KHMFOKI) SI'K(TK(>IIKLI()(}KAl'n, 141 



llocciilus !)(' shoAvii on such a [)h()t()<j,raph, since the line thoy pnxhicc 

 is not broad (Mioti<2h to cntoi' the second slit. If the slit were set 

 neai'er to the center of the line the piiotograph should i-epi-esent 

 a section of the lloccidus correspondino- to a hi<i;hei' level, wliei'e a 

 narrower line is ])roduced. It is evident that while none of the 

 higher and raivr calcium va})or could he shown in this })hotograph, 

 il ini<j,ht nevertheless include reaions lyinu' helow it, where the 

 «"dciiHn Aai)or is dense enou<>h to [)roduce a broader line. However, 

 since the calcium vapor is risino; from a re<2;ion of hi<'h pressui'e to 

 one of a much lower pressure, it nnist exj)and as it rises, and there- 

 fore a section at any level should, in <>eneral, be of a la)'<>er area 

 than a section of the same flocculus at any lower level. As a con- 

 sequence of the increasin<i- extent of the vapor with the altitude, and 

 the increase of brightness observed when passing from K^ to K., 

 a photograph corresponding to a given level is not necessarily af- 

 fected in any considerable degree l)y the existence of the denser vapor 

 below, except in cases where the high-level vapor does not lie innne- 

 diately above the low-level vapor. LoAv-level phenomena, even 

 when very bright, may be wholly concealed by general excess of 

 radiation, or in some cases by absorption, of the calcium vapor at 

 high levels. Moreover, it is of course to be understood that the 

 term '' level "' is not used here in a strict sense. A section of a 

 large flocculus photographed ^fith K. might, for example, correspond 

 to a nuu^h greater height above the photosphere than that of the 

 minute flocculi shown on the same photograph. It must never be for- 

 gotten, when examining the photographs, that composite effects 

 are very likely to be present. 



Such considerations regarding the possibility of photographing 

 sections of the flocculi at different levels are borne out by the photo- 

 graphs, as will be seen by reference to the accompanying illustra- 

 tions. Plates VII and viii represent the spot group of 1903, April 

 i2i), as photographed with four different settings of the second slit. 

 These were taken within such time limits and in such an order tliat, 

 as no distinctly eruptive phenomena were present, the principal dif- 

 ferences between tlie photographs are therefore to be attributed to 

 differences in the extent and brightness of the vapor at various levels, 

 and not to clianges going on in the sun at the time. 



The assumption that these photographs represent sections of .the 

 calcium flocculi at different elevations seems to be the simplest and 

 most satisfactory way of explaining the results obtained. Essen- 

 tially conclusive evidence in favor of this assumption is afforded by 

 the photographs of the recent great sun spot. (Pis. xii to xviii.) 



"With the aid of this additional means of research we may return 

 to a consideration of the structure of the flocculi. It has already 

 been remarked that the general surface of the sun appears to be cov- 



