352 Mr. J. N. Lockyer on Spectroscopic [Mar. 18, 



On the 21st I caught a trace of magnesium ; but it was late in the clay, 

 and I was compelled to cease observing by houses hiding the sun. 



On the 28th 1 was more fortunate. If anything, the evidences of intense 

 action were stronger than on the 21st, and after one glance at the F line I 

 turned at once to the magnesium lines. I saw them appearing short and 

 faint at the base of the chromosphere. My work on the spots led me to 

 imagine that I should find sodium-vapour associated with the magnesium ; 

 and on turning from b to D I found this to be the case. I afterwards re- 

 versed barium in the same way. The spectrum of the chromosphere 

 seemed to be full of lines, and I do not think the three substances I have 

 named accounted for ail of them. The observation was one of excessive 

 delicacy, as the lines were short and very thin. The prominence was a 

 small one, about twice the usual height of the chromosphere ; but the 

 hydrogen lines towered high above those due to the newly injected materials. 

 The lines of magnesium extended perhaps one-sixth of the height of the F 

 line, barium a little less, and sodium least of all. 



We have, then, the following facts : — 



I. The lines of sodium, magnesium, and barium, when observed in a 

 spot, are thicker than their usual Fraunhofer lines. 



II. The lines of sodium, magnesium, and barium, when observed in the 

 chromosphere, are thinner than their usual Fraunhofer lines. 



A series of experiments bearing upon these observations is now in 

 progress at the College of Chemistry, and will form the subject of a commu- 

 nication from Dr. Frankland and myself. I may at once, however, remark 

 that we have here additional evidence of a fact I asserted in 1865 on tele- 

 scopic evidence — the fact, namely, that a spot is the seat of a downrush, 

 a downrush to a region, as we now know, where the selective absorption of 

 the upper strata is different from what it would be (and, indeed, is elsewhere) 

 at a higher level. 



Messrs. De La Rue, Stewart, and Loewy, who brought forward the theory 

 of a downrush about the same time as my observations were made in 1865, 

 at once suggested as one advantage of this explanation that all the grada- 

 tions of darkness, from the faculae to the central umbra, are thus supposed 

 to be due to the same cause, namely, the presence to a greater or less extent 

 of a relatively cooler absorbing atmosphere. This I think is now spectro- 

 scopically established ; we have, in fact, two causes for the darkening of a 

 spot : — 



I. The general absorption of the chromosphere, thicker here than else- 

 where, as the spot is a cavity. 



II. The greater selective absorption of the lower sodium, barium, mag- 

 nesium stratum, the surface of its last layer being below the ordinary- 

 level. 



Messrs. De La Rue, Stewart, and Loewy also suggested, in their e Re- 

 searches on Solar Physics,' that if the photosphere of the sun be the plane 

 of condensation of gaseous matter, the plane may be found to be subject to 



