On the Chief Line in the Spectrum of the Nebulce. 195 



equally on both sides, the chief line thickened only on the more re- 

 frangible side. This result is shown in fig. 4. 



FIG. 4. Diagram showing the appearance of the three principal lines in the 

 spectrum of the nebula in Orion as observed in the Westgate 30-inch reflector. 



This was confirmed by Messrs. Fowler and Baxandall at Keusing- 

 ton, with the 10-inch equatorial on October 31st and November 1st, 

 and again by Mr. Fowler, with the 30-inch, on November 2nd. It may 

 be noted also that I got momentary glimpses of many bright lines 

 between F and G on October 31st. Messrs. Fowler and Coppen have 

 since made some very careful observations of the Ring nebula in 

 Lyra, and also record the chief line as having a fringe on the more 

 refrangible side. A line less refrangible than 500 in the neighbour- 

 hood of b was suspected ; this may turn out to be the carbon fluting 

 near 517 : the absence of the hydrogen line in the 10-inch was im- 

 portant as indicating that the nebula is in an advanced stage of 

 condensation, approaching that of the nebula in Andromeda. 



In the observations with the siderostat arrangement, as pointed 

 out in the extracts from the observatory note-book, the chief line 

 was noted by Mr. Fowler and Lieutenant Bacon to have a decided 

 fringe on the more refrangible side. 



It may be remarked that high dispersion is not so likely to show 

 the fluted character of the chief line as low, for the more the 

 fringe is dispersed the fainter it must become. 



In consequence of the brilliancy of the Orion nebula, the fluted 

 appearance of the chief line would be more manifest than in any other 

 nebulae, and the absence of the fringe when the line is seen in the 

 spectra of fainter nebulae is therefore not antagonistic to the view that 

 the line may be the remnant of the magnesium fluting. This must 

 not be misinterpreted. Given two nebulae, exactly alike in every 

 respect but temperature, then the line, if visible in both, would appear 

 more like a compound fluting in that nebula of which the temperature 



