250 CAMPBELL— CROCKER ECLIPSE EXPEDITION. 



undoubtedly contains the Fraunhofer lines. The interpretation is 

 that the outer corona is shining, possibly in part by its own light, 

 but chiefly by virtue of the photospheric rays falling upon the outer 

 coronal materials. SHde No. 19 was obtained in 1918. The broad 

 spectrum is that of the sun itself. The spectrum of the corona to 

 the west of the sun is in the upper narrow band. The green bright 

 line is shown on the extreme right, and the ultra-violet bright line 

 at 3601A is very close to the extreme left end. The bright points 

 on the lower edge of the coronal spectrum are from the promi- 

 nences. The radiations in tw^o long bright lines at H and K un- 

 doubtedly originate chiefly in the prominences, and the great length 

 of these lines is due to the dift'usion of the calcium light in its own 

 atmosphere. The exposure was in effect too short to record the 

 spectrum of the outer corona. No dark lines are visible. Spec- 

 trograms secured by us at earlier eclipses have recorded dark lines 

 in the outer corona. 



Trustworthy observations of this kind require an absolutely clear 

 sky. It is dangerous to draw conclusions from spectrograms ob- 

 tained through light clouds. This is illustrated by slide No. 20, 

 copied from the coronal spectrogram secured in Sumatra in 1901. 

 The Fraunhofer lines are shown strongly in the outer corona, a>id 

 likeivise upon the dark moon! Much of the Fraunhofer effect is 

 undoubtedly due to the diffusion of the sun's photospheric rays by 

 the thin clouds which covered the sky at that tmie. 



The observed polarization effects are in harmony with the spec- 

 trographic as to the nature of the continuous coronal radiations. 



Rotation of the Corona. 



A few observers have sought to determine the rotational speed 

 of the corona as a Doppler-Fizeau effect, by measuring the accurate 

 wave-lengths of the green bright line to the east and to the west of 

 the sun. If the corona is rotating with a speed approximating that 

 of the sun's underlying surface, then the corona adjoining the east 

 limb of the sun should be approaching us at the rate of two km. per 

 second, and the corresponding coronal structure west of the sun 

 should be receding with an equal speed. It cannot be said that the 

 observations for rotation have been successful, though the dift'er- 



