100 THE CHEMISfRY OF THE SUN. [CHAP. 



spectrum as a dark band, of greater or less width according to 

 the size of the spot, running along the whole length of the 

 spectrum, and crossed, as are the other parts of the spectrum, by 

 the Fraunhofer lines; the nucleus is seen as a narrow band 

 darker than the rest. Besides this continuous absorption, another 

 phenomenon is seen. Some (often many) of the Fraunhofer 

 lines in the parts where they are crossed by the spot spectrum, 

 are considerably thickened, the widening being greatest in the 

 nucleus and gradually fading away in the penumbra. 



In Fig. 38 this appearance is well depicted. The spectroscope 

 has been so placed that its slit bisects two spots, in each of 

 which many lines are seen widened, notably the two D lines of 



FIG. 38. Spectrum of sun-spot, showing the widening of the D lines. . 



sodium. It is not always the same set of lines that are widened. 

 In some spots the sodium lines are most affected, in others the 

 iron lines, and so on. 



I was induced to start this method of observation in 1866 for 

 the following reason : 



Two rival theories had been suggested to explain how it 

 was that a sun-spot is dark. 1 One school, led by M. Faye, 

 believed that the interior of the sun is a nebulous gaseous mass 

 of feeble radiating power at a temperature of dissociation, and 

 that surrounding this is a highly radiating photosphere. On 

 this hypothesis, in a sun-spot, the nebulous interior mass is 



1 See Proc. Roy. Soc. 1866, vol. xv. p. 256. 



