194 Mr. J. N. Lockyer. 



(April 3, 1889), Professor Vogel says this appearance of the line was 

 probably due to a slit not sufficiently narrow. He says that he re- 

 examined this line in his observations with the great Vienna refractor, 

 and that it did not then appear otherwise than denned and narrow." 



I fancy that Dr. Huggins and Professor Vogel must know that 

 widening the slit does not generally cause a well-defined line to 

 become less defined on one side only. Again, the fluting would very 

 probably be seen little better with the Vienna refractor than with 

 that at Bothkamp ; for I find that the brightness of the nebula in 

 the former is to that in the latter only about as 13 to 10. 



Since my paper of November, 1887, was written other observers 

 besides Dr. and Mrs. Huggins have had their attention directed to 

 nebular spectra, with special reference to the character of the chief 

 line. 



My first observations of the nebula of Orion from this point of view 

 were made at Westgate-on-Sea in October, 1888, by means of a 

 12- inch mirror that had been kindly placed at my disposal by 

 Mr. Common. The image of the nebula being allowed to float 

 slowly over the slit, I distinctly got the impression that the line in 

 question varied in its behaviour from the other lines, and that at the 

 points where it was brightest it extended most towards the blue end 

 of the spectrum. The observations were repeated at Kensington 

 with the 10-inch equatorial by Mr. Fowler, Demonstrator of As- 

 tronomy, and Mr. Baxandall, and they arrived at the conclusion that 

 the chief line had a decidedly fluted appearance. 



This observation is further borne out by Mr. Taylor, who, referring 

 to an observation made in November. 1888,* states : 



" The 5001 line is by far the brightest in the spectrum. It is 

 never seen sharp, but, with the narrowest slit, always has a fluffy 

 appearance, this being much more marked on the blue than on the red 

 edge. This was most carefully examined for evidence of structure, 

 but. the line was always found to be single, and no decided evidence of 

 fluting structure could be made out." It is clear from this observa- 

 tion that the line fades away towards the violet end of the spectrum, 

 although the actual compound structure of the magnesium fluting is 

 not visible. I shall presently have to refer to an experiment which 

 shows that the compound structure would not be likely to be visible. 



I have quite recently (October 29) observed the spectrum of the 

 nebula in Orion with my 30-inch reflector at Westgate-on-Sea, using 

 an enlarged form of pocket spectroscope with a dispersion which 

 does not split D, and the observation is, to my mind, final. I found 

 that in certain parts of he nebula the lines were knotted, and in 

 others broken ; but in the former case, whilst the F line thickened 



* 'Monthly Notices, E.A.S.,' vol. 49, p. 125. 



