54 



Dr. W. Hugging and Mrs. Huggins. [May 2, 



the left eye, in such a position that the image seen in the instrument 

 with the right eye was apparently projected on the paper. If I had 

 noticed any peculiarity about A, 5004, it would certainly have been 

 noted."* 



In an early observation of the dumb-bell nebula Professor Yogel, 

 indeed (' Beobachtungen zu Bothkamp,' p. 59, 1872), describes this 

 line as less defined towards the violet side. In a letter (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 defined and narrow. 



The other line in the spectrum of the nebulae upon which 

 Mr. Lockyer mainly relies for the presence of magnesium is the line 

 shown in my photographic spectrum of 1882, f and to which I assigned 

 the wave-length of about 3730. Mr. Lockyer says of this line : J " In 

 the Bunsen as ordinarily employed the fluting at 500 far eclipses the 

 other parts of the spectrum in brilliancy, and at this temperature, as 

 already observed by Messrs. Liveing and Dewar, the ultra-violet line 

 visible is that at 373." Passing by a minor point, which Liveing and 

 Dewar have already pointed out,§ namely, that their observation was 

 made at the higher temperature of burning magnesium, this statement 

 is insufficiently complete, for what occurs at this part of the 

 spectrum, and is characteristic of the magnesium-flame spectrum, is a 

 triplet, of which the line given by Liveing and Dewar at about 3730 

 is the least refrangible member. 



In the accompanying Diagram I give a representation of this triplet 

 at the wave-lengths given by Liveing and Dewar, namely X 3730, 3724 

 and 3720. In the photograph of 1888, in which the strong line can be 

 seen distinct from the lines near it, the line is found to be very near 

 the middle line of the triplet. I have therefore assigned to this line the 

 position of about X 3724. This line appears pretty strong, and there- 

 fore if it were really one of the lines of the triplet, the other two 

 members of the triplet should have appeared on the plate. On one 

 side of the star-spectra this line is a little broader than on the other 



* Mr. Taylor, late of the South Kensington Laboratories, observing at Sir Henry 

 Thompson's observatory in November, 1888, says : " 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 line was most carefully examined for evidence of structure, but 

 was always found to be single, and no decided evidence of fluting structure could be 

 made out. It may be that greater dispersion may show structure, but with the 

 dispersion used here no structure could be seen." ' Monthly Notices R.A.S.,' 

 vol. 49, p. 125. 



f ' Koy. Soc. Proc.,' vol. 33, p. 425. 



I ' Eoy. Soc. Proc.,' vol. 43, 1887, p. 122. 



§ ' Eoy. Soc. Proc.,' vol. 44, 1888, p. 244. 



