56 F. ^Y. Yery — Nebxilosity around Nova Persei. 



their trajectory where they begin to recede from us with 

 increasing velocity. If the actual velocity is that of light, the 

 component of motion in the line of sight will soon reach a high 

 value — let us say 100,000 km. per sec. — when, even if the 

 original ionic radiation were rich in violet and ultra-violet 

 light, the waves of ether must be so lengthened by the motion 

 of recession that they no longer impress the photographic 

 plate. Consequently, at this radial distance from the star, and 

 in like manner on the opposite side at t, the nebula fades out ; 

 but at nearly the same radial distance, particles which have 

 passed undetected on account of their recession along lines 

 from the star's north pole, reach, at t' and^>', positions where 

 the motion of recession changes to one of approach. Yiolet 

 light begins to emanate from these regions. Soon the motion 



of approach becomes so vigorous that even red or infra-red 

 rays, if they exist, will have their wave-lengths so shortened 

 that they can be photographed. On the supposition that the 

 nebula is a gigantic corona with symmetrical sheaves of fila- 

 ments around both poles, diverging under angles of something 

 over 60° — one capable of being photographed, the other not — 

 the puzzling phenomena of appearance and disappearance at 

 the outer ring are explained. They are demanded by the 

 theory, instead of being anomalies. The spectroscope alone 

 can decide whether these hypothetical changes of wave-length 

 are real ; and as yet this evidence is wanting. If the spectro- 

 scope should decide against the change, the supposition of 

 direct ionic luminosity would have to be given up, but not 

 necessarily the ionic theory, since there remains the hypothesis 

 of phosphorescence of quiescent matter under ionic impact. 



Perrine's observation of March 29, 1901, indicates the exist- 

 ence, at that date, of two nebulous rings, with radii in the 

 ratio of 1:2, and an arc on the N.E. side, which perhaps is 

 the sole record of a third and wider ring. The three radii hav- 

 ing approximately the ratio 1:2:4, may correspond to ions 



