REPORT OF EXECUTIVE COMMITTEE. 127 



The results of this work are embodied in a paper "On the condi- 

 tions which govern the appearance of spark lines in arc spectra. ' ' 



Dr. Crew makes the comment on this paper that, in addition to 

 the solution of the original problem, it contains also the explanation 

 of the hitherto anomalous fact that an atmosphere of hydrogen 

 introduces spark lines into arc spectra. Both the phenomenon and 

 the explanation may be of considerable importance in astrophysics. 



3. As to the third problem, namely, to find the order, in point of 

 time, in which the lines of Mg and zinc make their appearance, the 

 situation has not changed since the last report, when it was stated 

 that unexpected difficulties arose when the attempt was made to 

 pass from the carbon spark to the metallic spark. In fact, the me- 

 tallic spark cools down so quickly that the entire phenomenon is 

 over in something like one one-thousandth of a second. 



George E. Hale, Mount Wilson, Cal. Grant No. 152. For experi- 

 ments on the use of fused quartz for the construction of optical 

 mirrors. $3 , 000. 



The recent developments of astrophysical research have shown 

 the necessity of constructing horizontal reflecting telescopes of great 

 focal length, especially for photographic observations of the sun. 

 The most serious difficulty in accomplishing this appears to lie in 

 the fact that the form of the mirrors employed in the coelostat tele- 

 scope changes through the expansion caused by the sun's heat. 

 This tends to injure the definition of the solar image, and thus to 

 prevent the accomplishment of the highest class of work. 



In 1903 Dr. Elihu Thompson suggested that if the mirrors could 

 be made of fused quartz the difficulty should practically disappear, 

 since the expansion of fused quartz by heat is only about one-tenth 

 that of glass. A grant made by the Carnegie Institution permitted 

 experiments in this direction to be undertaken, with the advice and 

 cooperation of Dr. Thompson. The immediate supervision of the 

 work was intrusted to Prof. G. W. Ritchey, superintendent of in- 

 strument construction at the Yerkes Observatory. After it had 

 been decided to erect the Snow telescope on Mount Wilson, it became 

 necessary for Professor Ritchey to accompany the expedition to 

 California, in order that he might take charge of the construction 

 of the new instruments required in the investigation. It was there- 

 fore decided to make the quartz experiments in Pasadena, where the 

 Edison Electric Company kindly offered suitable space in its power- 



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