136 NOTES ON HALLEY's COMET. 



careful examination shows that even in this short interval there 

 were certain changes in the tail. A well-defined streamer on the 

 south side of the main tail apparently moved slightly, but quite 

 perceptibly, away from the latter. This might either mean tn^c 

 the streamer was actually separating from the main body of the 

 tail, or that the tail was rotating. Other photographs strengthen 

 the impression that the tail had a rotary motion, and that the 

 actual paths of particles along the tail were spiral curves. A con- 

 tinuous series of photographs taken at short intervals would 

 probably throw much light upon the nature of the rapid changes 

 which take place in a comet's tail. 



The present and widely-accepted theory as to the origin 

 oi the tail of a comet is that it is formed of particles which 

 have been projected from the nucleus with equal velocity 

 in all directions. The particles which have been ejected 

 from that hemisphere of the nucleus facing the sun are 

 driven back by large repulsive forces emanating from the sun, and 

 form paraboloidal envelopes having the appearance of a "sheath" 

 over the nucleus. The particles expelled from the nucleus on the 

 side away from the sun take no part in the formation of these 

 envelopes. The "sheaths" are very well-marked on many of 

 the photographs of Halley's Comet. They are rarely symmetrical 

 but generally brighter and longer on the south side. On some 

 occasions the central line or axis of the tail was darker than other 

 parts, suggesting that the tail was hollow and formed mainly by 

 the continuations of the sheath around the head. On other occa- 

 sions the axis of the tail was relatively bright as though the tail 

 were mainly composed of particles projected from the nucleus on 

 the side away from the sun. 



The most debateable questions in connection with the tail of 

 Halley's Comet have arisen in connection with the predicted pas- 

 sage of the earth through it. This was to have occurred on the 

 i8th — igth May, 1910, and naturally all preparations were made 

 to observe a phenomenon of so rare a nature. What was actually 

 observed was that the main tail of the comet remained almost 

 stationary in the Eastern sky until May 21st, after which date the 

 tail presumably diffused away. On the evening of May 20th, the 

 nucleus, with a short tail, was seen in its expected position in the 

 Western sky, but absolutely no connection could be traced betVeen 

 it and the great shaft of light seen the following morning in the 

 Eastern sky. On the i8th and 19th of May, certain optical 

 meteorological phenomena {e.g., the appearance of Bishop's rings 

 about the sun and moon), were so widely observed as to render a 

 connection between their formation and the near vicinity of the 

 comet's tail not improbable. 



The full explanation of these various occurrences has yet to 

 be given. In regard to the meteorological phenomena observed, it 

 is very likely that some of the numerous streamers lying on the 

 south side of the comet's tail crossed the path of the Earth ; but 

 it was quite certain that the main body of the tail lay to the North 

 of the Ecliptic and could not have encountered the Earth. The 

 greater difficulty is to explain the persistence in the Eastern sky 



