54 CARNEGIE INSTITUTION 



slopes of the mountain. It is not surprising, therefore, that the sun's 

 image, as seen from the summit of Mount Wilson, is apparently bet- 

 ter defined than at any other point hitherto tested with a telescope. 



But for the successful prosecution of solar research another condi- 

 tion must be fulfilled. The phenomena on the sun's surface are 

 constantly changing in form, not only from hour to hour, but from 

 second to second in the violent eruptions which are numerous dur- 

 ing the period of greatest solar activity. In order to study these 

 changing phenomena intelligently, it is necessary that they be kept 

 constantly under observation. In observatories subject to frequent 

 clouds and storms the progress of such solar changes cannot be 

 steadily watched. At some critical moment clouds frequently inter- 

 pose to prevent further work. In an investigation of the solar rota- 

 tion, for example, it is of great importance that the position of a 

 sun spot or a facula be determined day after day without interrup- 

 tion. In actual practice many of the photographs made at existing 

 observatories are rendered almost useless on account of the cloud} 7 

 periods which separate them from other photographs. Mount Wil- 

 son has the unique advantage of combining extraordinary perfec- 

 tion of definition with such freedom from clouds as to permit con- 

 tinuous work for months at a time. 



Summing up, we may therefore say that, even with existing 

 methods of research, important advances in our knowledge of the 

 sun could be attained by providing for observations ( i ) of the solar 

 heat radiation from some high elevation, and (2) of the phenomena 

 of the sun's surface from a site such as Mount Wilson. In the be- 

 ginning the work should be divided between two sites, but it is prob- 

 able that the higher station could be given up after the relation of 

 the atmospheric absorption at the two stations should become known. 

 During the same period it would also be necessary to provide for 

 simultaneous observations from a third point many thousands of 

 feet below the high station, to measure the absorptive effect of a 

 known atmospheric layer, in order that the total atmospheric absorp- 

 tion may be determined and eliminated. 



New Types ok Reflecting Telescopes and Their Use in Conjunction 

 with Laboratory Instruments. 



The exceptional opportunity which exists at the present time for 

 advancing our knowledge of the sun by no means depends solety, 

 however, upon the possibility of eliminating a large part of the dis- 

 turbances due to our atmosphere. Even greater possibilities for 



