116 THE CHEMISTRY OF THE SUN. [CHAP. 



them, and are not seen so frequently near the sun's poles as near 

 the equator. 



2. Nebulous. -Those which are perfectly tranquil, so far as 

 wave-length evidence goes. They are often high, are persistent, 

 and not very bright. These do not as a rule accompany 

 spots. 



This classification was afterwards accepted by Secchi, Zollner, 

 Sporer, and Young, and also by Kespighi, who, however, 

 divides the eruptive class into a great many sub-classes. 



Now, not only are these differences in the forms of the promi- 

 nences associated with differences in their chemical constitution, 

 but by observing the form of each prominence, first by means of 

 a line due to one substance and afterwards by means of lines 

 due to others, we can get actual sections of the prominence 

 showing the parts occupied by the substances producing the 

 different lines. The importance of such observations we shall 

 find it difficult to overrate. 



Detailed observations of the chromosphere showed that its free 

 edge was not only subject to those great local disturbances which 

 form the prominences, but that along its whole length it was 

 subject to the same action in a smaller degree. The edge of the 

 chromosphere is never smooth, but irregular, and the forms of the 

 irregularities are found to conform to one or other of the two 

 types to which we have seen the prominences may be 

 referred. Sometimes it is marked by gentle undulations, at 

 other times thrown into sharp jet-like projections. 



Since the introduction of the new method, several workers, 

 pre-eminent amongst whom Professors Kespighi, Secchi and 

 Tacchini must be mentioned, have carried on the observation 

 either of the forms and details or the chemical composition of 

 the prominences with indefatigable perseverance, so that as the 

 result of their labours we have an almost daily record of all the 

 prominences visible. 



It was at first thought impossible that prominences could 



