1 8 POPULAR SCIENCE MONTHLY. 



lines, which are much narrower and better defined. These lines, which 

 We designate as H 2 and K 2 , are the ones habitually employed in pho- 

 tographing the flocculi with the spectroheliograph. Superposed upon 

 these bright lines are still narrower dark lines, due to the absorption of 

 cooler calcium vapor at higher elevations (H 3 , K 3 ). It will be seen 

 that the evidence of the existence of calcium vapor at various densities 

 in the sun is complete, and that we may here find a way of photo- 

 graphing the vapor at low levels without admitting to the photographic 

 plate any light that comes from the rarer vapors at higher levels. It 

 is simply necessary to set the second slit of the spectroheliograph near 

 the edge of the broad H t or K 1 bands, in order to obtain a picture 

 showing only that vapor which is dense enough to produce a band of 

 width sufficient to reach this position of the slit. No light from the 

 rarer vapors above can enter the second slit under these circumstances, 

 since they are incapable of producing a band of the necessary breadth. 

 Light from the denser vapors below will, however, enter the slit. But 

 since it happens, within certain limits, that the vapor grows brighter 

 and also expands as it rises above the photosphere, it seems to follow 

 that a photograph will generally represent a section of the vapor at 

 the level corresponding to the position of the line on the slit, and that 

 little confusion will result from the presence of the denser but less 

 brilliant vapors lying below. 



The great sun-spot of October, 1903, afforded an opportunity to 

 try this method in a very satisfactory manner. Sections of the calcium 

 vapor in the neighborhood of this spot-group, corresponding to the 

 two different levels photographed on October 9, are shown in Fig. 9.* 

 The manner in which the vapor at the H 2 level overhangs the edge 

 of the sun-spot is very striking, and thorough study should throw 

 some light on the conditions which exist in such regions. For it 

 is possible, not only to photograph sections of the vapor at various 

 levels, but also to ascertain by the displacement of the H 2 line, as pho- 

 tographed with a powerful spectroscope, the direction and velocity of 

 motion of the vapor which constitutes the flocculus. It is commonly 

 found that the vapor is moving upward at a velocity of about one kilo- 

 meter per second, though the velocity varies considerably at different 



* Although these photographs have heen arranged for comparison with the 

 stereoscope, it is to be understood that no stereoscopic effect in the ordinary 

 sense will be obtained in examining them. The purpose of using the stereo- 

 scope is simply to allow the images to be superposed, thus permitting them 

 to be seen at the same point in rapid succession by simply moving a card 

 so as to cover alternately the two lenses of the stereoscope. In this way the 

 sun-spot may be examined, first as it appears at the low level of the denser 

 vapor and then as it appears at the higher level of the rarer vapor. Thus 

 the manner in which the calcium flocculi overhang the penumbra, and some- 

 times the umbra, of spots can be observed. 



