On the Chief Line in the Spectrum of the Nebula. 191 



should briefly state what I understand by a fluting, and I cannot do 

 this better than by referring to observations of a candle flame which 

 anyone can make. A pocket spectroscope and a lens are all that are 

 needed to follow my remarks. If the image of the base of the flame 

 be projected on to the slit, bright flutings are seen in the green 

 (near 6), citron, blue, and violet. That in the green is the brightest, 

 and is seen to consist of three apparent bright lines with faint fringes 

 on their more refrangible sides. The different members of the group 

 gradually diminish in brightness, the least refrangible being the 

 brightest. Such a group as this I look upon as a compound fluting, 

 and each member itself as a simple fluting, since with high disper- 

 sion the fringes break up into series of fine lines very close to- 

 gether. If now the image of the flame be gradually raised, so that 

 the base passes off and portions nearer the centre are brought on 

 to the slit, the fainter members of the group gradually disappear, 

 and when a certain point is reached only the brightest, least refran- 

 gible, simple fluting is left. This I look upon as the " remnant of a 

 fluting," whether the fluting was in the first instance simple or com- 

 pound. 



The compound fluting of magnesium near 500 is very similar to 

 that of carbon. It consists of a series of bright lines of gradually 

 diminishing brightness and increasing distances apart towards the 

 more refrangible end, and each has a fringe on the more refrangible 

 side. The first maximum (the least refrangible) is brighter than the 

 others, and the fringe close to it is brighter than the second maxi- 

 mum, and so when " the remnant of the magnesium fluting near 500 " 

 is referred to, the first maximum with that portion of its fringe 

 which is brighter than the second maximum is meant. 



Before I give the observations of the character of the chief 

 nebula line in historical sequence, I quote Dr. Huggins's statement : 

 " My own observations of the line, since my discovery of it in 1864, 

 with different spectroscopes up to a dispersion equal to eight prisms 

 of 60, show the line to become narrow as the slit is made narrow, 

 and to be sharply and perfectly defined at both edges." 



The following are the first recorded observations : 



In 1864, the spectrum of the Dumb-bell nebula in Vulpecula was 

 observed, and it was noted* that the light of this nebula, after "' pass- 

 ing through the prisms, remained concentrated in a bright line, cor- 

 responding to the brightest of the three lines represented in fig. 5, 

 Plate X. This line appeared nebulous at the edges" 



Similarly, it was recorded in 1866 of the spectrum of General 

 Catalogue No. 4403 : f" The spectrum of this nebula indicates 

 that it posesses a gaseous constitution. One bright line only was 



* ' Phil. Trans.,' 1864, p. 441. 

 t ' Phil. Trans.,' 1866, p. 385. 



