430 
MR. W. HUGGINS AND DR. W. A. MILLER ON 
vary in nature as the constituents of the star vary ; and observation has shown that the 
stars do differ from the sun and from each other in respect of the elements of which 
they consist. The light of each star therefore will be diminished by the loss of those 
rays which correspond in refrangibility to the bright lines which the constituents 
of each atmosphere would, in the incandescent state, be capable of emitting. In pro- 
portion as these dark lines preponderate in particular parts of the spectrum, so will the 
colours in which they occur be weaker, and consequently the colours of other refrangi- 
bilities will predominate. 
Of this the spectrum of a Orionis affords a good example. The green and blue parts of 
the spectrum are comparatively dark, from the numerous and close groups of dark lines. 
In the orange they are less strong. Hence it might be anticipated that the light of the 
star would be characterized by “ an orange tinge,” as noted by Smyth. (3 Pegasi is 
described by Smyth as “deep yellow;” and the appearance exhibited by its spectrum, 
which closely resembles that of a Orionis, though much fainter, supports the same 
view. 
Aldebaran is recorded by Smyth as of a “ pale rose tint.” In the spectrum of this star, 
with the exception of the hydrogen line C, there are but few strong lines in the red, 
whilst the orange portion is considerably subdued by dark lines, which are less nume- 
rous in the green and blue. Sirius, on the contrary, is “brilliant white” (Smyth); and 
the continuous brightness of the spectrum, with the exception of five strong lines, is, as 
compared with Aldebaran and a, Orionis, unaffected by the dark lines which cross it. 
The spectrum is indeed crowded with numerous fine lines ; but the intensity of these 
lines is extremely feeble as contrasted with those of the stars just mentioned. It may 
be that the length of the stellar atmosphere through which the light passes is less, 
relatively to the intensity of radiation from the photosphere, and so is insufficient to 
produce lines of the same degree of blackness as would be produced if the atmosphere 
were denser. The great intensity, however, of the light of Sirius would rather lead to 
the conclusion that the atmosphere of vapours is itself highly incandescent. If so, 
might it not to some extent replace with its own light, the light which it has absorbed 
from the photosphere behind it ? It matters little, however, for the present purpose, 
whether or not either of these suppositions be adopted. There is at all events a most 
striking difference between the effect on the colour of the star of the closely grouped 
and very dark lines in the green and blue portions of the spectrum of a Orionis and of 
the corresponding portion of the spectrum of Sirius, in which the dark lines are faint 
and wholly unequal to produce any noticeable subduing of the blue and green rays. 
We have not yet had an opportunity of testing by experiment whether this hypo- 
exceedingly high temperature to convert them into vapour, renders untenable the supposition, which might 
otherwise have been entertained, that the orange and red tints of the light of these stars might be due to an 
inferior degree of incandescence of the photosphere as compared with the temperature of the stars the light of 
which is white. 
