142 RUMFORD spp:ctroheliograph. 



ered with columns of bright calcium vapor, varying in diameter 

 from less than a second to several seconds of arc, sej)arated by darker 

 spaces, which correspond in appearance to the darker spaces that 

 separate the photospheric " grains." The summits of these columns 

 seem to lie in the second stratum, corresponding to the bright lines 

 H„ and K,. 



In the larger fiocculi the surmise of a structure composed of ex- 

 panding columns of calcium vapor seems to be borne out by the pho- 

 tographs. Compare, for example, figures 1 and 2 in plate vii. At 

 the lower level (fig. 1) the flocculus is resolved into a series of well- 

 defined elements, of comparatively small area. At the higher level 

 (fig. 2) the area of the entire flocculus is greatly increased, and there 

 seems to be evidence (hardly visible in the cut) that the columns 

 comjiosing it have arched over, so that they are no longer seen end 

 on. Few photographs are sufficiently well defined to bring out such 

 details, and it can not be said Avitli certainty that the eifects seen at 

 the higher level are always due to separation and bending of the col- 

 umns, as well as to expansion of each of the individual columns. In 

 any event, the increase in area at this level is sometimes very great, 

 in many cases sufficient to cover not only the penumbra, but also 

 entire spots. Another illustration of the expansion at increasing 

 altitudes may be seen in the four photographs, corresponding to dif- 

 ferent levels, which are reproduced in plates vii and viii. A much 

 finer illustration is afl'orded by the photographs in plates xi to xvjii 

 of the recent great sun spot. 



HYDROGEN FLOCCULI. 



The method of photographing the sini with the aid of the dark 

 Fraunhofer lines has already been explained. The spectroheliograph 

 is employed exactly as in the case of the bright calcium lines, but the 

 dispersion is increased sufficiently to insure that the Avidth of the 

 dark lines shall be greater than that of the second slit. Under 

 such circumstances photographs corresponding to the hydrogen lines, 

 or to any other dark lines of sufficient width, may be obtained. 



The first photograpli made with a dark hydrogen line {Hfi) was 

 taken with the Eumford spectroheliograph on May IG, 1903. On 

 developiug the plate we were surprised to find a structure differing 

 materially from that obtained with H. and K^. Closer examination 

 and a comparison of the photograph with a K. photograph made on 

 the same day showed that the bright calcium flocculi were replaced 

 on the Up photograj)h by dark structures of similar, though by no 

 means identical, form. There could be no doubt about the adjust- 

 ment of the Hfi line on the second slit, since a prominence was shown 

 on the photograph extending above the sun's limb. At a point near 

 a sun spot a brilliant object appeared. The same bright object was 



