( 221 ) 



Recently it has been found by Prof. Half, that, while the II,-, 

 H r and /7,-lines give very similar results, photographs with the 

 much stronger line H v are widely different in some respects ! ). 

 Bright floccnli appear on these plates at points where no corresponding 

 objects are shown by H \ Moreover, the dark H y - floccnli, while 

 showing a general agreement in position and form with those of 

 H$, are stronger and more extensive. In some instances, however, 

 small areas appear dark in Hs which are absent or fainter in H y . 



Such differences are of the same character as those observed 

 between photographs made with the slit in the broad calcium-bands 

 H or K at various distances from the central line. They may find 

 a corresponding explanation if we assume that the rays, used in the 

 H u photographs, were on the average refracted to a higher degree 

 than those, used in the i^-photographs, but both by the same system 

 of density gradients. It is not improbable, therefore, that in the 

 wings of H y waves may be selected so as to give spectroheliograph 

 results, closely resembling // f ;-plates. 



That also lines of different elements may give very similar results 

 with the spectroheliograph, is exemplified by the case of calcium 

 and iron. Among the beautiful collection of photographs secured on 

 Mount Wilson I saw several iron-(A 404:5)-plates rather closely resem- 

 bling certain calcium-tfiTJ-plates of the same daily series. As the 

 atomic weights of calcium and iron are not so very different, and 

 their levels of maximum density therefore probably not far apart, 

 the refraction caused by these elements may bring out the density 

 gradients of nearly the same layer of the solar atmosphere. It will 

 do so by showing a similar distribution of the light in the two 

 photographs — provided that rays of the same refrangibility are 

 used in both cases. And this condition may be fulfilled by setting 

 the camera slit on corresponding regions of the spectrum, in the 

 sense as illustrated by Fig. 3, if we imagine it now to bear on the 

 calcium-(/f)-line and the iron-(/. 4()45)-line. 



With a calcium- and a hydrogen-line such similarity could not be 

 found. 



Far more evidence will of course be required before we shall be 

 able to decide whether or not anomalous dispersion is the principal 

 agent in determining the flocculent appearance of the solar disk. 



bilities regarding the relative position of density gradients and source of light. 

 Consequently a H-;-p!ate, obtained with the camera-slit centrally, so as to embrace 

 the whole width of that rather narrow dispersion band, will scarcely differ, at 

 first sight, from a photograph made with only one of the wings. 



2 ) Hale, "Solar vortices", Contrib. from the Mt. Wilson Solar Obs. No. U 2G. 



15* 



