viii.j THEORIES OF SUN-SPOTS. 101 



revealed through an opening in the photosphere, caused by 

 an upward current, and the spot is black by reason of the 

 feeble radiating power of the nebulous mass. The other school, 

 comprising Messrs. De La Rue, Stewart, and Loewy, referred 

 the appearances connected with sun-spots to the effects, cooling 

 and absorptive, of an inrush or descending current of the sun's 

 external atmosphere, which must be colder than the subjacent 

 photosphere. 



By some observations communicated to the Eoyal Astrono- 

 mical Society l in 1865, 1 was led independently to this latter con- 

 clusion. The observations indicated that instead of a spot being 

 caused by an upward current, it is caused by a downward one, and 

 that the results, or, at all events, the concomitants of the down- 

 ward current, are a dimming and possible vaporisation of the 

 cloud masses carried down. I was led to hold that the current 

 had a downward direction by the fact that one of the cloud- 

 masses observed, passed in succession, in the space of about two 

 hours, through the various orders of brightness exhibited by 

 faculae, general surface, and penumbrse. 



If we had been dealing with defective radiation, we should 

 still have been dealing with radiation, and should have expected 

 to see bright lines ; but no obvious bright lines were seen in the 

 spectrum of the spot ; what we did see was the thickening and 

 darkening of certain lines and the continuous absorption. In 

 the case of the lines of sodium it was very marked ; so that we 

 were perfectly justified in saying that the sun-spot was really 

 not produced by any defect of radiation, but was truly and 

 really produced by an increased amount of absorption. 



4. Spectrum of the Limb. 



The darkness of the spectrum at the limb is very different 

 from the darkness in a sun-spot. 



1 Monthly Notices Roy. Ast. Soe. vol. xxv. p. 237. 



