60 F. W. Very — J^ebulosity around Nova Persei. 



the distance between the poles of the body. The ions are so 

 minute that the differential repulsion on opposite poles of a 

 corpuscle is very small. Morever, if an ion consists of a cir- 

 culation of electricity, everywhere at right angles to a circular 

 axis, the corpuscle has no free magnetic poles. It is conceiv- 

 able that an instantaneous variation in the magnetic field may 

 produce within a limited space a gradient of magnetic force 

 which is steep relatively to the dimensions of an ion, and that 

 the corpuscle is carried along at the rate of motion of the mag- 

 netic impulse. Unless there is some such process, it is difficult 

 to find a sufficient cause for the deviation of the motion from 

 a radial direction by a magnetic field as feeble as that at the 

 extreme nebular distances. Something remains to be dis- 

 covered in regard to the way in which magnetic forces of con- 

 siderable strength are exhibited far from their sources. The 

 volcanic explosion of Mt. Pelee produced an instantaneous mag- 

 netic impulse recognizable all over the earth. Rapid changes in 

 large sun-spots are often accompanied by magnetic storms upon 

 the earth, especially if the earth is near a solar radius through 

 the spot, or the tangent plane at the spot's position. A continuous 

 outpouring of magnetic energy in these cases, and to the 

 immensely greater distances of the Nova Persei nebula, would 

 dissipate energy at a startling rate ; but a series of instantaneous 

 impulses, separated by relatively large intervals of time, while 

 entirely competent to guide the ionic movements, would also 

 economize energy. Besides this, there is a further economy in 

 the magneto-electric impulse which is not shared by the electro- 

 magnetic wave, for the former travels along the magnetic lines 

 of force from pole to pole, and is part of a regenerative system, 

 while the electromagnetic waves pass away in ever expanding 

 spheres and are dissipated. The ionic hypothesis of the origin 

 of the nebula is attended by difficulties, but at least it offers 

 the possibility of a complete explanation. 



In conclusion, I think that we may summarize the evidence 

 in regard to the cause of the nebulosity around Nova Persei 

 as follows : 



Lockyer's hypothesis does not fit the facts. The first two 

 radiation hypotheses are ruled out by the continued visibility 

 of the rings in spite of the gradual cessation of motion. The 

 reflection hypothesis of Kapteyn and Seeliger and its modifica- 

 tion by Hinks are further discredited by the impossibility of 

 an adequate albedo in a widely dispersed or nebulous material. 

 Of the two ionic hypotheses, that of luminous diamagnetic 

 corpuscles under magnetic control, and moving with velocities 

 of the same order as that of light, is favored by the appearances 

 and disappearances at the outer ring, but only spectroscopic 

 observation can decide between them authoritatively. 



Arcturus, Virginia. 



