618 Professor George E. Bale [May 14, 



the results would throw much light on the nature of sun-spots. For 

 many years, however, this hope was not realised. The first mono- 

 chromatic images of the sun were made with the K line of calcium. 

 If we compare such an image with a direct photograph of the sun, 

 made in the ordinary way, we see that the sun-spots are surrounded 

 and frequently covered by vast clouds of luminous calcium vapour. 

 These attain elevations of several thousand miles above the sun's 

 surface, but they must not be confused with the prominences, which 

 ascend to much higher elevations. When observed at the sun's limb, 

 the bright calcium flocculi, as these luminous clouds are called, are 

 so low, in comparison with the prominences, that- they can hardly be 

 detected as elevations. Thus our knowledge of the calcium flocculi 

 must be derived mainly from the study of spectroheliograph plates, 

 which show them in projection on the disk. I must not omit to 

 mention, however, that the calcium vapour rises to the highest parts 

 of the prominences, and that this higher a ad cooler vapour frequently 

 indicates its presence on spectroheliograph plates in the phenomena 

 of dark flocculi. These are relatively inconspicuous, however, and 

 need not be discussed here.* 



It soon appeared that the average photograph of bright calcium 

 flocculi could not be counted upon to indicate the existence of definite 

 streams or currents in the solar atmosphere. In 1903 the hydrogen 

 flocculi were photographed for the first time. By comparing these 

 flocculi with the corresponding calcium flocculi we see that, in general, 

 dark regions on the hydrogen image agree approximately in form with 

 bright regions on the calcium image. This might appear to indicate 

 that hydrogen is absent in the regions where calcium is most abun- 

 dant. An investigation of the question, however, does not lead to 

 this conclusion. Dark hydrogen flocculi seem to mark those regions 

 on the sun's disk where hydrogen is present as an absorbing medium, 

 which reduces the intensity of the light coming through it from 

 below. In certain areas, where the temperature is higher or the 

 condition of radiation otherwise different, the hydrogen flocculi are 

 bright. In many cases eruptions are in progress at these points, but 

 in others the difference in brightness is apparently not the direct 

 result of eruptive action. 



The hydrogen flocculi, thus photographed with the lines H/3, 

 Hy, or H8, differ in many respects from the calcium flocculi. Not only 

 do they usually appear dark, where the calcium flocculi are bright : 

 their forms exhibit striking peculiarities, which are absent or much 

 less conspicuous in the case of calcium. The appearance of the 

 calcium flocculi resembles that of floating cumulus clouds in our own 

 atmosphere, whose capricious changes in form reveal tiie operation of 

 no simple law. But the hydrogen flocculi, on the contrary, exhibit 

 a definiteness of structure in striking contrast to this appearance. 



* Eruptive prominences are also recorded on the disk^as bright flocculi. 



