108 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



of their rare occurrence, are almost instantaneous, so far as each 

 particular phenomenon is concerned ; and, secondly, when the 

 duration is, say, four or five or six minutes, which happens but 

 very rarely, although a great deal of work may be done, only 

 a very small part of the more interesting regions of the solar 

 atmosphere is uncovered. 



How then can we avail ourselves of this method ? It should 

 be perfectly clear that if instead of observing a spot we allow 

 the slit to lie on the edge of the sun, and then sweep round it, 

 if the method is competent to abolish the illumination of our 

 atmosphere to make the bright lines visible that here and 



FIG. 41. Line C (red), with radial slit. 



there if the slit travels over a prominence it will admit the light 

 of the prominence ; and if we have the image of the sun very 

 accurately focused on the slit, the size of the image of the sun 

 and the length of our slit being known, the length of the slit 

 illuminated by the prominence will enable us readily to deter- 

 mine the exact height of the prominence. Further, if it should 

 happen that there is a sort of vaporous sea round the sun 

 usually invisible, but which this new method will reveal, it 

 would follow that we shall get the depth of the sea sounded for 

 us by the length of the line. 



Again, if we do sweep round the sun in this way, and if these 

 prominences really do give us lines, we have exactly the same 



