On Photographs of the Spectrum of the Nebula in Orion. 199 



note-book. The observations were made by Mr. Fowler (who was 

 assisted by Mr. Coppen) on February 5 : 



" Made further observations of the nebula of Orion with 4-prism 

 Steinheil spectroscope. First compared nebula spectrum with spec- 

 trum of spark between iron poles close to slit, the secondary collimator 

 not being used at all. 



" Results. 495 nebula line exactly coincident with iron line 

 4956'8. At the same time, the 500 nebula line was certainly less 

 refrangible than the nitrogen lines. 



" Next adjusted collimator and put hydrogen tube and iron spark 

 successively in front of slit. 



" Results. 3rd nebula line coincident with F line of hydrogen. 



" 495 line coincident with iron 4956'8. 



" 500 line less refrangible than nitrogen lines. 



" 500 line exactly coincident with magnesium fluting, whether the 

 magnesium was burned behind the slit of collimator or at the centre 

 of siderostat mirror." 



It will be seen that these observations entirely confirm those which 

 I have already communicated to the Society, and also carry the work 

 a step further in the determination of the actual wave-length of the 

 nebula line near X495 by the siderostat and collimator method. 



"Preliminary Note on Photographs of the Spectrum of the 

 Nebula in Orion." By J. NORMAN LocKYER, F.R.S. Re- 

 ceived and read February 13, 1890. 



In other communications to the Society, I have shown that the 

 chief nebula line coincides absolutely in position with the remnant of 

 the fluting seen in the flame of burning magnesium near X 500, with 

 the highest dispersion we could command at South Kensington. 

 Attempts have recently been made, therefore, with the 30-inch 

 reflector at Westgate-on-Sea, to obtain photographs of the spectrum 

 of the nebula, using magnesium as the term of comparison. The 

 objects sought were, primarily, to determine whether there was a 

 line in the nebula correspondiug with one of the lines of the magne- 

 sium triplet about X 373, and to obtain as complete a photographic 

 record as possible of the spectrum between this triplet and X 500. 

 With this view, Mawson's instantaneous plates were used, these 

 having been found to be fairly sensitive to the green. The exposures 

 have been carried up to four hours, and five photographs have already 

 been taken, some of them with shorter exposures than that named, in 

 consequence of the sky becoming clouded or irregularities in the 

 driving clock, which is not yet completely finished. One plate only 

 was exposed for four hours, on February 11, but, unfortunately, in 

 YOL. XLVIII. p 



