112 THE CHEMISTRY OF THE SUN. [CHAP. 



layer, first of all, a layer of hydrogen with its prominences, and 

 then at the bottom of this layer another layer of magnesium, 

 which wells up sometimes where the prominences are most 

 active. 



The different lines we see in our instruments when we observe 

 the solar prominences are not all alike. Some of the lines vary 

 very much from the appearance of the c line of hydrogen, for 

 instance. In fact, in one line, the F line (see Fig. 43) we get a 

 trumpet-shaped appearance. The line widens as it approaches 

 the sun, so that it resembles an arrow head, resting on the thin 

 absorption line which forms the shaft. This is not only true of 

 the hydrogen lines, but of the lines due to the injection of other 

 substances into the sun's outer atmosphere from below. 



This artificial eclipse method does more than reveal the heights 

 and chemical nature of the prominences. A slight extension 

 of it enables us to see their actual forms and watch the 

 tremendous changes going on in them from time to time. The 

 first observation of this nature was made in 1868 1 by causing 

 the narrow slit, radial to the edge of the sun, to pass slowly over 

 the prominence. By this means a number of sections of varying 

 length was obtained, which, placed side by side, gave an idea 

 of its shape. It is sufficiently obvious that in this way a 

 perfect view of the prominence can only be obtained by moving 

 the slit with sufficient rapidity to allow of persistent images. 

 At the very outset Janssen and I attempted to accomplish 

 this, Janssen by giving a rotatory motion to a direct- vision 

 spectroscope, I by giving an oscillating motion to the slit, 

 in which plan I was followed by Young, 2 who afterwards ex- 

 panded it. Although these plans were successful, they were 

 subject to the disadvantages of reducing the quantity of light 

 and shaking the instrument. 



The method now adopted for viewing the forms of the promi- 

 nences is one of extreme simplicity, the principle of which was 

 1 Proc. Roy. Soc. No. 105, 1868. 2 Nature, Dec. 8, 1870. 



