MOUNT WILSON SOLAR OBSERVATORY. 235 



The following photographs have been taken with the 60-foot tower 

 telescope by Messrs. Ellerman, Nicholson, Joy, and Campbell, who 

 also made those taken with the Snow telescope: photoheliograms, 

 207; 5-foot spectroheliograms (Ha, of entire disk), 139; 13-foot spectro- 

 heliograms, 1,G09. 



IDENTITY OF THE DARK " FILAMENTS" WITH PROMINENCES. 



The hydrogen flocculi of the liigher solar atmosphere, including the 

 long dark objects which M. Deslandres has called ''filaments," were 

 first photographed with the Rumford spectroheliograph of the Yerkes 

 Observatory in 1903. Their essential identity with prominences was 

 at once suspected, and was confirmed by observations near and at the 

 limb made at that time and in subsequent years. A good case in point 

 is illustrated in our paper on "Solar Vortices" in the Astrophysical 

 Journal for September 1908. 



The same conclusion has been reached by Evershed, Ricco, and 

 others who have investigated these objects, but it has been disputed by 

 Deslandres, who maintains that the identity has not been proved. 

 Without going into details, which will be given in full elsewhere, it 

 may be said that a great number of prominences photographed with 

 the 13-foot spectroheliograph fully bear out our original interpreta- 

 tion. Many beautiful examples of prominences photographed with the 

 center of Ha, while half projected against the disk and half rising 

 above the limb, occur in our collection of plates. 



STEREOGRAMS OF CALCIUM AND HYDROGEN FLOCCULI. 



The rapid changes in form characteristic of most phenomena of the 

 solar atmosphere interfere seriously with the preparation of reliable 

 stereograms. Several years ago we published stereograms of the cal- 

 cium flocculi, made with an interval of several hours, which show the 

 sphericity of the solar image and the rehef of the H2 flocculi. But these 

 were small scale images of a region in the solar atmosphere where the 

 changes in form are much less rapid than at the higher hydrogen level. 



To overcome the difficulty, it is necessary to limit the interval 

 between the photographs to the shortest possible time. Under the 

 conditions used in our work, an application of the criterion of Helm- 

 holtz indicates that the angle through which the sun rotates in 5 

 minutes should be sufficient to show true stereoscopic rehef. Combin- 

 ing properly a pair of photographs taken with this interval, we see the 

 dark hydrogen flocculi, and especially the long dark filaments which 

 have been identified with prominences, rising plainly above the surface. 



We must be on our guard, however, against false effects of relief 

 due to various causes. The true stereoscopic relief with such a time 

 interval is very small, and may sometimes be completely masked by 

 false effects. To check its reality, we have photographed a globe bear- 

 ing small objects in relief, and have found that by turning it through 



