lines, which are much narrower and better defined. These lines, which We designate as H2 and K2, are the ones habitually employed in photographing 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 (H3, K3). 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 photographing 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 H1 or K1 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 H2 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 H2 line, as photographed 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 kilometer per second, though the velocity varies considerably at different
- Although these photographs have been 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 stereoscope 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 sometimes the umbra, of spots can be observed.