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SMITHSONIAN MISCELI/ANEOUS COLI.ECTIONS 



VOL. 52 



it is probable that calcium vapor lying in the upper chromosphere, 

 below the level of prominences, may produce dark flocculi. 



Our discovery at the Yerkes Observatory of the dark calcium 

 flocculi was made soon after we had first photographed the hydrogen 

 flocculi and found them (in most cases) to be dark. On the hydro- 

 gen plates there occasionally appeared exceptionally dark flocculi, 

 and when one of these plates was compared with a calcium plate 

 taken at about the same time, a dark object, similar in form to that 

 shown by the hydrogen plate, was found to be present. We thus 

 have strong presumptive evidence, since the hydrogen and calcium 

 plates show these effects in the same way, that these particular 

 hydrogen flocculi are comparatively high-level phenomena. 



While it of course does not follow that the ordinary hydrogen 

 flocculi, which are not so dark as these exceptional ones, lie at the 

 same level, the very fact that they are dark suggests the view that 

 they are due to the absorptive effect of the cooler hydrogen in the 

 upper chromosphere. The bright hydrogen flocculi, so frequently 

 recorded in the neighborhood of Sun-spots, are supposed to be due 

 to radiation from hydrogen at a higher temperature. 



Assuming for the present the validity of this hypothesis, it ap- 

 pears that the ordinary dark hydrogen flocculi recorded in our daily 

 photographs of the Sun represent a higher level than the bright cal- 

 cium flocculi obtained in the daily series made with the Ho line. 

 Thus we might reasonably expect that the rotation period derived 

 from a study of the motion of these flocculi would differ from that 

 of the bright calcium flocculi. 



The measures of the daily change in longitude of the hydrogen 

 flocculi at present available are too few in number to give a reliable 

 determination of the solar rotation. Indeed, the marked proper 

 motions of these objects in all directions on the solar surface, and 

 their rapid change of form, will make it necessary to obtain a great 

 number of these measures before final conclusions can be drawn; 

 547 flocculi measured on 20 different plates give the results obtained 

 in the following table. 



