1866.] 



Observations of the Sun. 



257 



some obseryatious (referred to by tlie authors last named) wbicli had led 

 me independently to the same conclusion as the one announced by them. 

 The observations indicated that, instead of a spot being caused by an u^- 

 icard current, it is caused by a dowmuard one, and that the results, or, at 

 all events, the concomitants of the downward current are a dimming and 

 possible vaporization 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 facidcs, ge- 

 neral surface, B.nd pemnnbrcs. 



On March 4th of the present year I commenced a spectroscopic ob- 

 servation of sun-spots, with a view of endeavouring to test the two rival 

 theories, and especially of following up the observations before alluded to. 



The method I adopted was to apply a direct-vision spectroscop'e to my 

 6.|-inch equatoreal (by ^lessrs. Cooke and Sons) at some distance outside the 

 eyepiece, with its axis coincident with the axis of the telescope prolonged. 

 In front of the slit of the spectroscope was placed a screen on which the 

 image of the sun was received ; in this screen there was also a fine slit cor- 

 responding to that of the spectroscope. 



By this method it is possible to observe at one time the spectra of the 

 imibra of a spot and of the adjoining photosphere or penumbra ; unfor- 

 tunately, however, favourable conditions of spot {i. e. as to size, position 

 on the disk, and absence "of cloudy stratum"), atmosphere, and instru- 

 ment are rarely coincident. The conditions were by no means all I could 

 have desired when my first observations were made ; and, owing to the 

 recent absence of spots, I have had no opportunities of repeating my ob- 

 servations. Hence I should have hesitated still longer to lay them before 

 the Royal Society had not M. Faye again recently called attention to 

 the subject. 



On turning the telescope and spectrum-apparatus, driven by clock-work, 

 on to the sun at the date mentioned, in such a manner that the centre of 

 the umbra of the small spot then visible fell on the middle of the slit in the 

 screen, which, like the corresponding one in the spectroscope, was longer 

 than the diameter of the umbra, the solar spectrum was observed in the 

 field of view of the spectroscope with its central portion (con-esponding 

 to the diameter of the umbra falHng on the sUt) greatly enfeebled in 

 brilliancy. 



All the absorption-bands, however, visible in the spectrum of the pho- 

 tosphere, above and below, were visible in the spectrum of the spot ; they, 

 moreover, appeared thicker where they crossed the spot-spectrum. 



I was unable to detect the slightest indication of any bright bands, 

 although the spectrum was sufficiently feeble, I think, to have rendered 

 them unmistakeably visible had there been any. 



Should these observations be confirmed by observations of a larger spot 

 free from "cloudy stratum,'' it will follow, not only that the phenomena 



y2 



