SPECTRUM OF THE GREAT NEBULA IN ORION. 87 
Taste ITV.—Comparison of Orion Nebula with Stars of Groups IJ. and III, 
(Region K to \ 472 Angstrém)—(continued.) 

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Group IITy. 


Orion nebula. Group II.) Group IIIa. 
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| 
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| a Cygni, Rigel. Bellatrix, | 6 Orionis. | « Virginis. | 
o6 pease 30 4417 (3) 00 4417 (2) 
4426 (2) | 
ac eoeacte 36 o¢ 50 4437 (3) | 
4471 (4) | 4471 4471 4471 (1) | 4471 (4) | 4471 (6) | 4471 (5) | 4471 (4) 
oc lenrea oe 4481 (5) | 4481 (5) | 4481 (8) 
4495 (4) | 
4539 (2) | 4541 45.4] 4541 (2) 4541 (1) | 4541 (1) 
oe ES . 4549 (4) 
4555°5 (3 fe 4553 (3) 
4558°5 (3 
a - 4583 (4) | 
4627 (2) | 4629 4629 Aig) (8) || se 4629 (2) 
4715 oc fe 56 Be 4714 (3) 









In the accompanying map (fig. 4) an attempt is made to show the gradual change 
of the bright lines of the nebulz and bright-line stars into dark ones as condensation 
proceeds. Not all the lines seen in the various spectra, but only those which best 
illustrate the results of progressive condensation, are dealt with. In the spectrum of 
the Orion nebula and planetary nebule, the lines shown in the map are those which 
afterwards appear as bright lines in the bright-line stars, or as dark lines in some of 
the more condensed bodies. The lines mapped as belonging to the bright-line stars 
are those which appear twice in Professor PICKERING’S lists, to which reference has 
been made, except in the case of the important line at 4471, which has been 
included because it appears in P Cygni as well as in one of PICKERING’s types of 
bright-line stars. The dark lines shown in the spectra of stars of Groups II., II«., 
TIIg., Ily., 1V«., and IV£.,* are only those which show remarkable coincidences 
either amongst themselves or with the bright lines at the foot of the map. The 
approximate intensities of the various lines in the map are represented by their 
thicknesses. 
If we consider, first, the question of the hydrogen lines, it will be seen that they 
begin to thin out as bright lines in the bright-line stars, and make their appearance 
as thin dark lines in Group II. From Group II. to Group IV. they thicken pretty 
regularly ; but other causes besides temperature may possibly affect their apparent 
thickness, as I have previously pointed out.t 
In addition to the lines of hydrogen, other lines, including the H and K lines of 
* See ‘ Phil. Trans.,’ vol. 184, 1893, p. 725. 
+ Ibid., p. 688. 
