8TELLAK EVOLUTION. 1()1 



and are visihlc to tlu' nuked eye only when the diiHH't llu'Iit of the sun's 

 di.sk is cut ott' l)y the intm-position of the moon at a total eclipst\ But 

 methods have been devised by which tbey can be observed or photo- 

 graphed on any clear day through the agency of a modified form of 

 spectroscope. The prominences are constantly changing in form, 

 sometimes slowly, as in the case of this group (PI. IX. tig. 1), a photo- 

 graph of which, taken at the eclipse of May 28, 11 loo, by the astronomer 

 royal of England in Spain, is shown for comparison with the photo- 

 graph taken about two hours earlier l)v the Yerkes Observatory party 

 in North C'arolina. Here the change in the foim of the mass of gas 

 which constitutes the prominence is comparatively small, but that 

 violent forces are sometimes at work nuw be illustrated by photographs 

 of an eruptive prominence tak(>n at the Kenwood Obsei'vatory in 1895 

 (PI. IX, fig, 2). At the moment when the first photograph was made 

 the prominence had attained a height of 1(>0,000 miles and was rising 

 rapidl}. Eighteen minutes later another picture was taken; during 

 the interval the prominence hadl)een going upward at the rate of 6,000 

 miles a mimite, and when the exposure was made it had reached an 

 elevation of 280,000 miles. When looked for a few minutes later it 

 had completely disappeared. 



The constitution of the chromosphere, the sea of flame some lo,000 

 miles deep from which the prominences rise, increases in complexity 

 as the surface of the solar disk is approached. In its upper part only 

 the vapor of calcium and the light gases, hydrogen and helium, are 

 found. But in proceedi ng downward the vapors of magnesium, sodiinn, 

 iron, chromium, and last of all, carbon, are successively encountt'red. 

 At this ])artof the solar atmosphere the dark lines of the solar spectrum 

 take their rise through the effect of absorption. 



Time does not permit a detailed description of the phenomena of the 

 sun's disk. When photographed with an instrument which excludes 

 from the sensitive plate all light except that Avhich is characteristic of 

 the vapor of calcium, its surface is found to be dotted over with exten- 

 sive luminous regions. Associated with these are the sun spots, the 

 minute study of which has revealed some strikingly beautiful phenom- 

 ena, which have been most successfully drawn by Langley. The 

 surface of the sun in the regions devoid of spots is show n by the pho- 

 tographs of Janssen to consist of brilliant granules separated b}" darker 

 spaces. Much might l)e said of the peculiar law of rotation of the 

 sun, which causes a point near the e((uator to complete an axial rota- 

 tion in much less time than a point nearer the poles. Much might 

 also be said of the periodicity of sun spots, which at times are very 

 numerous and again, as at present, are absent from the sun's disk for 

 weeks together. But enough has already been told to indicate some 

 of the chief eharacteristics of this central star of the solar system, 



SM 1902—11 



