432 
ME. W. HUGGINS AND DE. W. A. MILLEE ON 
air upon the more refrangible portions of the spectrum ; in proportion to which absorp- 
tion the other parts of the spectrum become relatively exalted, and thus predominate 
more or less in the eye. 
This inequality of the absorptive action of the vapours of the atmosphere upon 
different parts of the spectrum becomes very evident if the eyepiece of the telescope 
be put out of focus (without the focus) so as to bring the blue and red rays to a focus 
in the centre of an expanded image of the star. In the case of B of /3 Cygni, the centre 
appears purple, surrounded with a margin of green. In proportion to the changes in 
the atmosphere by the passage of masses of vapour or thin cloud, will be the variations 
of these colours. The green becomes greener ; but the blue and the violet are affected 
in a much greater degree, at times fading almost completely ; then the colours resume 
their former tints and brightness. Several such changes may sometimes occur during 
one observation. 
The spectrum B observed under conditions of atmosphere in which the colour of the 
star was blue, was remarkable for the faintness of the orange and yellow portions com- 
pared with the rest of the spectrum. The diminished brightness of these parts appears 
to be produced by several groups of closely set fine lines, while towards the more refran- 
gible limit of the spectrum a few strong lines separated by considerable intervals are 
seen. 
The observation of this star, on account of the faintness of its spectrum, is so difficult 
and fatiguing to the eye that we have not been able to examine it more accurately or 
in greater detail. 
We have by the same method of observation examined the spectra of the components 
of a Herculis. The spectrum of A is remarkable for the great strength of the groups 
of lines in the green, blue, and violet ; fainter bands are visible in the yellow and orange, 
also two strong bands in the red. This arrangement of the bands of absorption agrees 
with the orange colour which strongly predominates in the light of this star. 
B is bluish green in colour. The more refrangible portions of its spectrum are 
very bright in consequence of the absence of any strong bands. The yellow and the 
orange parts are crossed by several groups of lines. — August 31, 1864.] 
The suggestive fact that stars of these more highly refrangible colours are always 
observed in close contiguity with much brighter stars, generally of an orange or red tint, 
would afford countenance to the supposition that these exceptional colours are due to 
some special physical conditions essentially connected with the stellar systems of which 
they seem to form a part. 
Arago* remarks, “Among the sixty or eighty thousand isolated stars, the positions 
of which are to be found in the catalogues of astronomers, there are none, I think, 
inscribed with any other indications in regard to colour, than white, red, and yellow. 
The physical conditions which determine the emission of blue and green light appear, 
then, to exist only in multiple stars.” 
* Popular Astronomy, translated by Smyth and Grant, vol. i. p. 295. 
