MOUNT WILSON OBSERVATORY. 197 



of lines being photographed in the spectrum of the prominences and 

 the upper chromosphere. Certain features connected with the radial 

 motion of one of the prominences appear to be of exceptional interest. 

 The negative secured wdth the prismatic spectrograph showed a faint 

 Une on one side of the solar image in the position of the green coronal 

 hne. WTiile no measures of the rotation of the corona are possible, a 

 determination of the wave-length of this line has been made which 

 appears to be of a high order of accuracy. 



AN ATTEMPT TO DETECT THE CORONA WITHOUT AN ECLIPSE. 



Two methods of detecting the corona in full sunlight were tried by 

 the Director many years ago. The first involved setting the second 

 sUt of a spectroheliograph on the center of a dark line in the solar 

 spectrum. This would produce a reduction in the brightness of the sky 

 without diminution in the light of the corona, assumed to be derived, 

 not from a bright line, but from the continuous spectrum. This 

 failed on Pike's Peak in 1893 and on Mount Etna in 1894. The 

 second method required the use of a sensitive bolometer, arranged 

 differentially to measure the relative radiation of the coronal streamers 

 and the sky between them. This also failed in several trials, and 

 Abbot's results at the total solar eclipse of 1900 indicated that the 

 coronal radiation was insufficient to be measured in sunlight with a 

 bolometer. 



At the recent total ecHpse of June 8, Dr. Joel Stebbins, using a 

 potassium photo-electric cell, obtained large deflections correspond- 

 ing to the total radiation of the corona (including, presumably, the 

 chromosphere and prominences). This suggested to the Director the 

 possible use of the photo-electric cell to detect the corona in full sun- 

 light, and when Dr. Stebbins was consulted he thought favorably of 

 the plan. A series of experiments has accordingly been made on 

 Mount Wilson by Mr. EUerman. 



Two photo-electric cells were mounted in a metallic box with 

 double waUs. The box was supported on the massive circular head of 

 the 75-foot spectrograph, at the focus of the 150-foot tower telescope. 

 When obser\'ing, the solar image (about 16.5 inches in diameter) is 

 made concentric with, the spectrograph head, which can be rotated 

 in position angle, carrying the photo-electric cells around the sun's 

 image. Both cells are exposed to the sky outside the sun through 

 adjustable apertures, the centers of which are equidistant (usually 

 about 2') from the sun's limb. The cells are so connected with the 

 electrometer as to give deflections corresponding to the difference in 

 radiation received by them. If one were on a coronal streamer and 

 the other on the sky between two streamers, the radiation would be 

 unequal, and it was hoped that we might detect by this means the 

 difference due to the added radiation of the corona. 



