On the Chief Line in the Spectrum of the Nebula. 1 79 



(in some observations) four prisms and a Cooke spectroscope of six- 

 teen prisms have been used. 



The comparisons so far employed by Dr. Huggins in his observa- 

 tions of the chief nebula line are the double green line of nitrogen, 

 a line of lead, and the bright fluting seen in the spectrum of burning 

 magnesium. The relative positions of these have been re-observed in 

 the laboratory. 



The exact wave-length of the brightest edge of the magnesium 

 fluting was first determined by means of a comparison photograph of 

 the Sun and burning magnesium. 



The dispersion and width of slit were such that practically all the 

 lines seen in Rowland's photographic map were shown in the photo- 

 graph. There was a slight shift, the amount of which could be deter- 

 mined by measuring the displacement of b; when this was allowed 

 for, the wave-length of the magnesium fluting was found to be 5006'5 

 on Angstrom's scale. This has since been confirmed by observations 

 with the four-prism Steinheil and the first order spectrum of the 

 Rowland grating. 



Comparison photographs have also been attempted with the Row- 

 land grating, but it was found that even with two hours' exposure 

 only the first four maxima of the magnesium fluting were obtained. 

 It was found difficult to keep the flame of the burning wire sufficiently 

 steady to ensure the light falling directly on the slit during the whole 

 time of exposure. 



It may be mentioned here that the secondary maxima of the fluting 

 succeed each other at gradually increasing distances apart. The 

 wave-lengths given by Messrs. Liveing and Dewar* and those deter- 

 mined from the photographic comparison referred to are as follow : 



Liveing and Dewar. 

 5006-4 



10-8 

 4995-6 



10-2 

 4085-4 



11-8 

 4973-6 



12-0 

 4961-6 



13-0 

 4948-6 



14-2 

 4934-4 



Lockyer. 

 5006-5 



10-4 

 4996-1 



11-0 

 4985-1 



11-6 

 4973-5 



12-2 

 4961-3 



12-9 

 4948-4 



14-0 

 49344 



The next observations were made with respect to the relative posi- 

 tions of the magnesium fluting and the less refrangible component of 



* ' Roy. Soc. Proc.,' vol. 44, p. 248. 



