248 CAMPBELL— CROCKER ECLIPSE EXPEDITION. 



A point of first importance in any method of studying the polar- 

 ized light consists in making sure that the apparatus itself does not 

 introduce polarization, and thus vitiate and render uncertain the 

 observational data. One should be especially on his guard when 

 using quartz and possibly other optical pieces in any way. Before 

 definitely adopting any form or design of analyzing apparatus, labo- 

 ratory tests should be made to ensure that this apparatus may he 

 trusted not to produce its own polarization effects. 



The Spectrum of the Corona. 



The spectrographic study of the corona at eclipses of the past 

 two or three decades has led to tolerably definite ideas as to the 

 quality of the coronal light. That there are shallow strata of incan- 

 descent gases overlying the photosphere and chromosphere of the 

 sun is certain. However, the use of the term strata in this connec- 

 tion may be misleading. Some of the strata seem to be fairly uni- 

 form in thickness over large arcs of the sun's limb, or change their 

 thickness very gradually in passing from one region to another, but 

 in other cases the thickness is very irregular. Slide No. 17 is an 

 exposure, effectively short, secured with a grating spectrograph in 

 1918, to record the so-called green coronal ring consisting of radia- 

 tions with wave-length 5303A. The ring is seen to be " lumpy." 

 Similar photographs of the ring at wave-length 4231A have shown 

 it to be more nearly uniform in thickness. The principal conden- 

 sations in the green ring are adjacent to prominences, but not in 

 coincidence with them. In the illustration the positions of the prin- 

 cipal prominences are indicated by the dotted line enclosures lying 

 ooitside of the green ring. The positions of the 5303 maxima are 

 indicated by the dotted line extending inward from the ring and 

 bearing numbers indicating the north and south solar latitudes. The 

 green condensations are slightly farther from the solar equator than 

 are the adjacent prominences. If the condensations are related to 

 the prominences, the relationship is not immediate and intimate. 

 There is poverty of green material near the north and south poles 

 of the sun. 



The faint dark bands passing through the green condensations 

 have been noted at earlier eclipses. The condensations are undoubt- 



