SPECTRUM OF THE GREAT NEBULA IN ORION. Ot 
Ill. THe Ortcins oF THE LINES. 
Tt will be seen from the table, that hydrogen enters largely into the composition of 
the vapours of the nebula. H, H,, H;, H., and the ultra-violet series, certainly as far 
as H, (new notation),* are all present. 
It is worthy of remark, however, that while, as previously stated, H, is the strongest 
line in the whole spectrum, and H,, H;, and H, are also strong, the ultra-violet 
hydrogen lines are amongst the weakest. 
Next to H,, the line \ 373 is the most intense. In 1887, I suggested that this line 
was one of the members of the triplet seen in the spectrum of burning magnesium. 
As I stated in a preliminary communication, the wave-length could not be finally 
determined from the photographs already obtained, but it was probably near ) 3729. 
This value, however, will require correction for motion in the line of sight. If Mr. 
KEELER'S valuest for the motion be accepted, and the earth’s orbital velocity be 
allowed for, the correction will be about 0:22 tenth metres towards the red. This will 
bring the nebular line slightly nearer the least refrangible member of the magnesium 
triplet. Further measures of photographs taken with higher dispersion are necessary 
in order to settle this point. 
The lines next in importance to those already mentioned, are near wave-lengths 
4471, 4495 and 3868. The first of these, the strongest between H, and H,, is 
probably the line observed by Dr. CopELAND in 1886. With reference to this line, 
I wrote as follows in a paper communicated to the Royal Society on Nov. 9, 1889.1 
“The observations of Dr. CopELAND have now, I think, established the identity of 
the yellow line, in the nebula of Orion at all events with D;. In a letter to 
Dr. CopELAnD, I suggested that the line at \ 447 was, in all probability, LoRENzont’s 
f of the chromosphere spectrum, seeing that it was associated both in the nebula and 
chromosphere with hydrogen and D,. This he believes to be very probable. The line 
makes its appearance in the chromosphere spectrum about 75 times to 100 appearances 
of D,, or the lines of hydrogen.” 
For the other strong lines near ) 3868 and ) 4495, no origins have been found. 
From the final reduction of the photographs, as given in the table, it appears that 
the line formerly said§ to be “near \ 4027,” is at 44025. It can, therefore, no 
longer be attributed to manganese. Its origin is at present unknown, but, as will 
appear later, it is a line frequently met with in the spectra of other celestial bodies. 
The line at \ 4690 referred to above, does not appear in the revised list, which only 
contains lines measured without great difficulty. Further, only a small proportion of 
the lines now mapped can be ascribed to metallic origins, but these, it will be seen, are 
* Voce, ‘ Ast. Nach.,’ 3198, 1893. 
+ ‘Roy. Soc. Proc.,’ vol. 49, p. 400, 1891. 
£ Ibid., vol. 47, p. 30. 
§ Ibid., vol. 48, p. 200. 
