576 Recent Results of Astronomical Research. [March 26, 



feature is that two or more of the streamers often run parallel to one 

 another, their undulations exactly corresponding with the crest of one 

 fitting over the crest of the other. As might be expected the undula- 

 tions become larger, both in length and in amplitude, as we proceed out- 

 wards from the head. Professor Wolf has published some interesting 

 data on this point. The thought suggests itself that the curves may 

 be spiral curves produced by a rotation of the nucleus of the comet 

 whilst it is discharging the streamers. The hypothesis is one which 

 can probably be tested by careful measurement ; meanwhile it appears 

 to be more probable that the undulations, like so many other features, 

 must be accounted for by changes, perhaps rhythmic changes, in the 

 force of expulsion of the material. 



Whatever may be the true cause of the phenomena of comets' 

 tails, it is at least clear that the source of the power which forms them 

 and which directs them is to be found in the sun. I am not sure 

 that the exceptional activity of this comet is not due to the physical 

 state of the sun at the time rather than to the constitution of the 

 object itself. Certainly progress in explaining the phenomena is 

 hampered by our partial ignorance of the electrical and physical 

 conditions of the sun's surface. Therefore I cannot close this 

 discourse without alluding to what is perhaps the greatest result of 

 any that recent years have afforded to astronomy : Professor Hale's 

 photographs of Solar Vortices and investigations of their magnetic 

 fields. Witli the great Tower telescope in the clear atmosphere of 

 Mount AVilson Observatory he obtained photographs of the remark- 

 able structure, revealing to our eyes tlie gigantic whirlwinds raging 

 over the solar surface, above the sun-spots. These photographs are 

 taken with the light of one particular wave-length the Ha line of 

 hydrogen. He lias gone farther and shown, I believe, to the satis- 

 faction of physicists, that the light passing up through these vortices 

 bears the sure marks of having passed through a strong magnetic 

 field, whose lines run perpendicular to the solar surface, and that, 

 according as the vortices rotate clockwise or counter-clockwise, the 

 lines of magnetic force run from or into the sun. The chain of 

 evidence seems to show that the field is produced by the rotation of 

 negatively charged material in these vortices. 



It would be hard to exaggerate the value of these latest revela- 

 tions as to the condition of the sun, far though they are from 

 satisfying our inquiries or enabling us to realise that power which 

 over all the millions of miles convulses the comet, and scatters its 

 trailing debris to the remotest parts. 



[A. S. E.] 



