SPECTRUM OF THE GREAT NEBULA IN ORION. 85 
(c) Relation to Stars of Groups II. and LIL. 
With further condensation, the interspaces between the meteorites will be reduced, 
and the bright-line stars will pass to stars with absorption spectra in which the dark 
lines correspond with the bright lines of the nebulee. There will, however, be inter- 
mediate stages (Group II. and the early stages of Group III.), as I have already 
pointed out.* At these stages, some of the high-temperature lines do not appear 
either as bright or dark lines, and this, no doubt, for the reason that the radiation 
from the interspaces is masked by the absorption of the vapours in the immediate 
neighbourhood of the meteoritic stones. In these stars the hydrogen lines are 
normally feeble dark lines, but the amount of radiating area in a cross section is so 
nearly equal to the amount of absorbing area that disturbances which, according to 
the meteoritic hypothesis, produce the increase of light in the variable stars of the 
group, are sufficient to make the hydrogen lines appear bright. 
When we pass to the more condensed bodies, we find a group of stars, of which 
y Cygni is a typical case, in which the dark lines are very numerous,t but different 
from those which appear in the solar spectrum. This difference, however, is not 
taken account of in VocEw’s classification of stellar spectra. It may be added that 
photographs of the spectra of other stars resembling y Cygni have been obtained since 
the date of the paper referred to. 
At a still further stage of condensation we get stars in which there are only a 
relatively small number of lines, and these are lines which appear in the nebule. 
This similarity became evident at an early stage of the discussion of the photographie 
spectrum of the Orion nebula, and a comparison with the spectrum of a Andromedze 
was given in the paper communicated to the Royal Society, in November, 1892.1 
The first suggestion of such a relation appears to have been made by Dr. ScHEINER, 
of Potsdam,§ who pointed out that the strong line at \ 4471, which had been observed 
in the Orion nebula by Dr. CopELAND, was also seen in the Potsdam photographs of 
the spectrum of Rigel. This line is one of the brightest in the nebula photographs 
now under discussion, and is seen in the spectra of a large number of stars of the type 
of Rigel and Bellatrix. 
The spectra of such stars in the region between K and 472 are described in my 
paper referred to above; and in Table IV. they are compared with the spectrum of 
the Orion nebula. It will be seen that out of 31 lines in the nebula in the region 
compared, 20 are coincident with stellar lines.|| 
* © Phil. Trans.,’ A, 1893, vol. 184, p. 711. + Ibid., p. 698. ab) Tilak, (0) CUS), 
§ ‘ Ast. Nach.,’ 2923, p. 328, 1889. 
|| It may be added that D,, which appears bright in the nebula, was observed by Mr. Fowter asa 
dark line in the spectrum of 7 and ¢ Orionis, on Dec. 12, 1893, and in Rigel on March 4, 1894. It does 
not, however, appear in the spectrum of Sirius or # Lyre. Ds has also been photographed asa dark line 
in the spectra of Band « Orionis, by Mr. Campsety at the Lick Observatory. ‘Astr. and Ast. Phys.,’ May, 
1894 p. 395. 
