78 REPORTS ON INVESTIGATIONS AND PROJECTS. 



facilities such photographs offer for detecting eruptive phenomena. Ac- 

 cordingly, our studies of the total area and brightness of the flocculi are 

 being made with special reference to their comparison with the records of 

 magnetic phenomena. 



In order to have at hand a means of at once detecting magnetic storms, 

 a special instrument, devised for this purpose, has been installed on 

 Mount Wilson, with the kind advice and cooperation of Dr. Bauer. This 

 recording variometer gives a continuous record of the magnetic varia- 

 tions; it is also provided with an alarm, which notifies us in case the 

 deflections become at all marked. We are thus enabled to take special 

 photographs of the sun at times when the disturbed state of the earth's 

 magnetism might lead to the supposition that active solar phenomena 

 could perhaps be detected with the spectroheliograph. The variometer 

 has been in regular use, in the charge of Mr. Ellerman, since last spring. 

 It promises to be a most effective part of our equipment. 



Special Investigations. 



Two special investigations were carried on by Professor Nichols dur- 

 ing the summer. The first of these was a study of the effect of the 

 ionization produced by X-rays on the absorption or radiation of a gas or 

 vapor. NOo, for example, absorbs X-rays strongly. When a continu- 

 ous source is observed through this gas, a very characteristic absorption 

 spectrum is produced. The question was to determine whether the 

 degree of the absorption, or its character, is in any wise altered when 

 X-rays are sent through the gas. The effect was studied both visually 

 and photographically. If any change takes place, it is small and can 

 only be determined from a more complete study of the photographs than 

 Professor Nichols has yet had an opportunity to make. This, and other 

 related investigations, will be continued by him, in the hope that light 

 will be thrown on the nature of the radiation processes that obtain in 

 solar prominences and in other celestial phenomena. 



The second investigation dealt with the question whether the "Rest- 

 strahlen," obtained after repeated reflections from rock-salt surfaces, 

 reach us in any appreciable amount from the sun. By the aid of repeated 

 reflections on rock-salt surfaces, the rays of other wave-length were 

 eliminated from the sunlight, which was then focused upon the vane of 

 a very sensitive radiometer. The purity of the observed beam was tested 

 by transmission through a rock-salt screen, and by comparative tests of 

 the light of an electric arc. The very small deflections observed indicate 

 that if any light of this wave-length reaches us from the sun, its intensity 

 is inconsiderable. The exact quantitative results, however, can not be 

 given until the reductions now in progress are completed. 



