dust in the atmosphere may interfere greatly witli 

 the work. It accordingly seemed that special attention should be 

 given to the question of dust in the atmosphere above Mount Wilson. 

 It has fortunately turned out, as will be shown later, that the presence 

 of any appreciable amount of dust in the air above the mountain is 

 so exceptional a phenomenon as to constitute no important objection 

 to Mount Wilson as an observaton^ site. 



