130 THE CHEMISTRY OF THE SUN. [CHAP. 



The experiment is managed as follows. The condenser of a 

 Duboscq lantern being removed, a combination of a slit and 

 lens is put in its place, the slit being placed inside the lantern 

 close to the carbons at the principal focus of the lens. The 

 carbons, about f- of an inch in diameter, have their ends filed 

 perfectly flat, and a very thin layer of metallic sodium is spread 

 on the lower pole. 



The rays of light proceeding through the slit fall upon the 

 lens, which renders them parallel, and then enter two hollow 

 prisms filled with carbon disulphide. They are next focused by 

 means of a lens of long focus upon a screen. 



When the two poles are brought together the sodium line is 

 seen to be very strongly reversed upon the screen, the line 

 being about 15 inches wide in a spectrum of between 3 and 4 

 yards long, and it remains so, as long as the carbons are nearly in 

 contact, till all the sodium is volatilised. By separating the poles 

 the absorption line can be thinned down until it becomes a fine 

 line, and then, by opening them a little more, the line can be 

 obtained as a bright one. 



The cause of this is perfectly obvious. The temperature is 

 practically the same all the time, but we have a very considerable 

 quantity of sodium vapour surrounding the incandescent poles 

 in the first instance. On the continued application of the heat 

 this sodium vapour goes away by degrees, and we gradually 

 deal with a smaller quantity, and as we deal with a smaller 

 quantity the line thins. We therefore are justified in saying 

 that when in a sun-spot we get the line of sodium considerably 

 thickened, that is due to the fact that there is a greater quantity 

 of sodium vapour present in that spot. 



These principles afford a satisfactory explanation not only of 

 the widening of F and other lines near the limb, but of those 

 bright patches or lozenges which are sometimes seen interrupting 

 or accompanying the dark hydrogen lines on the surface of the 

 sun itself, especially in the locality of spots. The fact that the 



