RUMFORD SPECTROHELIOGRAPH. 18V> 



that these minute fl,occuli are cohimus of caloium vapor, rising above 

 the cohnnns of condensed ^'apol■s of which the photospheric '' <rrains " 

 are the summits. 



On such an assumption it becomes interesting to inquire whether 

 the hirger calcium floccidi are made np of simihir cohnnns of calcium 

 vapor. As a rule, the seeing is hardly good enougli to permit a deci- 

 sion to be reached on this point. But under the best conditions there 

 appears to be distinct evidence of a filamentary structure, the fila- 

 ments seeming to spread out like the branches of a tree (fig. 2, pi. v). 

 It is evident that much light could be thrown on the qiu'stion if it 

 Mere possible to photograph sections of the flocculi at different eleva- 

 tions above the photosphere, since in this way the forui and size of 

 distinct cohnnns of calcium vapor, if such were present, could be 

 determined at different levels. 



FOK^r AND EXTENT OF CAI.CIUIM FLOCCULI AT VARTOt'S ELEVATIONS ABOVE 



THE PHOTOSPHERE. 



Fortunately, it is possil)le to accomplish this very result, if the 

 present mode of explaining (he photograi)hs may l)e regarded as 

 sound. VCv have already had occasion to consider some of the charac- 

 teristics of the IT and K revcn-sals of the chromos])here. In the solar 

 spectrum itself the appearance of the H and K lines clearly indicates 

 that calcium Aapor occurs under widely different conditions of inten- 

 sity at various levels above the photospheres It is a well-known fact 

 that if a considerable quantity of calcium vapor is introduced into an 

 electric arc, l)road bands, bright in the center and fading towai'd 

 l)oth edges, will appear at the j^osition of the H and K lines (fig. 2, 

 pi. hi)." The width of the bands may be taken as an approximate 

 measure of the density of the calcium vapor, which decreases toward 

 the outer part of the arc, where the Ijands are reduced to narrow lines. 

 The narrow dark lines at the center of the briglit bands are caused by 

 the al)sorption of the comparatively cool and rare calcium vaj)or in 

 the outer ])in-t of the arc. 



A similar condition of things undoubtedly exists in the sun. In 

 the first place, we have broad diffuse dark bands in the solar spectrum 

 at II and K, pi-oduced by comparatively dense calcium va})or close 

 to the ])hotosphere. For convenience of ivference these bands will 

 be called 11^ and K,^. As the l)right reversals at the base of the 

 chromosphere, when j)h()tographe<l at the sun's limb with a tan- 

 gential slit, or* at a total ecli[)se, are much narrower than these bands, 

 it may be concluded that the dense calcium vapor in the chronio- 



Pliotosraphed in tlie electric arc with tlie solar spectrograph of the ►Snow 

 liorizontal telescope.' The fxratiiig temporarily euiployed gives strong ghosts, 

 which are consiticuous in the photograph. 



