620 Professor George E. Hale [May 14, 



the prominences much more clearly than photographs taken with H8. 

 Moreover, since Ha is stronger than H8 in the upper chromosphere, 

 in regions where no prominences are present, the average level repre- 

 sented by this line will, in general, be higher than that represented 

 by H8. A comparison of two photographs of the sun's disk, made 

 with the lines in question, will suffice to make this clear. This enor- 

 mous group of prominences, stretching for several hundred thousand 

 miles across the sun, is much more clearly indicated by Ha than by 

 H8. In general, the hydrogen flocculi are stronger and more distinct 

 when photographed with Ha, and there are some regions which appear 

 bright with Ha and dark with H8. This latter peculiarity probably 

 has an important bearing upon the similar behaviour of hydrogen in 

 certain stars and nebulae, but a discussion of this question cannot be 

 undertaken here. 



The jSrst of the Ha photographs gave strong hopes of a substan- 

 tial advance in our knowledge of the solar atmosphere. The sharp- 

 ness and comparatively strong contrast of these flocculi, and the evi- 

 dences of definite structure and clearly defined stream lines which 

 they revealed were highly encouraging. The work was begun during 

 the disturbed weather of the rainy season, when the definition of the 

 solar image is never of the best. On April 30, 1908, the first photo- 

 graphs were secured under the fine atmospheric conditions which pre- 

 vail in the dry season. This direct photograph (Fig. 1) shows a 

 small and insignificant group of sun-spots, which would not seem, 

 without other indications, to merit special attention. The next 

 photograph (Fig. 2) shows that an enormous calcium flocculus occu- 

 pied this region of the sun, but its form was in no wise remarkable, 

 and afforded no evidence of the phenomena brought to light by the 

 Ha photograph (Fig. 3). The structure recorded with the aid of the 

 latter line recalls Langley's sun-spot drawing, and suggests the opera- 

 tion of some great force related to the sun-spot group. The same 

 cyclonic structure had been less satisfactorily recorded on the previous 

 day, but a comparison of the two photographs failed to indicate such 

 changes as motion along the apparent stream lines might be supposed 

 to produce. 



The close of the rainy season now permitted an active study of 

 the Ha flocculi to be undertaken. Many photographs were made 

 daily, and the almost constant association of apparent cyclonic storms 

 or vortices with sun-spots became evident. During several months 

 of the year in California an unbroken succession of clear days can be 

 counted upon, so that the changes of a given vortex can be followed 

 without interruption. The cyclonic storms were found to be of two 

 principal types : the first associated with groups of spots and repre- 

 sented in such photographs as those of April 30 and September 2 ; 

 the second associated with single spots, and resembling a simple vor- 

 tex, as illustrated in the photographs of September 9 and October 7, 

 1908 (Fig. 4). The appearance of these simple vorticles is such as 



